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

[Abhi Das]

Hi Al,

Did you get a chance to look into the RCU pathwalk stuff a bit more?
Any ideas on how to allow may_open() to indicate to inode_permission()
that it's part of a RESOLVE_CACHED lookup?

Cheers!
--Abhi


On Thu, Nov 9, 2023 at 4:23 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> 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.
>