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

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Message-ID: <ZAOnsOl+cXk9mTj5@ZenIV>
Date:   Sat, 4 Mar 2023 20:18:56 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Christian Brauner <brauner@...nel.org>, serge@...lyn.com,
        paul@...l-moore.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if
 possible

On Fri, Mar 03, 2023 at 09:39:11PM +0100, Mateusz Guzik wrote:

> the allocation routine does not have any information about the size
> available at compilation time, so has to resort to a memset call at
> runtime. Instead, should this be:
> 
> f = kmem_cache_alloc(...);
> memset(f, 0, sizeof(*f));
> 
> ... the compiler could in principle inititalize stuff as indicated by
> code and emit zerofill for the rest. Interestingly, last I checked
> neither clang nor gcc knew how to do it, they instead resort to a full
> sized memset anyway, which is quite a bummer.

For struct file I wouldn't expect a win from that, TBH.