From: Segher Boessenkool <segher@kernel.crashing.org>
To: Alexander Potapenko <glider@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Evgenii Stepanov <eugenis@google.com>,
Kees Cook <keescook@chromium.org>, Marco Elver <elver@google.com>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Vitaly Buka <vitalybuka@google.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-toolchains <linux-toolchains@vger.kernel.org>
Subject: Re: [PATCH] [RFC] Initialization of unused function parameters
Date: Tue, 14 Jun 2022 16:40:39 -0500 [thread overview]
Message-ID: <20220614214039.GA25951@gate.crashing.org> (raw)
In-Reply-To: <CAG_fn=UPoM3bafwu6inGPMjg1bPw3HSFM_KrE_hen_MN3fu2vA@mail.gmail.com>
Hi!
On Tue, Jun 14, 2022 at 10:19:53PM +0200, Alexander Potapenko wrote:
> ================
> char *kmalloc(int size);
>
> char *kmalloc_or_not(int flag, int size, char *p) {
> if (flag)
> return kmalloc(size);
> else
> return p;
> }
>
> char global[16];
>
> char *p(int flag) {
> char *c;
> int size;
> if (flag)
> return kmalloc_or_not(1, 4, c);
> else
> return kmalloc_or_not(0, size, global);
> }
> ================
Since C11, lvalue conversion of an automatic variable that does not have
its address taken is explicitly undefined behaviour (6.3.2.1/2). So in
function "p", both where "c" and where "size" are passed causes UB (so
that executing "p" always causes UB btw).
> In this example `size` is passed into kmalloc_or_not() initialized,
> however it is never used, so the code probably has defined behavior.
No such luck: the passing itself already causes UB.
GCC does not warn, it has already optimised the code to what you expect
by the time this warning is done. If you use -fno-inline it does warn
for both "c" and "size" (via -Wmaybe-uninitialized).
But it is still UB! All bets are off, no compiler can do any correct
translation of your program, since there *is none*.
Segher
next prev parent reply other threads:[~2022-06-14 21:49 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-14 14:48 [PATCH] [RFC] Initialization of unused function parameters Alexander Potapenko
2022-06-14 16:48 ` Linus Torvalds
2022-06-14 17:11 ` Nick Desaulniers
2022-06-14 17:24 ` Linus Torvalds
2022-06-14 18:08 ` Nick Desaulniers
2022-06-14 22:27 ` Peter Zijlstra
2022-06-14 18:07 ` Alexander Potapenko
2022-06-14 18:30 ` Linus Torvalds
2022-06-14 20:19 ` Alexander Potapenko
2022-06-14 20:43 ` Linus Torvalds
2022-06-14 21:40 ` Segher Boessenkool [this message]
2022-06-14 22:08 ` Evgenii Stepanov
2022-06-15 8:30 ` Alexander Potapenko
2022-06-15 16:46 ` Segher Boessenkool
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20220614214039.GA25951@gate.crashing.org \
--to=segher@kernel.crashing.org \
--cc=elver@google.com \
--cc=eugenis@google.com \
--cc=glider@google.com \
--cc=keescook@chromium.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-toolchains@vger.kernel.org \
--cc=nathan@kernel.org \
--cc=ndesaulniers@google.com \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
--cc=vitalybuka@google.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).