Re: [PATCH bpf-next 1/3] s390/bpf: Add orig_gpr2 to user_pt_regs

[Heiko Carstens <hca@linux.ibm.com>]

On Wed, Feb 02, 2022 at 06:23:46PM +0100, Christian Borntraeger wrote:
> Am 02.02.22 um 15:19 schrieb Vasily Gorbik:
> > > diff --git a/arch/s390/include/uapi/asm/ptrace.h b/arch/s390/include/uapi/asm/ptrace.h
> > > index ad64d673b5e6..b3dec603f507 100644
> > > --- a/arch/s390/include/uapi/asm/ptrace.h
> > > +++ b/arch/s390/include/uapi/asm/ptrace.h
> > > @@ -295,6 +295,7 @@ typedef struct {
> > >   	unsigned long args[1];
> > >   	psw_t psw;
> > >   	unsigned long gprs[NUM_GPRS];
> > > +	unsigned long orig_gpr2;
> > >   } user_pt_regs;
> > 
> > It could be a good opportunity to get rid of that "args[1]" which is not
> > used for syscall parameters handling since commit baa071588c3f ("[S390]
> > cleanup system call parameter setup") [v2.6.37], as well as completely
> > unused now, and shouldn't really be exported to eBPF. And luckily eBPF
> > never used it.
> > 
> > So, how about reusing "args[1]" slot for orig_gpr2 instead?
> 
> Since this is uapi we certainly have to careful. Reusing this could be ok, though.

I agree with Vasily: let's get rid of "args[1]", rename it to orig_gpr2,
and effectively move orig_gpr2 to user_pt_regs, while at the same time
reducing the size of struct pt_regs a bit.

This will also prevent future random usages of the args member; e.g. until
recently it was used to pass the last breaking event address to the
exception handler. However that usage has also been removed already.

Ilya, could you resend with this proposed change, please?