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snapuserd: Address alignment fault on 32-bit systems 

When the scratch space is mmap'ed, the metadata buffer
will be un-aligned. This may lead to alignment fault
on 32-bit systems. Address this by temporarily copying
it to buffer.

No perf impact as this code path is not in I/O path
and the copy is a for the size of metadata buffer which
is 8k.

Bug: 206426215
Test: Full and Incremental OTA on pixel
1: Compile snapuserd as 32 bit and reproduced the bug on pixel.
2: With fix - OTA applied successfully.
3: Reboot the device when merge was in-flight as the fix is primarily
in that path.
4: Verify merge completion and data integrity post merge.

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I63c0d862057ebf138c9d1696a942030e30598739
This commit is contained in:
Akilesh Kailash 2021-12-01 12:06:51 +00:00
parent 904cae37ce
commit 14e9504d2e
2 changed files with 14 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
fs_mgr/libsnapshot/snapuserd/dm-snapshot-merge/snapuserd_readahead.cpp
View File

@ -246,9 +246,15 @@ bool ReadAheadThread::ReconstructDataFromCow() {
int num_ops = 0;
int total_blocks_merged = 0;
// This memcpy is important as metadata_buffer_ will be an unaligned address and will fault
// on 32-bit systems
std::unique_ptr<uint8_t[]> metadata_buffer =
std::make_unique<uint8_t[]>(snapuserd_->GetBufferMetadataSize());
memcpy(metadata_buffer.get(), metadata_buffer_, snapuserd_->GetBufferMetadataSize());
while (true) {
struct ScratchMetadata* bm = reinterpret_cast<struct ScratchMetadata*>(
(char*)metadata_buffer_ + metadata_offset);
(char*)metadata_buffer.get() + metadata_offset);
// Done reading metadata
if (bm->new_block == 0 && bm->file_offset == 0) {

8
fs_mgr/libsnapshot/snapuserd/user-space-merge/snapuserd_readahead.cpp
View File

@ -121,9 +121,15 @@ bool ReadAhead::ReconstructDataFromCow() {
int num_ops = 0;
int total_blocks_merged = 0;
// This memcpy is important as metadata_buffer_ will be an unaligned address and will fault
// on 32-bit systems
std::unique_ptr<uint8_t[]> metadata_buffer =
std::make_unique<uint8_t[]>(snapuserd_->GetBufferMetadataSize());
memcpy(metadata_buffer.get(), metadata_buffer_, snapuserd_->GetBufferMetadataSize());
while (true) {
struct ScratchMetadata* bm = reinterpret_cast<struct ScratchMetadata*>(
(char*)metadata_buffer_ + metadata_offset);
(char*)metadata_buffer.get() + metadata_offset);
// Done reading metadata
if (bm->new_block == 0 && bm->file_offset == 0) {