[PATCH bpf-next v3 2/4] libbpf: Access first syscall argument with CO-RE direct read on arm64

[Andrii Nakryiko]

On Fri, Aug 30, 2024 at 9:17 PM Pu Lehui <pulehui at huaweicloud.com> wrote:
>
> From: Pu Lehui <pulehui at huawei.com>
>
> Currently PT_REGS_PARM1 SYSCALL(x) is consistent with PT_REGS_PARM1_CORE
> SYSCALL(x), which will introduce the overhead of BPF_CORE_READ(), taking
> into account the read pt_regs comes directly from the context, let's use
> CO-RE direct read to access the first system call argument.
>
> Suggested-by: Andrii Nakryiko <andrii at kernel.org>
> Signed-off-by: Pu Lehui <pulehui at huawei.com>
> ---
>  tools/lib/bpf/bpf_tracing.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf_tracing.h b/tools/lib/bpf/bpf_tracing.h
> index e7d9382efeb3..051c408e6aed 100644
> --- a/tools/lib/bpf/bpf_tracing.h
> +++ b/tools/lib/bpf/bpf_tracing.h
> @@ -222,7 +222,7 @@ struct pt_regs___s390 {
>
>  struct pt_regs___arm64 {
>         unsigned long orig_x0;
> -};
> +} __attribute__((preserve_access_index));
>
>  /* arm64 provides struct user_pt_regs instead of struct pt_regs to userspace */
>  #define __PT_REGS_CAST(x) ((const struct user_pt_regs *)(x))
> @@ -241,7 +241,7 @@ struct pt_regs___arm64 {
>  #define __PT_PARM4_SYSCALL_REG __PT_PARM4_REG
>  #define __PT_PARM5_SYSCALL_REG __PT_PARM5_REG
>  #define __PT_PARM6_SYSCALL_REG __PT_PARM6_REG
> -#define PT_REGS_PARM1_SYSCALL(x) PT_REGS_PARM1_CORE_SYSCALL(x)
> +#define PT_REGS_PARM1_SYSCALL(x) (((const struct pt_regs___arm64 *)(x))->orig_x0)

It would probably be best (for consistency) to stick to using
__PTR_PARM1_SYSCALL_REG instead of hard-coding orig_x0 here, no? I'll
fix it up while applying. Same for patch #1 and #4.

It would be great if you can double-check that final patches in
bpf-next/master compile and work well for arm64, s390x, and RV64 (as I
can't really test that much locally).



>  #define PT_REGS_PARM1_CORE_SYSCALL(x) \
>         BPF_CORE_READ((const struct pt_regs___arm64 *)(x), __PT_PARM1_SYSCALL_REG)
>
> --
> 2.34.1
>