block: Add flat VMDK extent layout

Defines and implements the layout of a VMDK extent, the region
of storage that is used by the virtual disk. Each line in the
descriptor file's extent section describes one extent.

Signed-off-by: Sumedh Alok Sharma <sumsharma@microsoft.com>

block/src/formats/vmdk/descriptor.rs

@@ -8,10 +8,10 @@
 //! of `FLAT` extent lines, and a disk database (DDB). Only the `monolithicFlat`
 //! and `twoGbMaxExtentFlat` create types are recognized.
 
-use std::collections::HashMap;
 use std::fs::File;
 use std::io;
 use std::os::unix::fs::FileExt;
+use std::path::Path;
 use std::str::Lines;
 
 use crate::AlignedFile;
@@ -33,7 +33,6 @@ pub enum VMDKDiskType {
 /// Flat VMDK extent line fields.
 /// Format of each extent line:
 /// `<access> <sectors> <type> "<file>" [offset]`.
-#[allow(dead_code)]
 #[derive(Debug, Default)]
 pub struct VmdkExtentHeader {
     pub access: String,
@@ -44,14 +43,9 @@ pub struct VmdkExtentHeader {
 }
 
 /// Descriptor header fields.
-#[allow(dead_code)]
 #[derive(Debug, Default)]
 pub struct VmdkDescriptorHeader {
-    pub version: u32,
-    pub cid: u32,
-    pub parent_cid: u32,
     pub create_type: VMDKDiskType,
-    pub parent_filename_hint: String,
 }
 
 /// Ordered list of extents.
@@ -59,11 +53,49 @@ pub struct VmdkDescriptorHeader {
 pub struct VmdkDescriptorExtents {
     pub extents: Vec<VmdkExtentHeader>,
 }
-/// Disk database.
-#[allow(dead_code)]
+
+/// Parsed flat VMDK descriptor
+///
+/// extents_list: ordered extent list
+/// base_path: descriptor file's parent directory
 #[derive(Debug, Default)]
-pub struct VmdkDescriptorDdb {
-    pub entries: HashMap<String, String>,
+pub struct VmdkDescriptor {
+    pub base_path: String,
+    pub extents_list: VmdkDescriptorExtents,
+}
+
+impl VmdkDescriptor {
+    pub fn new(file: &File, path: &Path) -> io::Result<Self> {
+        // The descriptor's directory anchors the relative extent filenames.
+        let base_path = path
+            .parent()
+            .ok_or_else(|| {
+                io::Error::new(
+                    io::ErrorKind::InvalidInput,
+                    "Cannot retrieve parent directory of the file",
+                )
+            })?
+            .to_string_lossy()
+            .to_string();
+
+        // Valid descriptor file must be much larger than 4 bytes.
+        if file.metadata()?.len() < 4 {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                "Invalid VMDK descriptor file: file is empty or too small",
+            ));
+        }
+
+        let content = read_descriptor(file)?;
+        let mut lines = content.lines();
+        let (_header, last_line) = parse_header(&mut lines)?;
+        let extents_list = parse_extents(&mut lines, last_line)?;
+
+        Ok(Self {
+            base_path,
+            extents_list,
+        })
+    }
 }
 
 // Read the whole descriptor into memory through an `AlignedFile` and return it
@@ -115,35 +147,24 @@ pub(crate) fn parse_header<'a>(
             break;
         }
         let parts: Vec<&str> = line.split('=').map(|s| s.trim()).collect();
-        if parts.len() == 2 {
-            match parts[0] {
-                "version" => header.version = parts[1].parse().unwrap_or(0),
-                "CID" => header.cid = u32::from_str_radix(parts[1], 16).unwrap_or(0),
-                "parentCID" => header.parent_cid = u32::from_str_radix(parts[1], 16).unwrap_or(0),
-                "createType" => {
-                    // Tools such as qemu-img quote the value, e.g.
-                    // createType="monolithicFlat", strip the quotes.
-                    header.create_type = match parts[1].trim_matches('"') {
-                        "monolithicFlat" => VMDKDiskType::MonolithicFlat,
-                        "twoGbMaxExtentFlat" => VMDKDiskType::TwoGbMaxExtentFlat,
-                        _ => VMDKDiskType::CreateTypeUnsupported,
-                    }
-                }
-                "parentFileNameHint" => header.parent_filename_hint = parts[1].to_string(),
-                _ => {}
-            }
+        if parts.len() == 2 && parts[0] == "createType" {
+            // Tools such as qemu-img quote the value, strip quotes for comparison.
+            header.create_type = match parts[1].trim_matches('"') {
+                "monolithicFlat" => VMDKDiskType::MonolithicFlat,
+                "twoGbMaxExtentFlat" => VMDKDiskType::TwoGbMaxExtentFlat,
+                _ => VMDKDiskType::CreateTypeUnsupported,
+            };
         }
     }
 
