linux-kernel - Re: [PATCH 2/6] tools/nolibc: disable function sanitizer for _start

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Message-ID: <96891976-1f46-4080-91af-1a810fe4498c@t-8ch.de>
Date: Sat, 19 Apr 2025 12:10:24 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Willy Tarreau <w@....eu>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, Shuah Khan <shuah@...nel.org>, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/6] tools/nolibc: disable function sanitizer for
 _start_c()

On 2025-04-19 11:06:31+0200, Willy Tarreau wrote:
> On Wed, Apr 16, 2025 at 08:40:17PM +0200, Thomas Weißschuh wrote:
> > Both constructors and main() may be executed with different function
> > signatures than they are actually using.
> > This is intentional but trips up UBSAN.
> > 
> > Disable the function sanitizer of UBSAN in _start_c().
> > 
> > Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> > ---
> >  tools/include/nolibc/crt.h | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/tools/include/nolibc/crt.h b/tools/include/nolibc/crt.h
> > index c4b10103bbec50f1a3a0a4562e34fdbd1b43ce6f..961cfe777c3564e705dfdd581de828b374d05b0b 100644
> > --- a/tools/include/nolibc/crt.h
> > +++ b/tools/include/nolibc/crt.h
> > @@ -7,6 +7,8 @@
> >  #ifndef _NOLIBC_CRT_H
> >  #define _NOLIBC_CRT_H
> >  
> > +#include "compiler.h"
> > +
> >  char **environ __attribute__((weak));
> >  const unsigned long *_auxv __attribute__((weak));
> >  
> > @@ -25,6 +27,9 @@ extern void (*const __fini_array_end[])(void) __attribute__((weak));
> >  
> >  void _start_c(long *sp);
> >  __attribute__((weak,used))
> > +#if __nolibc_has_feature(undefined_behavior_sanitizer)
> > +	__attribute__((no_sanitize("function")))
> > +#endif
> 
> I'm wondering if it wouldn't be more reliable with:
> 
>   #if __nolibc_has_attribute(no_sanitize)
> 	__attribute__((no_sanitize("function")))
>   #endif
> 
> Because in the end, what you want is to always place that attribute
> whenever it's supported, no ?

That doesn't work because GCC does knows no_sanitize but not
no_sanitize("function").
Also no_sanitize is not specific to UBSAN but works for all sanitizers.

> 
> >  void _start_c(long *sp)
> >  {
> >  	long argc;
> 
> Willy