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

[Menglong Dong]

On Fri, Jun 9, 2023 at 5:12 AM Yonghong Song <yhs@xxxxxxxx> wrote:
>
>
>
> On 6/7/23 8:17 PM, Menglong Dong wrote:
> > 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?
>
> This is the stack_size, you should ensure stack pointer is 16-byte aligned.

Oh, I see. Considering the begin of the stack frame
should already be 16-byte aligned, what we should
do here is to make the size of the current stack frame
16-byte aligned. Then, rsp will be 16-byte aligned.

Am I right?

Which means the code should be:

+       if (nr_regs > 6 && (flags & BPF_TRAMP_F_CALL_ORIG)) {
+               stack_size += (nr_regs - 6) * 8;
+               stack_size = ALIGN(stack_size, 16);
+       }

Then, the size of current stack frame will be:
stack_size + 8(rbp) + 8(rip)

This is the example that I refer to:
https://godbolt.org/z/7o9nh4nbc

>
> >
> > Thanks!
> > Menglong Dong