Re: [PATCH] fs: RESOLVE_CACHED final path component fix

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Nov 09, 2023 at 08:08:44PM +0100, Andreas Gruenbacher wrote:
> Jens,
> 
> since your commit 99668f618062, applications can request cached lookups
> with the RESOLVE_CACHED openat2() flag.  When adding support for that in
> gfs2, we found that this causes the ->permission inode operation to be
> called with the MAY_NOT_BLOCK flag set for directories along the path,
> which is good, but the ->permission check on the final path component is
> missing that flag.  The filesystem will then sleep when it needs to read
> in the ACL, for example.
> 
> This doesn't look like the intended RESOLVE_CACHED behavior.
> 
> The file permission checks in path_openat() happen as follows:
> 
> (1) link_path_walk() -> may_lookup() -> inode_permission() is called for
> each but the final path component. If the LOOKUP_RCU nameidata flag is
> set, may_lookup() passes the MAY_NOT_BLOCK flag on to
> inode_permission(), which passes it on to the permission inode
> operation.
> 
> (2) do_open() -> may_open() -> inode_permission() is called for the
> final path component. The MAY_* flags passed to inode_permission() are
> computed by build_open_flags(), outside of do_open(), and passed down
> from there. The MAY_NOT_BLOCK flag doesn't get set.
> 
> I think we can fix this in build_open_flags(), by setting the
> MAY_NOT_BLOCK flag when a RESOLVE_CACHED lookup is requested, right
> where RESOLVE_CACHED is mapped to LOOKUP_CACHED as well.

No.  This will expose ->permission() instances to previously impossible
cases of MAY_NOT_BLOCK lookups, and we already have enough trouble
in that area.  See RCU pathwalk patches I posted last cycle; I'm
planning to rebase what still needs to be rebased and feed the
fixes into mainline, but that won't happen until the end of this
week *AND* ->permission()-related part of code audit will need
to be repeated and extended.

Until then - no, with the side of fuck, no.