Re: [PATCH] bpf, x86: allow function arguments up to 12 for TRACING

[Menglong Dong]

On Wed, May 31, 2023 at 4:01 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Tue, May 30, 2023 at 12:44:23PM +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'.
> >
> > It works well for the FENTRY and FEXIT, I'm not sure if there are other
> > complicated cases.
> >
> > Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> > ---
> >  arch/x86/net/bpf_jit_comp.c | 88 ++++++++++++++++++++++++++++++++-----
>
> please add selftests for this.. I had to add one to be able to check
> the generated trampoline
>

Okay!

BTW, I failed to compile the latest selftests/bpf with
the following errors:

progs/verifier_and.c:58:16: error: invalid operand for instruction
        asm volatile ("                                 \

The version of clang I used is:

clang --version
Debian clang version 14.0.6
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Does anyone know the reason?

Thanks!
Menglong Dong

> jirka
>
>