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

["Nicholas Piggin" <npiggin@gmail.com>]

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?

Thanks,
Nick