     Ok((header, last_comment_line))
 }
 
-pub(crate) fn parse_extents_and_ddb(
+pub(crate) fn parse_extents(
     lines: &mut Lines<'_>,
     last_comment_line: &str,
-) -> io::Result<(VmdkDescriptorExtents, VmdkDescriptorDdb)> {
+) -> io::Result<VmdkDescriptorExtents> {
     let mut extents = VmdkDescriptorExtents::default();
-    let mut ddb = VmdkDescriptorDdb::default();
 
     if last_comment_line != VMDK_DESCRIPTOR_EXTENTS {
         return Err(io::Error::new(
@@ -152,66 +173,55 @@ pub(crate) fn parse_extents_and_ddb(
         ));
     }
 
-    let mut in_extents_section = true;
     for line in lines.by_ref() {
         if line.trim().is_empty() {
             continue;
         }
         if line.starts_with('#') {
             if line == VMDK_DESCRIPTOR_DDB || line == VMDK_DESCRIPTOR_DDB_2 {
-                in_extents_section = false;
-                continue;
+                break;
             }
             return Err(io::Error::new(
                 io::ErrorKind::InvalidData,
                 "Expected the DDB section comment line",
             ));
         }
-        if in_extents_section {
-            let parts: Vec<&str> = line.split_whitespace().collect();
-            if parts.len() == 4 || parts.len() == 5 {
-                let size_in_sectors = parts[1].parse::<u64>().map_err(|_| {
+        let parts: Vec<&str> = line.split_whitespace().collect();
+        if parts.len() == 4 || parts.len() == 5 {
+            let size_in_sectors = parts[1].parse::<u64>().map_err(|_| {
+                io::Error::new(
+                    io::ErrorKind::InvalidData,
+                    format!(
+                        "VMDK extent size '{}' is not a valid sector count",
+                        parts[1]
+                    ),
+                )
+            })?;
+            let offset_in_sectors = match parts.get(4) {
+                Some(offset) => offset.parse::<u64>().map_err(|_| {
                     io::Error::new(
                         io::ErrorKind::InvalidData,
-                        format!(
-                            "VMDK extent size '{}' is not a valid sector count",
-                            parts[1]
-                        ),
+                        format!("VMDK extent offset '{offset}' is not a valid sector count"),
                     )
-                })?;
-                let offset_in_sectors = match parts.get(4) {
-                    Some(offset) => offset.parse::<u64>().map_err(|_| {
-                        io::Error::new(
-                            io::ErrorKind::InvalidData,
-                            format!("VMDK extent offset '{offset}' is not a valid sector count"),
-                        )
-                    })?,
-                    None => 0,
-                };
-                let extent = VmdkExtentHeader {
-                    access: parts[0].to_string(),
-                    size_in_sectors,
-                    extent_type: parts[2].to_string(),
-                    filename: parts[3].trim_matches('"').to_string(),
-                    offset_in_sectors,
-                };
-                extents.extents.push(extent);
-            } else {
-                return Err(io::Error::new(
-                    io::ErrorKind::InvalidData,
-                    "Malformed VMDK extent line",
-                ));
-            }
+                })?,
+                None => 0,
+            };
+            extents.extents.push(VmdkExtentHeader {
+                access: parts[0].to_string(),
+                size_in_sectors,
+                extent_type: parts[2].to_string(),
+                filename: parts[3].trim_matches('"').to_string(),
+                offset_in_sectors,
+            });
         } else {
-            let parts: Vec<&str> = line.split('=').map(|s| s.trim()).collect();
-            if parts.len() == 2 {
-                ddb.entries
-                    .insert(parts[0].to_string(), parts[1].to_string());
-            }
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                "Malformed VMDK extent line",
+            ));
         }
     }
 
-    Ok((extents, ddb))
+    Ok(extents)
 }
 
 /// Returns true when `prefix` begins with the `# Disk DescriptorFile` header.
@@ -240,8 +250,8 @@ pub fn is_flat_vmdk(f: &mut File) -> io::Result<bool> {
         _ => return Ok(false),
     }
 
-    let extents = match parse_extents_and_ddb(&mut lines, last_line) {
-        Ok((extents, _ddb)) => extents,
+    let extents = match parse_extents(&mut lines, last_line) {
+        Ok(extents) => extents,
         Err(e) if e.kind() == io::ErrorKind::InvalidData => return Ok(false),
         Err(e) => return Err(e),
     };
@@ -265,26 +275,54 @@ mod tests {
         Ok((header, last.to_string()))
     }
 
-    fn parse_body(
-        last_comment: &str,
-        body: &str,
-    ) -> io::Result<(VmdkDescriptorExtents, VmdkDescriptorDdb)> {
+    fn parse_body(last_comment: &str, body: &str) -> io::Result<VmdkDescriptorExtents> {
         let mut lines = body.lines();
-        parse_extents_and_ddb(&mut lines, last_comment)
+        parse_extents(&mut lines, last_comment)
     }
 
