projects/linux·commit da2dccd7

btrfs: fix hole expansion when writing at an offset beyond EOF

da2dccd7 · Filipe Manana <[email protected]> · 2025-02-05 17:36:48 +00:00

At btrfs_write_check() if our file's i_size is not sector size aligned and
we have a write that starts at an offset larger than the i_size that falls
within the same page of the i_size, then we end up not zeroing the file
range [i_size, write_offset).

The code is this:

    start_pos = round_down(pos, fs_info->sectorsize);
    oldsize = i_size_read(inode);
    if (start_pos > oldsize) {
        /* Expand hole size to cover write data, preventing empty gap */
        loff_t end_pos = round_up(pos + count, fs_info->sectorsize);

        ret = btrfs_cont_expand(BTRFS_I(inode), oldsize, end_pos);
        if (ret)
            return ret;
    }

So if our file's i_size is 90269 bytes and a write at offset 90365 bytes
comes in, we get 'start_pos' set to 90112 bytes, which is less than the
i_size and therefore we don't zero out the range [90269, 90365) by
calling btrfs_cont_expand().

This is an old bug introduced in commit 9036c10208e1 ("Btrfs: update hole
handling v2"), from 2008, and the buggy code got moved around over the
years.

Fix this by discarding 'start_pos' and comparing against the write offset
('pos') without any alignment.

This bug was recently exposed by test case generic/363 which tests this
scenario by polluting ranges beyond EOF with an mmap write and than verify
that after a file increases we get zeroes for the range which is supposed
to be a hole and not what we wrote with the previous mmaped write.

We're only seeing this exposed now because generic/363 used to run only
on xfs until last Sunday's fstests update.

The test was failing like this:

   $ ./check generic/363
   FSTYP         -- btrfs
   PLATFORM      -- Linux/x86_64 debian0 6.13.0-rc7-btrfs-next-185+ #17 SMP PREEMPT_DYNAMIC Mon Feb  3 12:28:46 WET 2025
   MKFS_OPTIONS  -- /dev/sdc
   MOUNT_OPTIONS -- /dev/sdc /home/fdmanana/btrfs-tests/scratch_1

   generic/363 0s ... [failed, exit status 1]- output mismatch (see /home/fdmanana/git/hub/xfstests/results//generic/363.out.bad)
       --- tests/generic/363.out	2025-02-05 15:31:14.013646509 +0000
       +++ /home/fdmanana/git/hub/xfstests/results//generic/363.out.bad	2025-02-05 17:25:33.112630781 +0000
       @@ -1 +1,46 @@
        QA output created by 363
       +READ BAD DATA: offset = 0xdcad, size = 0xd921, fname = /home/fdmanana/btrfs-tests/dev/junk
       +OFFSET      GOOD    BAD     RANGE
       +0x1609d     0x0000  0x3104  0x0
       +operation# (mod 256) for the bad data may be 4
       +0x1609e     0x0000  0x0472  0x1
       +operation# (mod 256) for the bad data may be 4
       ...
       (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/generic/363.out /home/fdmanana/git/hub/xfstests/results//generic/363.out.bad'  to see the entire diff)
   Ran: generic/363
   Failures: generic/363
   Failed 1 of 1 tests

Fixes: 9036c10208e1 ("Btrfs: update hole handling v2")
CC: [email protected]
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Filipe Manana <[email protected]>
Signed-off-by: David Sterba <[email protected]>
Commit
da2dccd7451de62b175fb8f0808d644959e964c7
Parents
acc18e1c

Changes

1 files changed · +1 −3

FileStatusAddDelContent
fs/btrfs/file.cmodified+1−3View file

fs/btrfs/file.c

View file
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 36f51c3..ed3c0d6 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1039,7 +1039,6 @@ int btrfs_write_check(struct kiocb *iocb, size_t count)
loff_t pos = iocb->ki_pos;
int ret;
loff_t oldsize;
- loff_t start_pos;
/*
* Quickly bail out on NOWAIT writes if we don't have the nodatacow or
@@ -1066,9 +1065,8 @@ int btrfs_write_check(struct kiocb *iocb, size_t count)
inode_inc_iversion(inode);
}
- start_pos = round_down(pos, fs_info->sectorsize);
oldsize = i_size_read(inode);
- if (start_pos > oldsize) {
+ if (pos > oldsize) {
/* Expand hole size to cover write data, preventing empty gap */
loff_t end_pos = round_up(pos + count, fs_info->sectorsize);