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

[Amir Goldstein <amir73il@gmail.com>]

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);
-- 
2.54.0