     // Two-stage parse, as `VmdkDescriptor::new` chains it.
-    fn parse_full(
-        input: &str,
-    ) -> io::Result<(
-        VmdkDescriptorHeader,
-        VmdkDescriptorExtents,
-        VmdkDescriptorDdb,
-    )> {
+    fn parse_full(input: &str) -> io::Result<(VmdkDescriptorHeader, VmdkDescriptorExtents)> {
         let mut lines = input.lines();
         let (header, last) = parse_header(&mut lines)?;
-        let (extents, ddb) = parse_extents_and_ddb(&mut lines, last)?;
-        Ok((header, extents, ddb))
+        let extents = parse_extents(&mut lines, last)?;
+        Ok((header, extents))
+    }
+
+    #[test]
+    fn new_is_unaffected_by_shared_file_offset() {
+        use std::io::{Read, Seek, SeekFrom, Write};
+
+        use vmm_sys_util::tempfile::TempFile;
+
+        // A minimal but complete monolithicFlat descriptor.
+        let descriptor_text = "# Disk DescriptorFile\n\
+                               version=1\n\
+                               createType=monolithicFlat\n\
+                               # Extent description\n\
+                               RW 2097152 FLAT \"disk-flat.vmdk\"\n\
+                               # The Disk Data Base\n\
+                               ddb.adapterType = \"ide\"\n";
+
+        let tmp = TempFile::new().unwrap();
+        let mut file: &File = tmp.as_file();
+        file.write_all(descriptor_text.as_bytes()).unwrap();
+
+        // Advance the shared OS file offset off zero, mimicking an earlier
+        // image-type probe. `&File` is `Copy`, so this moves the same fd's
+        // offset that `VmdkDescriptor::new` will see.
+        file.seek(SeekFrom::Start(0)).unwrap();
+        let mut scratch = [0u8; 8];
+        file.read_exact(&mut scratch).unwrap();
+        assert_ne!(file.stream_position().unwrap(), 0);
+
+        // `read_descriptor` reads positionally (anchored at offset 0), so the
+        // advanced offset must not affect the parse.
+        let descriptor = VmdkDescriptor::new(file, tmp.as_path()).unwrap();
+        assert_eq!(descriptor.extents_list.extents.len(), 1);
+        assert_eq!(
+            descriptor.extents_list.extents[0].filename,
+            "disk-flat.vmdk"
+        );
     }
 
     #[test]
@@ -294,7 +332,7 @@ mod tests {
                           ddb.adapterType = \"ide\"\n\
                           ddb.geometry.sectors = \"63\"\n";
 
-        let (extents, ddb) = parse_body("# Extent description", body).unwrap();
+        let extents = parse_body("# Extent description", body).unwrap();
 
         assert_eq!(extents.extents.len(), 1);
         let e = &extents.extents[0];
@@ -302,15 +340,6 @@ mod tests {
         assert_eq!(e.size_in_sectors, 2_097_152);
         assert_eq!(e.extent_type, "FLAT");
         assert_eq!(e.filename, "disk-flat.vmdk");
-
-        assert_eq!(
-            ddb.entries.get("ddb.adapterType").map(String::as_str),
-            Some("\"ide\"")
-        );
-        assert_eq!(
-            ddb.entries.get("ddb.geometry.sectors").map(String::as_str),
-            Some("\"63\"")
-        );
     }
 
     #[test]
@@ -321,7 +350,7 @@ mod tests {
                           # The Disk Data Base\n\
                           ddb.adapterType = \"lsilogic\"\n";
 
-        let (extents, _ddb) = parse_body("# Extent description", body).unwrap();
+        let extents = parse_body("# Extent description", body).unwrap();
 
         assert_eq!(extents.extents.len(), 3);
         assert_eq!(extents.extents[0].filename, "disk-s001.vmdk");
@@ -334,7 +363,7 @@ mod tests {
         // 5-field form: <access> <sectors> <type> <file> <offset>
         let body: &str = "RW 2097152 FLAT \"disk-flat.vmdk\" 0\n";
 
-        let (extents, _ddb) = parse_body("# Extent description", body).unwrap();
+        let extents = parse_body("# Extent description", body).unwrap();
 
         assert_eq!(extents.extents.len(), 1);
         assert_eq!(extents.extents[0].filename, "disk-flat.vmdk");
@@ -345,7 +374,7 @@ mod tests {
         let body: &str = "RDONLY 2097152 FLAT \"ro.vmdk\"\n\
                           NOACCESS 1048576 FLAT \"noaccess.vmdk\"\n";
 
-        let (extents, _ddb) = parse_body("# Extent description", body).unwrap();
+        let extents = parse_body("# Extent description", body).unwrap();
 
         assert_eq!(extents.extents.len(), 2);
         assert_eq!(extents.extents[0].access, "RDONLY");
@@ -394,7 +423,7 @@ mod tests {
         let body: &str = "RW 2097152 FLAT \"disk-flat.vmdk\"\n\
                           \n\
                           # The Disk Data Base\n";
-        let (extents, _ddb) = parse_body("# Extent description", body).unwrap();
+        let extents = parse_body("# Extent description", body).unwrap();
         assert_eq!(extents.extents.len(), 1);
         assert_eq!(extents.extents[0].filename, "disk-flat.vmdk");
     }
@@ -416,9 +445,6 @@ mod tests {
 
         let (header, last) = parse_hdr(input).unwrap();
 
