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@weissschuh.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
 void _start_c(long *sp)
 {
 	long argc;

-- 
2.49.0

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@weissschuh.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 ?

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

Willy

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@weissschuh.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

On Sat, Apr 19, 2025 at 12:10:24PM +0200, Thomas Weißschuh wrote:
> 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@weissschuh.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.

OK, thanks for explaining!

Willy