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SubjectRe: [PATCH bpf-next v3 1/3] bpf, x86: allow function arguments up to 12 for TRACING
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On 6/8/23 7:12 PM, Menglong Dong wrote:
> On Fri, Jun 9, 2023 at 5:07 AM Yonghong Song <yhs@meta.com> wrote:
>>
>>
>>
>> On 6/7/23 5:59 AM, menglong8.dong@gmail.com wrote:
>>> From: Menglong Dong <imagedong@tencent.com>
>>>
>>> For now, the BPF program of type BPF_PROG_TYPE_TRACING can only be used
>>> on the kernel functions whose arguments count less than 6. This is not
>>> friendly at all, as too many functions have arguments count more than 6.
>>
>> Since you already have some statistics, maybe listed in the commit message.
>>
>>>
>>> Therefore, let's enhance it by increasing the function arguments count
>>> allowed in arch_prepare_bpf_trampoline(), for now, only x86_64.
>>>
>>> For the case that we don't need to call origin function, which means
>>> without BPF_TRAMP_F_CALL_ORIG, we need only copy the function arguments
>>> that stored in the frame of the caller to current frame. The arguments
>>> of arg6-argN are stored in "$rbp + 0x18", we need copy them to
>>> "$rbp - regs_off + (6 * 8)".
>>
>> Maybe I missed something, could you explain why it is '$rbp + 0x18'?
>>
>> In the current upstream code, we have
>>
>> /* Generated trampoline stack layout:
>> *
>> * RBP + 8 [ return address ]
>> * RBP + 0 [ RBP ]
>> *
>> * RBP - 8 [ return value ] BPF_TRAMP_F_CALL_ORIG or
>> *
>> BPF_TRAMP_F_RET_FENTRY_RET flags
>> *
>> * [ reg_argN ] always
>> * [ ... ]
>> * RBP - regs_off [ reg_arg1 ] program's ctx pointer
>> *
>> * RBP - nregs_off [ regs count ] always
>> *
>> * RBP - ip_off [ traced function ] BPF_TRAMP_F_IP_ARG flag
>> *
>> * RBP - run_ctx_off [ bpf_tramp_run_ctx ]
>> */
>>
>> Next on-stack argument will be RBP + 16, right?
>>
>
> Sorry for the confusing, it seems there should be
> some comments here.
>
> It's not the next on-stack argument, but the next next on-stack
> argument. The call chain is:
>
> caller -> origin call -> trampoline
>
> So, we have to skip the "RIP" in the stack frame of "origin call",
> which means RBP + 16 + 8. To be clear, there are only 8-byte
> in the stack frame of "origin call".

Thanks. It does make sense now. So we have
caller -> origin call -> (5 nops changed to a call) -> trampoline
8 bytes 8 bytes
and inside trampoline we have 8 bytes in stack with 'push rbp'.
Yes, it would be great there is an explanation in the code.

>
> Thanks!
> Menglong Dong
>
>
>>>
>>> For the case with BPF_TRAMP_F_CALL_ORIG, we need prepare the arguments
>>> in stack before call origin function, which means we need alloc extra
>>> "8 * (arg_count - 6)" memory in the top of the stack. Note, there should
>>> not be any data be pushed to the stack before call the origin function.
>>> Then, we have to store rbx with 'mov' instead of 'push'.
>>>
>>> We use EMIT3_off32() or EMIT4() for "lea" and "sub". The range of the
>>> imm in "lea" and "sub" is [-128, 127] if EMIT4() is used. Therefore,
>>> we use EMIT3_off32() instead if the imm out of the range.
>>>
>>> It works well for the FENTRY and FEXIT, I'm not sure if there are other
>>> complicated cases.
>>
>> MODIFY_RETURN is also impacted by this patch.
>>
>>>
>>> Reviewed-by: Jiang Biao <benbjiang@tencent.com>
>>> Signed-off-by: Menglong Dong <imagedong@tencent.com>
>> [...]

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