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

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Date:   Fri, 29 Sep 2023 16:57:29 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        Jann Horn <jannh@...gle.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 Fri, 29 Sept 2023 at 14:39, Mateusz Guzik <mjguzik@...il.com> wrote:
>
> So to be clear, obtaining the initial count would require a dedicated
> accessor.

Please, no.

Sequence numbers here are fundamentally broken, since getting that
initial sequence number would involve either (a) making it something
outside of 'struct file' itself or (b) require the same re-validation
of the file pointer that the non-sequence number code needed in the
first place.

We already have the right model in the only place that really matters
(ie fd lookup). Using that same "validate file pointer after you got
the ref to it" for the two or three other cases that didn't do it (and
are simpler: the exec pointer in particular doesn't need the fdt
re-validation at all).

The fact that we had some fd lookup that didn't do the full thing that
a *real* fd lookup did is just bad. Let's fix it, not introduce a
sequence counter that only adds more complexity.

          Linus