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Message-ID: <20220614214039.GA25951@gate.crashing.org>
Date: Tue, 14 Jun 2022 16:40:39 -0500
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Alexander Potapenko <glider@...gle.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Evgenii Stepanov <eugenis@...gle.com>,
Kees Cook <keescook@...omium.org>,
Marco Elver <elver@...gle.com>,
Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
Vitaly Buka <vitalybuka@...gle.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: Re: [PATCH] [RFC] Initialization of unused function parameters
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
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