Re: [RFC][PATCH] getname_maybe_null() - the third variant of pathname copy-in

[Christian Brauner <brauner@kernel.org>]

On Wed, Oct 16, 2024 at 06:09:08AM +0100, Al Viro wrote:
> On Tue, Oct 15, 2024 at 04:05:15PM +0200, Christian Brauner wrote:
> > Looks good.
> > 
> > Fyi, I'm using your #base.getname as a base for some other work that I'm
> > currently doing. So please don't rebase #base.getname anymore. ;)
> > 
> > Since you have your #work.xattr and #work.stat using it as base it seems
> > pretty unlikely anyway but I just thought I mention explicitly that I'm
> > relying on that #base.getname branch.
> 
> FWIW, I see a problem with that sucker.  The trouble is in the
> combination AT_FDCWD, "", AT_EMPTY_PATH.  vfs_empty_path() returns

Yeah, we're aware.

> false on that, so fstatat() in mainline falls back to vfs_statx() and
> does the right thing.  This variant does _not_.
> 
> Note that your variant of xattr series also ended up failing on e.g.
> getxattrat() with such combination:
>         if (at_flags & AT_EMPTY_PATH && vfs_empty_path(dfd, pathname)) {
>                 CLASS(fd, f)(dfd);
>                 if (!f.file)
>                         return -EBADF;
>                 audit_file(f.file);
>                 return getxattr(file_mnt_idmap(f.file), file_dentry(f.file),
>                                 name, value, size);
>         }
> 
>         lookup_flags = (at_flags & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
> 
> retry:
>         error = user_path_at(dfd, pathname, lookup_flags, &path);
> 
> ended up calling user_path_at() with empty pathname and nothing like LOOKUP_EMPTY
> in lookup_flags.  Which bails out with -ENOENT, since getname() in there does
> so.  My variant bails out with -EBADF and I'd argue that neither is correct.
> 
> Not sure what's the sane solution here, need to think for a while...

Fwiw, in the other thread we concluded to just not care about AT_FDCWD with "".
And so far I agree with that.