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

[Menglong Dong]

On Thu, Jun 8, 2023 at 4:09 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Wed, Jun 07, 2023 at 08:59:09PM +0800, menglong8.dong@xxxxxxxxx wrote:
> > From: Menglong Dong <imagedong@xxxxxxxxxxx>
> >
> > 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?

Thanks!
Menglong Dong