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block: qcow: Clear autoclear features on writable open

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QCOW2 v3 autoclear_features field contains bits for features whose
metadata becomes invalid when the image is modified by software that
doesn't understand them. Defined bits:

- Bit 0: Bitmaps extension
- Bit 1: Raw external data

Cloud-hypervisor doesn't support bitmaps or external data files, so
all autoclear bits are cleared on writable open. This signals other
tools that these features' data may be stale.

Readonly opens preserve autoclear bits unchanged.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
This commit is contained in:
Anatol Belski
2026-01-28 20:13:20 +01:00
committed by Rob Bradford
parent 57fd672db6
commit 4545fe113e
1 changed files with 28 additions and 5 deletions
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33
block/src/qcow/mod.rs
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@@ -194,6 +194,7 @@ const DEFAULT_REFCOUNT_ORDER: u32 = 4;
const V2_BARE_HEADER_SIZE: u32 = 72;
const V3_BARE_HEADER_SIZE: u32 = 104;
const AUTOCLEAR_FEATURES_OFFSET: u64 = 88;
// bits 0-8 and 56-63 are reserved.
const L1_TABLE_OFFSET_MASK: u64 = 0x00ff_ffff_ffff_fe00;
@@ -719,6 +720,24 @@ impl QcowHeader {
IncompatFeatures::from_bits_truncate(self.incompatible_features)
.contains(IncompatFeatures::CORRUPT)
}
/// Clear all autoclear feature bits for QCOW2 v3 images.
///
/// These bits indicate features that can be safely disabled when modified
/// by software that doesn't understand them.
pub fn clear_autoclear_features<F: Seek + Write + FileSync>(
&mut self,
file: &mut F,
) -> Result<()> {
if self.version == 3 && self.autoclear_features != 0 {
self.autoclear_features = 0;
file.seek(SeekFrom::Start(AUTOCLEAR_FEATURES_OFFSET))
.map_err(Error::WritingHeader)?;
u64::write_be(file, 0).map_err(Error::WritingHeader)?;
file.fsync().map_err(Error::WritingHeader)?;
}
Ok(())
}
}
fn max_refcount_clusters(refcount_order: u32, cluster_size: u32, num_clusters: u32) -> u64 {
@@ -1023,11 +1042,15 @@ impl QcowFile {
qcow.find_avail_clusters()?;
if !IncompatFeatures::from_bits_truncate(qcow.header.incompatible_features)
.contains(IncompatFeatures::DIRTY)
&& is_writable
{
qcow.header.set_dirty_bit(qcow.raw_file.file_mut(), true)?;
if is_writable {
if !IncompatFeatures::from_bits_truncate(qcow.header.incompatible_features)
.contains(IncompatFeatures::DIRTY)
{
qcow.header.set_dirty_bit(qcow.raw_file.file_mut(), true)?;
}
qcow.header
.clear_autoclear_features(qcow.raw_file.file_mut())?;
}
Ok(qcow)