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

[Menglong Dong <menglong8.dong@gmail.com>]

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!
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>
> [...]