netdev - Re: [PATCH bpf-next v3 1/3] bpf, x86: allow function arguments up to 12 for TRACING

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Date: Thu, 8 Jun 2023 14:12:49 -0700
From: Yonghong Song <yhs@...a.com>
To: Menglong Dong <menglong8.dong@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: davem@...emloft.net, dsahern@...nel.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev,
        song@...nel.org, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, sdf@...gle.com, x86@...nel.org,
        imagedong@...cent.com, benbjiang@...cent.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 1/3] bpf, x86: allow function arguments up to
 12 for TRACING



On 6/7/23 8:17 PM, Menglong Dong wrote:
> On Thu, Jun 8, 2023 at 4:09 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
>>
>> On Wed, Jun 07, 2023 at 08:59:09PM +0800, menglong8.dong@...il.com wrote:
>>> From: Menglong Dong <imagedong@...cent.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.
>>>
>>> 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)".
>>>
>>> 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'.
>>
>> x86-64 psABI requires stack to be 16-byte aligned when args are passed on the stack.
>> I don't see this logic in the patch.
> 
> Yeah, it seems I missed this logic......:)
> 
> I have not figure out the rule of the alignment, but after
> observing the behavior of the compiler, the stack seems
> should be like this:
> 
> ------ stack frame begin
> rbp
> 
> xxx   -- this part should be aligned in 16-byte
> 
> ------ end of arguments in stack
> xxx
> ------ begin of arguments in stack
> 
> So the code should be:
> 
> +       if (nr_regs > 6 && (flags & BPF_TRAMP_F_CALL_ORIG)) {
> +                stack_size = ALIGN(stack_size, 16);
> +                stack_size += (nr_regs - 6) * 8;
> +       }
> 
> Am I right?

This is the stack_size, you should ensure stack pointer is 16-byte aligned.

> 
> Thanks!
> Menglong Dong