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Date:   Thu, 28 Sep 2023 15:25:56 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Mateusz Guzik <mjguzik@gmail.com>, viro@zeniv.linux.org.uk,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2] vfs: shave work on failed file open
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> Which all seems to be the case already, so with the put_cred() not
> needing the RCU delay, I thing we really could do this patch (note:

So I spent a good chunk of time going through this patch.

Before file->f_cred was introduced file->f_{g,u}id would have been
accessible just under rcu protection. And file->f_cred->f_fs{g,u}id
replaced that access. So I think the intention was that file->f_cred
would function the same way, i.e., it would be possible to go from file
to cred under rcu without requiring a reference.

But basically, file->f_cred is the only field that would give this
guarantee. Other pointers such as file->f_security
(security_file_free()) don't and are freed outside of the rcu delay
already as well.

This patch means that if someone wants to access file->f_cred under rcu
they now need to call get_file_rcu() first.

Nothing has relied on this rcu-only file->f_cred quirk/feature until now
so I think it's fine to change it.

Does that make sense?

Please take a look at:
https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/commit/?h=vfs.misc&id=e3f15ee79197fc8b17d3496b6fa4fa0fc20f5406
for testing.