linux-kernel - Re: [PATCH] [RFC] Initialization of unused function parameters

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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