Re: Build regressions/improvements in v6.2-rc4

[Guenter Roeck]

On Mon, Jan 16, 2023 at 05:40:00PM +0100, Heiko Carstens wrote:
> On Mon, Jan 16, 2023 at 01:36:34PM +0100, Geert Uytterhoeven wrote:
> > On Mon, 16 Jan 2023, Geert Uytterhoeven wrote:
> > > JFYI, when comparing v6.2-rc4[1] to v6.2-rc3-8-g1fe4fd6f5cad346e[3], the summaries are:
> > >  - build errors: +1/-5
> > 
> >   + /kisskb/src/include/linux/fortify-string.h: error: '__builtin_memcpy' reading 128 bytes from a region of size 0 [-Werror=stringop-overread]:  => 57:33
> > 
> > s390x-gcc11/s390-allmodconfig
> > 
> > /kisskb/src/arch/s390/kernel/setup.c: In function 'setup_lowcore_dat_on':
> > /kisskb/src/include/linux/fortify-string.h:57:33: error: '__builtin_memcpy' reading 128 bytes from a region of size 0 [-Werror=stringop-overread]
> >    57 | #define __underlying_memcpy     __builtin_memcpy
> >       |                                 ^
> > /kisskb/src/include/linux/fortify-string.h:578:9: note: in expansion of macro '__underlying_memcpy'
> >   578 |         __underlying_##op(p, q, __fortify_size);                        \
> >       |         ^~~~~~~~~~~~~
> > /kisskb/src/include/linux/fortify-string.h:623:26: note: in expansion of macro '__fortify_memcpy_chk'
> >   623 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
> >       |                          ^~~~~~~~~~~~~~~~~~~~
> > /kisskb/src/arch/s390/kernel/setup.c:526:9: note: in expansion of macro 'memcpy'
> >   526 |         memcpy(abs_lc->cregs_save_area, S390_lowcore.cregs_save_area,
> >       |         ^~~~~~
> > 
> > Looks like this was "'__builtin_memcpy' offset [0, 127] is out of the bounds
> > [0, 0]" before.
> 
> Thanks for reporting. Of course this doesn't happen with gcc-12, and
> this code will be rewritten with the next merge window anyway.
> But to workaround this with gcc-11, we could go with the below:
> 

This is because of

#define S390_lowcore (*((struct lowcore *) 0))

and is fixed with something like

#define S390_lowcore (*((struct lowcore *) absolute_pointer(0)))

See commit f6b5f1a56987 ("compiler.h: Introduce absolute_pointer macro").
The problem is only seen with gcc 11.2. I don't see it with 11.3 or 12.2.

Guenter

> diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
> index 2b6091349daa..696c9e007a36 100644
> --- a/arch/s390/kernel/setup.c
> +++ b/arch/s390/kernel/setup.c
> @@ -508,6 +508,7 @@ static void __init setup_lowcore_dat_on(void)
>  {
>  	struct lowcore *abs_lc;
>  	unsigned long flags;
> +	int i;
>  
>  	__ctl_clear_bit(0, 28);
>  	S390_lowcore.external_new_psw.mask |= PSW_MASK_DAT;
> @@ -523,8 +524,8 @@ static void __init setup_lowcore_dat_on(void)
>  	abs_lc = get_abs_lowcore(&flags);
>  	abs_lc->restart_flags = RESTART_FLAG_CTLREGS;
>  	abs_lc->program_new_psw = S390_lowcore.program_new_psw;
> -	memcpy(abs_lc->cregs_save_area, S390_lowcore.cregs_save_area,
> -	       sizeof(abs_lc->cregs_save_area));
> +	for (i = 0; i < 16; i++)
> +		abs_lc->cregs_save_area[i] = S390_lowcore.cregs_save_area[i];
>  	put_abs_lowcore(abs_lc, flags);
>  }
>