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

[Pu Lehui]

On 2024/9/5 4:21, Andrii Nakryiko wrote:
> 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).

I check that locally with cross-platform vmtest on RV64, it looks good:

Summary: 569/3944 PASSED, 104 SKIPPED, 0 FAILED

and BPF CI meet happy on arm64, s390x.


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