-        assert_eq!(header.version, 1);
-        assert_eq!(header.cid, 0xffff_fffe);
-        assert_eq!(header.parent_cid, 0xffff_ffff);
         assert!(matches!(header.create_type, VMDKDiskType::MonolithicFlat));
         assert_eq!(last, "# Extent description");
     }
@@ -447,15 +473,11 @@ mod tests {
                            # The Disk Data Base\n\
                            ddb.adapterType = \"ide\"\n";
 
-        let (header, extents, ddb) = parse_full(input).unwrap();
+        let (header, extents) = parse_full(input).unwrap();
 
         assert!(matches!(header.create_type, VMDKDiskType::MonolithicFlat));
         assert_eq!(extents.extents.len(), 1);
         assert_eq!(extents.extents[0].access, "RW");
-        assert_eq!(
-            ddb.entries.get("ddb.adapterType").map(String::as_str),
-            Some("\"ide\"")
-        );
     }
 
     #[test]
@@ -468,7 +490,7 @@ mod tests {
                            RW 4192256 FLAT \"disk-s002.vmdk\"\n\
                            # The Disk Data Base\n";
 
-        let (header, extents, _ddb) = parse_full(input).unwrap();
+        let (header, extents) = parse_full(input).unwrap();
 
         assert!(matches!(
             header.create_type,
@@ -497,7 +519,7 @@ mod tests {
                            ddb.virtualHWVersion = \"4\"\n\
                            ddb.adapterType = \"ide\"\n";
 
-        let (header, extents, ddb) = parse_full(input).unwrap();
+        let (header, extents) = parse_full(input).unwrap();
 
         assert!(matches!(header.create_type, VMDKDiskType::MonolithicFlat));
         assert_eq!(extents.extents.len(), 1);
@@ -505,9 +527,5 @@ mod tests {
         assert_eq!(extents.extents[0].size_in_sectors, 6_291_456);
         assert_eq!(extents.extents[0].extent_type, "FLAT");
         assert_eq!(extents.extents[0].filename, "t-flat.vmdk");
-        assert_eq!(
-            ddb.entries.get("ddb.adapterType").map(String::as_str),
-            Some("\"ide\"")
-        );
     }
 }

