On Fri, Jun 9, 2023 at 5:07 AM Yonghong Song <yhs@xxxxxxxx> wrote: > > > > On 6/7/23 5:59 AM, 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. > > 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@xxxxxxxxxxx> > > Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx> > [...]
