Re: [PATCH bpf-next v4 06/14] libbpf: Add PT_REGS_SYSCALL_REGS macro

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Tue, 2022-02-08 at 14:08 -0800, Andrii Nakryiko wrote:
> On Mon, Feb 7, 2022 at 9:16 PM Ilya Leoshkevich <iii@linux.ibm.com>
> wrote:
> > 
> > Depending on whether or not an arch has ARCH_HAS_SYSCALL_WRAPPER,
> > syscall arguments must be accessed through a different set of
> > registers. Provide PT_REGS_SYSCALL_REGS macro to abstract away
> > that difference.
> > 
> > Reported-by: Heiko Carstens <hca@linux.ibm.com>
> > Co-developed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> > Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > ---
> 
> Again, there was nothing wrong with the way you did it in v3, please
> revert to that one.

I've realized that, even though fully correct, v3 looked somewhat
ad-hoc: it defined PT_REGS_SYSCALL_REGS for different architectures
without explaining why this particular arch has this parciular way to
access syscall arguments.

So I've decided to switch to the existing terminology, as Naveen
proposed [1]:

- arches that select ARCH_HAS_SYSCALL_WRAPPER in Kconfig get a
  __BPF_ARCH_HAS_SYSCALL_WRAPPER in bpf_tracing.h

- syscall handler calling convention is (at least partially) determined
  by whether or not an arch has a sycall wrapper as described in
  ARCH_HAS_SYSCALL_WRAPPER help text

I can, of course, switch back to v3 - both patches look functionally
identical - but this one seems to be a bit easier to understand.

[1]
https://lore.kernel.org/bpf/1643991537.bfyv1b2oym.naveen@linux.ibm.com/#t

> 
> >  tools/lib/bpf/bpf_tracing.h | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> > 
> > diff --git a/tools/lib/bpf/bpf_tracing.h
> > b/tools/lib/bpf/bpf_tracing.h
> > index 82f1e935d549..7a015ee8fb11 100644
> > --- a/tools/lib/bpf/bpf_tracing.h
> > +++ b/tools/lib/bpf/bpf_tracing.h
> > @@ -64,6 +64,8 @@
> > 
> >  #if defined(bpf_target_x86)
> > 
> > +#define __BPF_ARCH_HAS_SYSCALL_WRAPPER
> > +
> >  #if defined(__KERNEL__) || defined(__VMLINUX_H__)
> > 
> >  #define __PT_PARM1_REG di
> > @@ -114,6 +116,8 @@
> > 
> >  #elif defined(bpf_target_s390)
> > 
> > +#define __BPF_ARCH_HAS_SYSCALL_WRAPPER
> > +
> >  /* s390 provides user_pt_regs instead of struct pt_regs to
> > userspace */
> >  #define __PT_REGS_CAST(x) ((const user_pt_regs *)(x))
> >  #define __PT_PARM1_REG gprs[2]
> > @@ -142,6 +146,8 @@
> > 
> >  #elif defined(bpf_target_arm64)
> > 
> > +#define __BPF_ARCH_HAS_SYSCALL_WRAPPER
> > +
> >  /* arm64 provides struct user_pt_regs instead of struct pt_regs to
> > userspace */
> >  #define __PT_REGS_CAST(x) ((const struct user_pt_regs *)(x))
> >  #define __PT_PARM1_REG regs[0]
> > @@ -344,6 +350,17 @@ struct pt_regs;
> > 
> >  #endif /* defined(bpf_target_defined) */
> > 
> > +/*
> > + * When invoked from a syscall handler BPF_KPROBE, returns a
> > pointer to a
> > + * struct pt_regs containing syscall arguments, that is suitable
> > for passing to
> > + * PT_REGS_PARMn_SYSCALL() and PT_REGS_PARMn_CORE_SYSCALL().
> > + */
> > +#ifdef __BPF_ARCH_HAS_SYSCALL_WRAPPER
> > +#define PT_REGS_SYSCALL_REGS(ctx) ((struct pt_regs
> > *)PT_REGS_PARM1(ctx))
> > +#else
> > +#define PT_REGS_SYSCALL_REGS(ctx) ctx
> > +#endif
> > +
> >  #ifndef ___bpf_concat
> >  #define ___bpf_concat(a, b) a ## b
> >  #endif
> > --
> > 2.34.1
> >