block/src/formats/vmdk/flat.rs

@@ -0,0 +1,637 @@
+// Copyright © 2026, Microsoft Corporation
+//
+// SPDX-License-Identifier: Apache-2.0
+
+//! Flat VMDK extent layout: opens the data extents referenced by the
+//! descriptor and maps the virtual disk onto them.
+
+#![allow(dead_code)]
+
+use std::ffi::{CString, OsStr};
+use std::fs::{File, OpenOptions};
+use std::io;
+use std::os::unix::ffi::OsStrExt;
+use std::os::unix::fs::OpenOptionsExt;
+use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
+use std::path::{Component, Path};
+use std::sync::Arc;
+
+use log::warn;
+
+use crate::formats::vmdk::descriptor::VmdkDescriptor;
+use crate::{AlignedFile, DiskTopology};
+
+const VMDK_SECTOR_SIZE: u64 = 512;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum ExtentAccess {
+    /// "RW": readable and writable.
+    ReadWrite,
+    /// "RDONLY": readable only, writes must be rejected.
+    ReadOnly,
+    /// "NOACCESS": cannot be accessed, reads and writes must be rejected.
+    NoAccess,
+}
+
+/// A single Flat VMDK extent
+///
+/// `twoGbMaxExtentFlat` images concatenate several of these to form the full
+/// virtual disk, `monolithicFlat` images have exactly one.
+#[derive(Debug)]
+pub(crate) struct VmdkExtent {
+    /// Open, alignment-aware handle to this extent's data file. `None` for
+    /// `NoAccess` extents, which are never opened because they cannot be
+    /// accessed.
+    ///
+    /// The handle is wrapped in an [`AlignedFile`]
+    pub file: Option<AlignedFile>,
+    /// Access mode declared for this extent in the descriptor.
+    pub access: ExtentAccess,
+    /// First virtual-disk offset (in bytes) backed by this extent.
+    pub virtual_start: u64,
+    /// Length (in bytes) of the virtual-disk range backed by this extent.
+    pub length: u64,
+    /// Starting offset (in bytes) within the backing file for this extent.
+    /// Non-zero when several extents reference the same file at growing
+    /// offsets (e.g. a >2GB file split under `twoGbMaxExtentFlat`).
+    pub file_base_offset: u64,
+}
+
+#[derive(Debug)]
+pub struct FlatVmdk {
+    descriptor: Arc<VmdkDescriptor>,
+    // Open handle to the VMDK descriptor file.
+    descriptor_file: Arc<File>,
+    // All opened data extents, in virtual-disk order.
+    extents: Arc<Vec<VmdkExtent>>,
+    size: u64,
+}
+
+#[repr(C)]
+struct OpenHow {
+    flags: u64,
+    mode: u64,
+    resolve: u64,
+}
+
+// Splits an untrusted extent `filename` into its `Normal` path components for
+// the fallback walk, rejecting any `..`/`.` traversal.
+fn extent_components(filename: &str) -> io::Result<Vec<&OsStr>> {
+    let mut components = Vec::new();
+    for component in Path::new(filename).components() {
+        match component {
+            Component::Normal(name) => components.push(name),
+            Component::RootDir => {}
+            _ => {
+                return Err(io::Error::new(
+                    io::ErrorKind::InvalidData,
+                    format!(
+                        "VMDK extent filename '{filename}' must not contain '..' or '.' path \
+                         components"
+                    ),
+                ));
+            }
+        }
+    }
+    if components.is_empty() {
+        return Err(io::Error::new(
+            io::ErrorKind::InvalidData,
+            format!("VMDK extent filename '{filename}' is empty"),
+        ));
+    }
+    Ok(components)
+}
+
+// Opens a single VMDK data extent for the descriptor whose directory is
+// base_path.
+//
+// The extent name may be relative to the descriptor or an absolute path. The
+// only difference between the two is:
+//   - relative -> colocated with descriptor file
+//   - absolute -> the filesystem root
+// The symlink policy rejects the final component if it is a symlink (O_NOFOLLOW).
+//
+// Resolution prefers openat2(2). On kernels without it (< 5.6, ENOSYS) or
+// where it is blocked (EPERM, e.g. a seccomp filter), it falls back to a
+// per-component openat walk.
+fn open_extent(
+    base_path: &str,
+    filename: &str,
+    writable: bool,
+    direct: bool,
+) -> io::Result<AlignedFile> {
+    let anchor = if Path::new(filename).is_absolute() {
+        "/"
+    } else {
+        base_path
+    };
+
+    let dir = OpenOptions::new()
+        .read(true)
+        .custom_flags(libc::O_DIRECTORY | libc::O_CLOEXEC)
+        .open(anchor)?;
+
+    match open_extent_openat2(dir.as_raw_fd(), filename, writable, direct) {
+        Ok(file) => Ok(AlignedFile::new(file, direct)),
+        Err(e) if matches!(e.raw_os_error(), Some(libc::ENOSYS) | Some(libc::EPERM)) => {
+            let components = extent_components(filename)?;
+            open_extent_walk(dir, &components, writable, direct)
+        }
+        Err(e) => Err(e),
+    }
+}
+
+fn open_extent_openat2(
+    dir_fd: RawFd,
+    filename: &str,
+    writable: bool,
+    direct: bool,
+) -> io::Result<File> {
+    let cname = CString::new(filename).map_err(|_| {
+        io::Error::new(
+            io::ErrorKind::InvalidData,
+            "VMDK extent filename contains an interior NUL byte",
+        )
+    })?;
+
+    let access = if writable {
+        libc::O_RDWR
+    } else {
+        libc::O_RDONLY
+    };
+
+    let mut flags = access | libc::O_CLOEXEC | libc::O_NOFOLLOW;
+    if direct {
+        flags |= libc::O_DIRECT;
+    }
+    let how = OpenHow {
+        flags: flags as u64,
+        mode: 0,
+        resolve: 0,
+    };
+
+    // SAFETY: FFI syscall. `cname` is NUL-terminated and outlives the call,
+    // `how` is a correctly sized `open_how` passed by pointer, and `dir_fd` is a
+    // valid directory fd.
+    let ret = unsafe {
