Re: [PATCH RFC bpf-next 2/7] x86/ftrace: implement DYNAMIC_FTRACE_WITH_JMP

Menglong Dong
2025-11-15T02:12:36Z

On Sat, Nov 15, 2025 at 12:39 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Fri, 14 Nov 2025 17:24:45 +0800
> Menglong Dong <menglong8.dong@gmail.com> wrote:
>
> > --- a/arch/x86/kernel/ftrace_64.S
> > +++ b/arch/x86/kernel/ftrace_64.S
> > @@ -285,8 +285,18 @@ SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL)
> > ANNOTATE_NOENDBR
> > RET
> >
> > +1:
> > + testb $1, %al
> > + jz 2f
> > + andq $0xfffffffffffffffe, %rax
> > + movq %rax, MCOUNT_REG_SIZE+8(%rsp)
> > + restore_mcount_regs
> > + /* Restore flags */
> > + popfq
> > + RET
> > +
> > /* Swap the flags with orig_rax */
> > -1: movq MCOUNT_REG_SIZE(%rsp), %rdi
> > +2: movq MCOUNT_REG_SIZE(%rsp), %rdi
> > movq %rdi, MCOUNT_REG_SIZE-8(%rsp)
> > movq %rax, MCOUNT_REG_SIZE(%rsp)
> >
>
> So in this case we have:
>
> original_caller:
> call foo -> foo:
> call fentry -> fentry:
> [do ftrace callbacks ]
> move tramp_addr to stack
> RET -> tramp_addr
> tramp_addr:
> [..]
> call foo_body -> foo_body:
> [..]
> RET -> back to tramp_addr
> [..]
> RET -> back to original_caller

Nice flow chart, which I think we can put in the commit log.

>
> I guess that looks balanced.

Yes, it is balanced.

>
> -- Steve
>
>