linux-kernel - Re: [PATCH v2] vfs: shave work on failed file open

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Date:   Thu, 28 Sep 2023 10:21:56 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        Mateusz Guzik <mjguzik@...il.com>, viro@...iv.linux.org.uk,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] vfs: shave work on failed file open

On Thu, 28 Sept 2023 at 07:44, Jann Horn <jannh@...gle.com> wrote:
>
> The issue I see with the current __fget_files_rcu() is that the
> "file->f_mode & mask" is no longer effective in its current position,
> it would have to be moved down below the get_file_rcu() call.

Yes, you're right.

But moving it down below the "re-check that the fdt pointer and the
file pointer still matches" should be easy and sufficient.

> There are also some weird get_file_rcu() users in other places like
> BPF's task_file_seq_get_next and in gfs2_glockfd_next_file that do
> weird stuff without the recheck, especially gfs2_glockfd_next_file
> even looks at the inodes of files without taking a reference (which
> seems a little dodgy but maybe actually currently works because inodes
> are also RCU-freed?).

The inodes are also RCU-free'd, but that is indeed dodgy.

I think it happens to work, and we actually have a somewhat similar
pattern in the RCU lookup code (except with dentry->d_inode, not
file->f_inode), because as you say the inode data structure itself is
rcu-free'd, but more importantly, that code does the "get_file_rcu()"
afterwards.

And yes, right now that works fine, because it will fail if the file
f_count goes down to zero.

And f_count will go down to zero before we really tear down the inode with

        file->f_op->release(inode, file);

and (more importantly) the dput -> dentry_kill -> dentry_unlink_inode
-> release.

So that get_file_rcu() will currently protect against any "oh, the
inode is stale and about to be released".

But yes, that protection would be broken by SLAB_TYPESAFE_BY_RCU,
since then the "f_count is zero" is no longer a final thing.

It's fixable by having the same "double check the file table" that I
do think we should do regardless. That get_file_rcu() pattern may
*work*, but it's very very dodgy.

                Linus