+        libc::syscall(
+            libc::SYS_openat2,
+            dir_fd,
+            cname.as_ptr(),
+            &how as *const OpenHow,
+            size_of::<OpenHow>(),
+        )
+    };
+    if ret < 0 {
+        return Err(io::Error::last_os_error());
+    }
+    // SAFETY: `openat2` returned a fresh descriptor we now own exclusively.
+    Ok(unsafe { File::from_raw_fd(ret as RawFd) })
+}
+
+fn open_extent_walk(
+    mut dir: File,
+    components: &[&OsStr],
+    writable: bool,
+    direct: bool,
+) -> io::Result<AlignedFile> {
+    let last = components.len() - 1;
+    for (i, name) in components.iter().enumerate() {
+        let cname = CString::new(name.as_bytes()).map_err(|_| {
+            io::Error::new(
+                io::ErrorKind::InvalidData,
+                "VMDK extent filename contains an interior NUL byte",
+            )
+        })?;
+
+        let flags = if i < last {
+            libc::O_RDONLY | libc::O_DIRECTORY | libc::O_CLOEXEC
+        } else {
+            // Final component: the extent file, opened with the declared access
+            // and cache mode, and O_NOFOLLOW so it may not be a symlink either.
+            let access = if writable {
+                libc::O_RDWR
+            } else {
+                libc::O_RDONLY
+            };
+            let mut flags = access | libc::O_NOFOLLOW | libc::O_CLOEXEC;
+            if direct {
+                flags |= libc::O_DIRECT;
+            }
+            flags
+        };
+
+        // SAFETY: `dir` is a valid open directory fd and `cname` is a
+        // NUL-terminated C string that outlives the call.
+        let fd = unsafe { libc::openat(dir.as_raw_fd(), cname.as_ptr(), flags) };
+        if fd < 0 {
+            return Err(io::Error::last_os_error());
+        }
+        // SAFETY: `fd` is a freshly opened descriptor we now own exclusively.
+        let opened = unsafe { File::from_raw_fd(fd) };
+
+        if i < last {
+            // Reassignment drops the previous directory `File`, closing that fd.
+            dir = opened;
+        } else {
+            return Ok(AlignedFile::new(opened, direct));
+        }
+    }
+
+    unreachable!("extent_components guarantees at least one component")
+}
+
+// Builds the error returned when a sector count/offset from the (untrusted)
+// descriptor, scaled to bytes, does not fit in a u64.
+fn overflow_error(what: &str) -> io::Error {
+    io::Error::new(
+        io::ErrorKind::InvalidData,
+        format!("VMDK {what} overflows a 64-bit byte count"),
+    )
+}
+
+impl FlatVmdk {
+    /// Opens a flat VMDK image from its already-open descriptor file.
+    pub fn new(file: File, path: &Path, direct: bool) -> io::Result<Self> {
+        let descriptor = VmdkDescriptor::new(&file, path)?;
+
+        if descriptor.extents_list.extents.is_empty() {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                "VMDK descriptor lists no extents",
+            ));
+        }
+
+        // Open every data extent and record the virtual-disk byte range it
+        // backs.
+        let mut extents: Vec<VmdkExtent> =
+            Vec::with_capacity(descriptor.extents_list.extents.len());
+        let mut virtual_start: u64 = 0;
+        for extent in &descriptor.extents_list.extents {
+            // A flat extent is a fixed, pre-allocated region, a zero-sector
+            // extent would back an empty virtual range that can never be read
+            // or written
+            if extent.size_in_sectors == 0 {
+                return Err(io::Error::new(
+                    io::ErrorKind::InvalidData,
+                    "VMDK flat extent has zero size",
+                ));
+            }
+
+            let length = extent
+                .size_in_sectors
+                .checked_mul(VMDK_SECTOR_SIZE)
+                .ok_or_else(|| overflow_error("extent size"))?;
+            let file_base_offset = extent
+                .offset_in_sectors
+                .checked_mul(VMDK_SECTOR_SIZE)
+                .ok_or_else(|| overflow_error("extent file offset"))?;
+            file_base_offset
+                .checked_add(length)
+                .ok_or_else(|| overflow_error("extent file range"))?;
+
+            // Open the backing file using exactly the access declared for this
+            // extent. The VMDK spec defines three values:
+            //   "RW"       -> read + write
+            //   "RDONLY"   -> read only
+            //   "NOACCESS" -> not accessible, do not open the file at all
+            let (access, extent_file) = match extent.access.as_str() {
+                "RW" => {
+                    let f = open_extent(&descriptor.base_path, &extent.filename, true, direct)?;
+                    (ExtentAccess::ReadWrite, Some(f))
+                }
+                "RDONLY" => {
+                    let f = open_extent(&descriptor.base_path, &extent.filename, false, direct)?;
+                    (ExtentAccess::ReadOnly, Some(f))
+                }
+                "NOACCESS" => (ExtentAccess::NoAccess, None),
+                other => {
+                    return Err(io::Error::new(
+                        io::ErrorKind::InvalidData,
+                        format!("unsupported VMDK extent access mode '{other}'"),
+                    ));
+                }
+            };
+
+            extents.push(VmdkExtent {
+                file: extent_file,
+                access,
+                virtual_start,
+                length,
+                file_base_offset,
+            });
+
+            virtual_start = virtual_start
+                .checked_add(length)
+                .ok_or_else(|| overflow_error("total virtual size"))?;
