Le 18/04/2022 à 21:44, Steven Rostedt a écrit :

On Mon, 18 Apr 2022 11:51:16 +0530
"Naveen N. Rao" [off-list ref] wrote:

quoted

quoted

--- a/arch/powerpc/kernel/trace/ftrace.c
+++ b/arch/powerpc/kernel/trace/ftrace.c

@@ -78,10 +78,7 @@ ftrace_modify_code(unsigned long ip, ppc_inst_t old, ppc_inst_t new)
  	}

  	/* replace the text with the new text */
-	if (patch_instruction((u32 *)ip, new))
-		return -EPERM;
-
-	return 0;
+	return patch_instruction((u32 *)ip, new);

I think the reason we were returning -EPERM is so that ftrace_bug() can

That is correct.

quoted

throw the right error message. That will change due to this patch,
though I'm not sure how much it matters. -EFAULT and -EPERM seem to
print almost the same error message.

In these cases it helps to know the type of failure, as the way to debug it
is different.

-EFAULT: It failed to read it the location. This means that the memory is
likely not even mapped in, or the pointer is way off.

-EINVAL: Means that what was read did not match what was expected (the code
was already updated, pointing to the wrong location, or simply the
calculation of what to expect is incorrect).

-EPERM: Means the write failed. What was read was expected, but the
permissions to write have not been updated properly.

Differentiating the three is crucial to looking at where the issue lies
when an ftrace_bug() triggers.

Apparently no caller really care about the value returned by 
patch_instruction(), the ones who check the return value just check that 
it's not 0.

So the most performant would be to have patch_instruction() return 
-EPERM instead of -EFAULT in case of failure.

Christophe