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

Commit Message

Naveen N Rao June 20, 2024, 6:54 p.m. UTC

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) <[e-mail]>
Signed-off-by: Naveen N Rao <[e-mail]>
---
 arch/powerpc/kernel/kprobes.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

Comments

Nicholas Piggin July 1, 2024, 8:40 a.m. UTC | #1

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) <[e-mail]>
> Signed-off-by: Naveen N Rao <[e-mail]>
> ---
>  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

Naveen N Rao July 1, 2024, 6:18 p.m. UTC | #2

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) <[e-mail]>
> > Signed-off-by: Naveen N Rao <[e-mail]>
> > ---
> >  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

Patch

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;
 	return !offset;
-#endif
 }
 
 /* XXX try and fold the magic of kprobe_lookup_name() in this */
 kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
 					 bool *on_func_entry)
 {
-	*on_func_entry = arch_kprobe_on_func_entry(offset);
+	*on_func_entry = arch_kprobe_on_func_entry(addr, offset);
 	return (kprobe_opcode_t *)(addr + offset);
 }