+        }
+
+        // The virtual disk size is the end offset of the last extent.
+        let total_disk_size = virtual_start;
+
+        Ok(Self {
+            descriptor: Arc::new(descriptor),
+            descriptor_file: Arc::new(file),
+            extents: Arc::new(extents),
+            size: total_disk_size,
+        })
+    }
+
+    pub fn virtual_block_size(&self) -> u64 {
+        self.size
+    }
+
+    /// Shared handle to the opened data extents, used to build the I/O worker.
+    pub fn extents(&self) -> Arc<Vec<VmdkExtent>> {
+        Arc::clone(&self.extents)
+    }
+
+    /// Host allocation size: the sum of every opened extent file's size.
+    /// `NoAccess` extents contribute 0 to the total.
+    pub fn physical_block_size(&self) -> u64 {
+        self.extents
+            .iter()
+            .map(|extent| {
+                extent
+                    .file
+                    .as_ref()
+                    .and_then(|f| f.metadata().ok())
+                    .map_or(0, |m| m.len())
+            })
+            .sum()
+    }
+
+    /// Sector/cluster geometry reported to the guest.
+    pub fn topology(&self) -> DiskTopology {
+        self.extents
+            .iter()
+            .find_map(|extent| extent.file.as_ref())
+            .map(|f| {
+                DiskTopology::probe(f.file()).unwrap_or_else(|_| {
+                    warn!("Unable to get VMDK extent topology. Using default topology");
+                    DiskTopology::default()
+                })
+            })
+            .unwrap_or_default()
+    }
+}
+
+// Expose the descriptor file's fd as the disk's representative fd.
+impl AsRawFd for FlatVmdk {
+    fn as_raw_fd(&self) -> RawFd {
+        self.descriptor_file.as_raw_fd()
+    }
+}
+
+impl Clone for FlatVmdk {
+    fn clone(&self) -> Self {
+        Self {
+            descriptor: Arc::clone(&self.descriptor),
+            descriptor_file: Arc::clone(&self.descriptor_file),
+            extents: Arc::clone(&self.extents),
+            size: self.size,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::fs;
+
+    use super::*;
+
+    #[test]
+    fn extent_components_allows_bare_name() {
+        // The common flat-VMDK case: a single co-located extent file.
+        let comps = extent_components("disk-flat.vmdk").unwrap();
+        assert_eq!(comps, [OsStr::new("disk-flat.vmdk")]);
+    }
+
+    #[test]
+    fn extent_components_rejects_traversal_and_empty() {
+        // `..`/`.` traversal and empty names are refused. (An absolute path is
+        // decomposed into its Normal components, the leading `/` is skipped and
+        // the caller anchors the walk at the filesystem root.)
+        extent_components("../../etc/passwd").unwrap_err();
+        extent_components("sub/../../escape").unwrap_err();
+        extent_components("extent-1.vmdk/../../").unwrap_err();
+        extent_components("./s001.vmdk").unwrap_err();
+        extent_components("").unwrap_err();
+    }
+
+    #[test]
+    fn extent_components_decomposes_absolute_path() {
+        // A leading `/` is skipped, the remaining Normal components are walked
+        // from the filesystem root by the caller.
+        let comps = extent_components("/var/lib/layer.erofs").unwrap();
+        assert_eq!(
+            comps,
+            [
+                OsStr::new("var"),
+                OsStr::new("lib"),
+                OsStr::new("layer.erofs")
+            ]
+        );
+    }
+
+    // Opens `path` as a directory anchor fd, mirroring how `open_extent` opens
+    // its anchor.
+    fn open_dir(path: &Path) -> File {
+        OpenOptions::new()
+            .read(true)
+            .custom_flags(libc::O_DIRECTORY | libc::O_CLOEXEC)
+            .open(path)
+            .unwrap()
+    }
+
+    // Returns false when `openat2(2)` is unavailable.
+    fn openat2_available(res: &io::Result<File>) -> bool {
+        !matches!(
+            res.as_ref().err().and_then(|e| e.raw_os_error()),
+            Some(libc::ENOSYS) | Some(libc::EPERM)
+        )
+    }
+
+    // Opens the same anchor + `filename` with BOTH extent-open implementations
+    // so a single scenario asserts they behave identically:
+    fn open_both(
+        base_path: &Path,
+        filename: &str,
+        writable: bool,
+        direct: bool,
+    ) -> (io::Result<File>, io::Result<AlignedFile>) {
+        let anchor: &Path = if Path::new(filename).is_absolute() {
+            Path::new("/")
+        } else {
+            base_path
+        };
+
+        let openat2_dir = open_dir(anchor);
+        let openat2_res = open_extent_openat2(openat2_dir.as_raw_fd(), filename, writable, direct);
+
+        let walk_dir = open_dir(anchor);
+        let walk_res = match extent_components(filename) {
+            Ok(components) => open_extent_walk(walk_dir, &components, writable, direct),
+            Err(e) => Err(e),
+        };
+
+        (openat2_res, walk_res)
+    }
+
+    // Asserts openat2.
+    fn check_openat2(res: &io::Result<File>, expect_ok: bool) {
+        if !openat2_available(res) {
+            return;
+        }
+        assert_eq!(
+            res.is_ok(),
+            expect_ok,
+            "openat2 result did not match expectation (expected ok = {expect_ok})"
+        );
+    }
+
+    // Asserts the per-component walk result.
+    fn check_walk(res: &io::Result<AlignedFile>, expect_ok: bool) {
+        assert_eq!(
+            res.is_ok(),
+            expect_ok,
+            "walk result did not match expectation (expected ok = {expect_ok})"
+        );
+    }
+
+    #[test]
+    fn open_extent_opens_regular_file() {
