Re: [RFC PATCH v3 01/11] powerpc/kprobes: Use ftrace to determine if a probe is at function entry

Naveen N Rao
2024-07-01T18:20:16Z

Hi Nick,
Thanks for the reviews!

On Mon, Jul 01, 2024 at 06:40:50PM GMT, Nicholas Piggin wrote:
> On Fri Jun 21, 2024 at 4:54 AM AEST, Naveen N Rao wrote:
> > Rather than hard-coding the offset into a function to be used to
> > determine if a kprobe is at function entry, use ftrace_location() to
> > determine the ftrace location within the function and categorize all
> > instructions till that offset to be function entry.
> >
> > For functions that cannot be traced, we fall back to using a fixed
> > offset of 8 (two instructions) to categorize a probe as being at
> > function entry for 64-bit elfv2, unless we are using pcrel.
> >
> > Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > Signed-off-by: Naveen N Rao <naveen@kernel.org>
> > ---
> > arch/powerpc/kernel/kprobes.c | 18 ++++++++----------
> > 1 file changed, 8 insertions(+), 10 deletions(-)
> >
> > diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> > index 14c5ddec3056..ca204f4f21c1 100644
> > --- a/arch/powerpc/kernel/kprobes.c
> > +++ b/arch/powerpc/kernel/kprobes.c
> > @@ -105,24 +105,22 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
> > return addr;
> > }
> >
> > -static bool arch_kprobe_on_func_entry(unsigned long offset)
> > +static bool arch_kprobe_on_func_entry(unsigned long addr, unsigned long offset)
> > {
> > -#ifdef CONFIG_PPC64_ELF_ABI_V2
> > -#ifdef CONFIG_KPROBES_ON_FTRACE
> > - return offset <= 16;
> > -#else
> > - return offset <= 8;
> > -#endif
> > -#else
> > + unsigned long ip = ftrace_location(addr);
> > +
> > + if (ip)
> > + return offset <= (ip - addr);
> > + if (IS_ENABLED(CONFIG_PPC64_ELF_ABI_V2) && !IS_ENABLED(CONFIG_PPC_KERNEL_PCREL))
> > + return offset <= 8;
>
> If it is PCREL, why not offset == 0 as well?

That's handled by the fallback code that is after the above line:
return !offset;

That addresses both pcrel, as well as 32-bit powerpc.

Thanks,
Naveen