Re: [PATCH v4 4/9] fprobe: rethook: Use ftrace_regs in fprobe exit handler and rethook

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 05 Sep 2023 09:17:02 +0200
Sven Schnelle <svens@linux.ibm.com> wrote:

> Masami Hiramatsu (Google) <mhiramat@kernel.org> writes:
> 
> > I found that this is not enough becuase s390/loongarch already implemented
> > their rethook, and as far as I can see, the s390 ftrace_regs does not save
> > the required registers for rethook. Thus, for such architecture, we need
> > another kconfig flag and keep using the pt_regs for rethook.
> 
> Looking into arch_rethook_trampoline() i think we save all required
> registers - which register do you think are missing? Or is there another
> function i should look at?

Yes, arch_rethook_trampoline() is good. It needs to save all registers.

In this series, I'm trying to change the pt_regs with ftrace_regs which will
reduce trampoline overhead if DYNAMIC_FTRACE_WITH_ARGS=y.

kprobe -> (pt_regs) -> rethook_try_hook()
fprobe -> (ftrace_regs) -> rethook_try_hook_ftrace() # new function

Thus, we need to ensure that the ftrace_regs which is saved in the ftrace
*without* FTRACE_WITH_REGS flags, can be used for hooking the function
return. I saw;

void arch_rethook_prepare(struct rethook_node *rh, struct pt_regs *regs, bool mcount)
{
        rh->ret_addr = regs->gprs[14];
        rh->frame = regs->gprs[15];

        /* Replace the return addr with trampoline addr */
        regs->gprs[14] = (unsigned long)&arch_rethook_trampoline;
}

gprs[15] is a stack pointer, so it is saved in ftrace_regs too, but what about
gprs[14]? (I guess it is a link register)
We need to read the gprs[14] and ensure that is restored to gpr14 when the
ftrace is exit even without FTRACE_WITH_REGS flag.

IOW, it is ftrace save regs/restore regs code issue. I need to check how the
function_graph implements it.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>