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block: qcow: Read the image magic positionally

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detect_image_type saved the cursor, rewound, read the magic, then
restored the cursor. Read the four magic bytes with read_exact_at at
offset 0 and decode with from_be_bytes, so the save, rewind, and
restore go away. BeUint moves to the test module, its only remaining
user in this file.

The result is unchanged.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
This commit is contained in:
Anatol Belski
2026-06-30 17:36:28 +02:00
committed by Rob Bradford
parent abc03f998a
commit 6a1dee64e7
1 changed files with 5 additions and 9 deletions
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14
block/src/formats/qcow/internal/mod.rs
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@@ -32,7 +32,7 @@ use header::{
offset_is_cluster_boundary,
};
use log::warn;
use qcow_raw_file::{BeUint, QcowRawFile};
use qcow_raw_file::QcowRawFile;
use refcount::RefCount;
use remain::sorted;
use thiserror::Error;
@@ -874,20 +874,15 @@ fn rebuild_refcounts(raw_file: &mut QcowRawFile, header: QcowHeader) -> BlockRes
/// Detect the type of an image file by checking for a valid qcow2 header.
pub fn detect_image_type(file: &mut AlignedFile) -> BlockResult<ImageType> {
let orig_seek = file
.stream_position()
.map_err(|e| BlockError::new(BlockErrorKind::Io, Error::SeekingFile(e)))?;
file.rewind()
.map_err(|e| BlockError::new(BlockErrorKind::Io, Error::SeekingFile(e)))?;
let magic = u32::read_be(file)
let mut magic_bytes = [0u8; 4];
file.read_exact_at(&mut magic_bytes, 0)
.map_err(|e| BlockError::new(BlockErrorKind::Io, Error::ReadingHeader(e)))?;
let magic = u32::from_be_bytes(magic_bytes);
let image_type = if magic == QCOW_MAGIC {
ImageType::Qcow2
} else {
ImageType::Raw
};
file.seek(SeekFrom::Start(orig_seek))
.map_err(|e| BlockError::new(BlockErrorKind::Io, Error::SeekingFile(e)))?;
Ok(image_type)
}
#[cfg(test)]
@@ -905,6 +900,7 @@ mod unit_tests {
AUTOCLEAR_FEATURES_OFFSET, DEFAULT_CLUSTER_BITS, DEFAULT_REFCOUNT_ORDER,
HEADER_EXT_BACKING_FORMAT, HEADER_EXT_END, V2_BARE_HEADER_SIZE, V3_BARE_HEADER_SIZE,
};
use super::qcow_raw_file::BeUint;
use super::util::ZERO_FLAG;
use super::*;
use crate::formats::qcow::{QcowDisk, QcowTempDisk};