linux-kernel - Re: [PATCH v6 2/3] treewide: Switch memcpy() users of 'task->comm' to a more safer implementation

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CAHk-=wixR7ZR+aebFsWX4qWZ84tMTmyNWLUPmTy3YvaNJGqd-Q@mail.gmail.com>
Date: Sat, 26 Jul 2025 16:37:33 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Kees Cook <kees@...nel.org>
Cc: Bhupesh <bhupesh@...lia.com>, akpm@...ux-foundation.org, kernel-dev@...lia.com, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-mm@...ck.org, oliver.sang@...el.com, lkp@...el.com, 
	laoar.shao@...il.com, pmladek@...e.com, rostedt@...dmis.org, 
	mathieu.desnoyers@...icios.com, arnaldo.melo@...il.com, 
	alexei.starovoitov@...il.com, andrii.nakryiko@...il.com, 
	mirq-linux@...e.qmqm.pl, peterz@...radead.org, willy@...radead.org, 
	david@...hat.com, viro@...iv.linux.org.uk, ebiederm@...ssion.com, 
	brauner@...nel.org, jack@...e.cz, mingo@...hat.com, juri.lelli@...hat.com, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/3] treewide: Switch memcpy() users of 'task->comm' to
 a more safer implementation

On Sat, 26 Jul 2025 at 16:19, Kees Cook <kees@...nel.org> wrote:
>
> That works for me! I just get twitchy around seeing memcpy used for strings. :) if we're gonna NUL after the memcpy, just use strscpy_pad().

I do worry a tiny bit about performance.

Because 'memcpy+set last byte to NUL' really is just a couple of
instructions when we're talking small constant-sized arrays.

strscpy_pad() isn't horrible, but it's still at another level. And
most of the cost is that "return the length" which people often don't
care about.

Dang, I wish we had some compiler trick to say "if the value isn't
used, do X, if it _is_ used do Y".

It's such a trivial thing in the compiler itself, and the information
is there, but I don't think it is exposed in any useful way.

In fact, it *is* exposed in one way I can think of:

   __attribute__((__warn_unused_result__))

but not in a useful form for actually generating different code.

Some kind of "__builtin_if_used(x,y)" where it picks 'x' if the value
is used, and 'y' if it isn't would be lovely for this.

Then you could do things like

    #define my_helper(x) \
        __builtin_if_used( \
                full_semantics(x), \
                simpler_version(x))

when having a return value means extra work and most people don't care.

Maybe it exists in some form that I haven't thought of?

Any compiler people around?

                 Linus