Re: [PATCH] btrfs: don't propagate EXTENT_FLAG_LOGGING to split extent maps

On Tue, 2026-06-30 at 12:58 -0700, Leo Martins wrote:
> When btrfs_drop_extent_map_range() splits an extent map, the new split
> maps inherit the original map's flags through a local 'flags' variable.
> Commit f86f7a75e2fb ("btrfs: use the flags of an extent map to identify
> the compression type") changed the EXTENT_FLAG_LOGGING clearing to
> operate on em->flags instead of that local 'flags' copy, so a split of
> an extent map that is currently being logged wrongly inherits
> EXTENT_FLAG_LOGGING.
> 
> The flag is then never cleared on the split, and when it is freed while
> still on the inode's modified_extents list (for example by the extent
> map shrinker) it trips the WARN_ON(!list_empty(&em->list)) in
> btrfs_free_extent_map() and leads to a use-after-free.
> 
> Clear EXTENT_FLAG_LOGGING from the local 'flags' copy used for the
> splits and only clear EXTENT_FLAG_PINNED from em->flags, restoring the
> behaviour prior to f86f7a75e2fb.
> 
> Fixes: f86f7a75e2fb ("btrfs: use the flags of an extent map to identify the compression type")
> Cc: Jeff Layton <[email protected]>
> Cc: Boris Burkov <[email protected]>
> Signed-off-by: Leo Martins <[email protected]>
> ---
>  fs/btrfs/extent_map.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
> index fce9c5cc0122..6ad7b39ae358 100644
> --- a/fs/btrfs/extent_map.c
> +++ b/fs/btrfs/extent_map.c
> @@ -866,13 +866,13 @@ void btrfs_drop_extent_map_range(struct btrfs_inode *inode, u64 start, u64 end,
>  			goto next;
>  		}
>  
> -		flags = em->flags;
>  		/*
>  		 * In case we split the extent map, we want to preserve the
>  		 * EXTENT_FLAG_LOGGING flag on our extent map, but we don't want
>  		 * it on the new extent maps.
>  		 */
> -		em->flags &= ~(EXTENT_FLAG_PINNED | EXTENT_FLAG_LOGGING);
> +		flags = em->flags & ~EXTENT_FLAG_LOGGING;
> +		em->flags &= ~EXTENT_FLAG_PINNED;
>  		modified = !list_empty(&em->list);
>  
>  		/*

Reviewed-by: Jeff Layton <[email protected]>