Messages in this thread | | | Date | Thu, 8 Jun 2023 21:29:17 -0700 | Subject | Re: [PATCH bpf-next v3 1/3] bpf, x86: allow function arguments up to 12 for TRACING | From | Yonghong Song <> |
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On 6/8/23 7:12 PM, Menglong Dong wrote: > 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. It does make sense now. So we have caller -> origin call -> (5 nops changed to a call) -> trampoline 8 bytes 8 bytes and inside trampoline we have 8 bytes in stack with 'push rbp'. Yes, it would be great there is an explanation in the code.
> > 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> >> [...]
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