Re: [PATCH 1/2] fs: preserve ACL_DONT_CACHE state in forget_cached_acl()

[Luis Henriques <luis@igalia.com>]

On Tue, Jul 14 2026, Amir Goldstein wrote:

> The ACL_DONT_CACHE state is meant to be a constant state for the inode
> for filesystems that want to opt out of posix acl caching.
>
> Commit facd61053cff1 ("fuse: fixes after adapting to new posix acl api")
> used this facility to opt out of posix acl caching for fuse inodes with
> fuse server that does not negotiate FUSE_POSIX_ACL (fc->posix_acl).
>
> The commit also takes care to gate the forget_all_cached_acls() call in
> fuse_set_acl() on fc->posix_acl because there is no need for it, but
> there are other placed in fuse code which call forget_all_cached_acls()
> unconditional to fc->posix_acl and those cause the loss of the
> ACL_DONT_CACHE state.
>
> This is not only a functional bug. Properly timed, a get_acl() from this
> fuse filesystem can return a stale cached value, as was observed in
> tests, because set_acl() does not invalidate the unintentional acl
> cache.
>
> We could fix this in fuse, but it actually makes no sense for the vfs
> helper forget_cached_acl() to invalidate the ACL_DONT_CACHE state, so
> let it not do that to fix fuse and future users of ACL_DONT_CACHE.
>
> Fixes: facd61053cff1 ("fuse: fixes after adapting to new posix acl api")
> Cc: stable@vger.kernel.org Signed-off-by: Amir Goldstein
> <amir73il@gmail.com>
> ---
>  fs/posix_acl.c | 7 +++++++ 1 file changed, 7 insertions(+)
>
> diff --git a/fs/posix_acl.c b/fs/posix_acl.c index
> b4bfe4ddf64ea..3dc62c1c27087 100644 --- a/fs/posix_acl.c +++
> b/fs/posix_acl.c @@ -93,6 +93,13 @@ static void
> __forget_cached_acl(struct posix_acl **p)
>  {
>  	struct posix_acl *old;
>  
> + /* + * ACL_DONT_CACHE is expected to be a "const" value and xchg it
> with + * ACL_NOT_CACHED would enable acl caching for the inode - + *
> clearly not what the caller has intended.  + */ + if (READ_ONCE(*p) ==
> ACL_DONT_CACHE) + return;
>  	old = xchg(p, ACL_NOT_CACHED); if (!is_uncached_acl(old))
>  		posix_acl_release(old);

Nice catch!  FWIW feel free to add my

Reviewed-by: Luis Henriques <luis@igalia.com>

Cheers,
-- 
Luís