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

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Ilya Leoshkevich wrote:
On Fri, 2022-02-04 at 08:21 +0000, Naveen N. Rao wrote:
Naveen N. Rao wrote:
> Hi Heiko,
> > Heiko Carstens wrote:
> > On Wed, Feb 02, 2022 at 12:41:58AM +0100, Ilya Leoshkevich wrote:
> > > user_pt_regs is used by eBPF in order to access userspace
> > > registers -
> > > see commit 466698e654e8 ("s390/bpf: correct broken uapi for
> > > BPF_PROG_TYPE_PERF_EVENT program type"). In order to access the
> > > first
> > > syscall argument from eBPF programs, we need to export
> > > orig_gpr2.
> > > > > > Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> > > ---
> > >  arch/s390/include/asm/ptrace.h      | 2 +-
> > >  arch/s390/include/uapi/asm/ptrace.h | 1 +
> > >  2 files changed, 2 insertions(+), 1 deletion(-)
> > > > > > diff --git a/arch/s390/include/asm/ptrace.h
> > > b/arch/s390/include/asm/ptrace.h
> > > index 4ffa8e7f0ed3..c8698e643904 100644
> > > --- a/arch/s390/include/asm/ptrace.h
> > > +++ b/arch/s390/include/asm/ptrace.h
> > > @@ -83,9 +83,9 @@ struct pt_regs {
> > >                         unsigned long args[1];
> > >                         psw_t psw;
> > >                         unsigned long gprs[NUM_GPRS];
> > > +                       unsigned long orig_gpr2;
> > >                 };
> > >         };
> > > -       unsigned long orig_gpr2;
> > >         union {
> > >                 struct {
> > >                         unsigned int int_code;
> > > 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;
> > > > Isn't this broken on nearly all architectures? I just checked
> > powerpc,
> > arm64, and riscv. While powerpc seems to mirror pt_regs as
> > user_pt_regs,
> > and therefore exports orig_gpr3, the bpf macros still seem to
> > access the
> > wrong location to access the first syscall parameter(?).
> > On powerpc, gpr[3] continues to be valid on syscall entry (so this > test > passes on powerpc), though orig_gpr3 will remain valid throughout.

Hmm.. we can't use orig_gpr3 since we don't use a syscall wrapper.
All system calls just receive the parameters directly.

- Naveen

Right, I ran into this yesterday as well.
I solved it in v2
(https://lore.kernel.org/bpf/20220204041955.1958263-1-iii@xxxxxxxxxxxxx/)
by introducing a macro that hides whether or not an arch uses a syscall
wrapper.

Thanks. I missed your patches. I will take a look.

- Naveen





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