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

> 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: [email protected]
> Signed-off-by: Amir Goldstein <[email protected]>
>
> diff --git a/fs/posix_acl.c b/fs/posix_acl.c
> index b4bfe4ddf64e..3dc62c1c2708 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;

Still on vacation this week but I took a glimpse:

If this isn't what the caller intended, having ACL_DONT_CACHE end up
should be treated like a bug. So shouldn't this then be a WARN_ON_ONCE()
and return?

-- 
Christian Brauner <[email protected]>