[PATCH bpf-next 1/2] libbpf: Fix accessing first syscall argument on RV64

[Pu Lehui]

On 2024/8/31 3:34, Andrii Nakryiko wrote:
> On Thu, Aug 29, 2024 at 6:32 AM Pu Lehui <pulehui at huaweicloud.com> wrote:
>>
[SNIP]
>>
>> -#define __PT_PARM1_SYSCALL_REG __PT_PARM1_REG
>> +#define __PT_PARM1_SYSCALL_REG orig_a0
>>   #define __PT_PARM2_SYSCALL_REG __PT_PARM2_REG
>>   #define __PT_PARM3_SYSCALL_REG __PT_PARM3_REG
>>   #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_CORE_SYSCALL(x) \
>> +       BPF_CORE_READ((const struct pt_regs___riscv *)(x), __PT_PARM1_SYSCALL_REG)
> 
> I feel like what we did for s390x is a bit suboptimal, so let's (try
> to) improve that and then do the same for RV64.
> 
> What I mean is that PT_REGS_PARMn_SYSCALL macros are used to read
> pt_regs coming directly from context, right? In that case we don't
> need to pay the price of BPF_CORE_READ(), we can just access memory
> directly (but we still need CO-RE relocation!).
> 
> So I think what we should do is
> 
> 1) mark pt_regs___riscv {} with __attribute__((preserve_access_index))
> so that normal field accesses are CO-RE-relocated
> 2) change PT_REGS_PARM1_SYSCALL(x) to be `((const
> struct_pt_regs___riscv *)(x))->orig_a0`, which will directly read
> memory
> 3) keep PT_REGS_PARM1_CORE_SYSCALL() as is
> 
> 
> But having written all the above, I'm not sure whether we allow CO-RE
> relocated direct context accesses (verifier might complain about
> modifying ctx register offset or something). So can you please check
> it either on s390 or RV64 and let me know? I'm not marking it as
> "Changes Requested" for that reason, because that might not work and
> we'll have to do BPF_CORE_READ().

Hi Andrii, thanks for your suggestion, it's really cool. I check that 
work for RV64, and send a new version:

https://lore.kernel.org/bpf/20240831041934.1629216-1-pulehui@huaweicloud.com

> 
>>
>>   #define __PT_RET_REG ra
>>   #define __PT_FP_REG s0
>> --
>> 2.34.1
>>