+        use vmm_sys_util::tempdir::TempDir;
+
+        let dir = TempDir::new_with_prefix("/tmp/vmdk-regular-test").unwrap();
+        let base = dir.as_path();
+        fs::write(base.join("disk-flat.vmdk"), b"data").unwrap();
+
+        let (openat2_res, walk_res) = open_both(base, "disk-flat.vmdk", false, false);
+        check_openat2(&openat2_res, true);
+        check_walk(&walk_res, true);
+    }
+
+    #[test]
+    fn open_extent_opens_file_in_subdirectory() {
+        use vmm_sys_util::tempdir::TempDir;
+
+        // A relative sub-path resolves beneath the descriptor directory.
+        let dir = TempDir::new_with_prefix("/tmp/vmdk-subdir-test").unwrap();
+        let base = dir.as_path();
+        fs::create_dir(base.join("extents")).unwrap();
+        fs::write(base.join("extents").join("s001.vmdk"), b"data").unwrap();
+
+        let (openat2_res, walk_res) = open_both(base, "extents/s001.vmdk", false, false);
+        check_openat2(&openat2_res, true);
+        check_walk(&walk_res, true);
+    }
+
+    #[test]
+    fn open_extent_opens_absolute_path_within_descriptor_dir() {
+        use vmm_sys_util::tempdir::TempDir;
+
+        let dir = TempDir::new_with_prefix("/tmp/vmdk-abs-in-test").unwrap();
+        let base = dir.as_path();
+        fs::write(base.join("gpt_meta_head.img"), b"data").unwrap();
+        let abs = base.join("gpt_meta_head.img");
+
+        let (openat2_res, walk_res) = open_both(base, abs.to_str().unwrap(), false, false);
+        check_openat2(&openat2_res, true);
+        check_walk(&walk_res, true);
+    }
+
+    #[test]
+    fn open_extent_opens_absolute_path_outside_descriptor_dir() {
+        use vmm_sys_util::tempdir::TempDir;
+
+        let desc_dir = TempDir::new_with_prefix("/tmp/vmdk-desc-test").unwrap();
+        let layer_dir = TempDir::new_with_prefix("/tmp/vmdk-layer-test").unwrap();
+        fs::write(layer_dir.as_path().join("layer.erofs"), b"data").unwrap();
+        let abs = layer_dir.as_path().join("layer.erofs");
+
+        let (openat2_res, walk_res) =
+            open_both(desc_dir.as_path(), abs.to_str().unwrap(), false, false);
+        check_openat2(&openat2_res, true);
+        check_walk(&walk_res, true);
+    }
+
+    #[test]
+    fn open_extent_rejects_symlinked_final_component_relative() {
+        use std::os::unix::fs::symlink;
+
+        use vmm_sys_util::tempdir::TempDir;
+
+        // A bare-named extent that is actually a symlink to a file the guest
+        // must never reach. Both implementations refuse it via O_NOFOLLOW.
+        let dir = TempDir::new_with_prefix("/tmp/vmdk-symlink-test").unwrap();
+        let base = dir.as_path();
+        let target = base.join("target-secret");
+        fs::write(&target, b"secret").unwrap();
+        symlink(&target, base.join("disk-flat.vmdk")).unwrap();
+
+        let (openat2_res, walk_res) = open_both(base, "disk-flat.vmdk", true, false);
+        check_openat2(&openat2_res, false);
+        check_walk(&walk_res, false);
+    }
+
+    #[test]
+    fn open_extent_rejects_symlinked_final_component_absolute() {
+        use std::os::unix::fs::symlink;
+
+        use vmm_sys_util::tempdir::TempDir;
+
+        // Even for absolute paths, the extent file itself may not be a symlink.
+        let dir = TempDir::new_with_prefix("/tmp/vmdk-abs-finalsym-test").unwrap();
+        let base = dir.as_path();
+        let target = base.join("target-secret");
+        fs::write(&target, b"secret").unwrap();
+        let link = base.join("extent-link.vmdk");
+        symlink(&target, &link).unwrap();
+
+        let (openat2_res, walk_res) = open_both(base, link.to_str().unwrap(), true, false);
+        check_openat2(&openat2_res, false);
+        check_walk(&walk_res, false);
+    }
+
+    #[test]
+    fn open_extent_follows_symlinked_intermediate_directory_relative() {
+        use std::os::unix::fs::symlink;
+
+        use vmm_sys_util::tempdir::TempDir;
+
+        // A relative path may traverse a symlinked intermediate directory
+        // (only the final component is guarded).
+        let real = TempDir::new_with_prefix("/tmp/vmdk-rel-real-test").unwrap();
+        fs::write(real.as_path().join("s001.vmdk"), b"data").unwrap();
+
+        let dir = TempDir::new_with_prefix("/tmp/vmdk-rel-symdir-test").unwrap();
+        let base = dir.as_path();
+        symlink(real.as_path(), base.join("sub")).unwrap();
+
+        let (openat2_res, walk_res) = open_both(base, "sub/s001.vmdk", false, false);
+        check_openat2(&openat2_res, true);
+        check_walk(&walk_res, true);
+    }
+
+    #[test]
+    fn open_extent_follows_symlinked_intermediate_directory_absolute() {
+        use std::os::unix::fs::symlink;
+
+        use vmm_sys_util::tempdir::TempDir;
+
+        // An absolute path may likewise traverse a symlinked intermediate
+        // directory (common in container deployments).
+        let real = TempDir::new_with_prefix("/tmp/vmdk-abs-realdir-test").unwrap();
+        fs::write(real.as_path().join("layer.erofs"), b"data").unwrap();
+
+        let dir = TempDir::new_with_prefix("/tmp/vmdk-abs-linkdir-test").unwrap();
+        let base = dir.as_path();
+        symlink(real.as_path(), base.join("link")).unwrap();
+
+        let via_symlink = base.join("link").join("layer.erofs");
+        let (openat2_res, walk_res) = open_both(base, via_symlink.to_str().unwrap(), false, false);
+        check_openat2(&openat2_res, true);
+        check_walk(&walk_res, true);
+    }
+}

block/src/formats/vmdk/mod.rs

@@ -8,5 +8,6 @@
 //! synchronous, extent-aware I/O.
 
 mod descriptor;
+mod flat;
 
 pub use descriptor::{has_descriptor_header, is_flat_vmdk};