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

[Al Viro]

On Thu, Nov 09, 2023 at 11:12:32PM +0100, Andreas Grünbacher wrote:
> Am Do., 9. Nov. 2023 um 23:00 Uhr schrieb Al Viro <viro@xxxxxxxxxxxxxxxxxx>:
> > 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.
> 
> True, lockdep wouldn't be happy.
> 
> >  See RCU pathwalk patches I posted last cycle;
> 
> Do you have a pointer? Thanks.

Thread starting with Message-ID: <20231002022815.GQ800259@ZenIV>
I don't remember if I posted the audit notes into it; I'll get around
to resurrecting that stuff this weekend, when the mainline settles down
enough to bother with that.