linux-kernel - RE: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

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Date:   Sun, 5 Mar 2023 17:23:29 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Mateusz Guzik <mjguzik@...il.com>, Borislav Petkov <bp@...e.de>
CC:     Alexander Potapenko <glider@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Christian Brauner <brauner@...nel.org>,
        "serge@...lyn.com" <serge@...lyn.com>,
        "paul@...l-moore.com" <paul@...l-moore.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>
Subject: RE: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if
 possible

From: Linus Torvalds
> Sent: 04 March 2023 20:48
> 
> On Sat, Mar 4, 2023 at 12:31 PM Mateusz Guzik <mjguzik@...il.com> wrote:
> >
> > Good news: gcc provides a lot of control as to how it inlines string
> > ops, most notably:
> >        -mstringop-strategy=alg
> 
> Note that any static decision is always going to be crap somewhere.
> You can make it do the "optimal" thing for any particular machine, but
> I consider that to be just garbage.
> 
> What I would actually like to see is the compiler always generate an
> out-of-line call for the "big enough to not just do inline trivially"
> case, but do so with the "rep stosb/movsb" calling convention.

I think you also want it to differentiate between requests that
are known to be a whole number of words and ones that might
be byte sized.

For the kmalloc+memzero case you know you can zero a whole
number of words - so all the checks memset has to do for
byte length/alignment can be removed.

The same is true for memcpy() calls used for structure copies.
The compiler knows that aligned full-word copies can be done.
So it shouldn't be calling a function that has to redo the tests.

	David

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