On Thu, 2016-02-25 at 14:17 +0100, Torsten Duwe wrote:

On Thu, Feb 25, 2016 at 01:28:25AM +1100, Michael Ellerman wrote:

quoted

We can make that process easier by marking the generated stubs with a
magic value, and then looking for that magic value. Altough this is not
as rigorous as the current method, I believe it is sufficient in
practice.

The actual magic value is sort of debatable; it should be "improbable"
enough. But this can be changed easily, for each kernel compile, even.

Yeah. Given the locations we're trying to patch are computed in the first place
from the mcount call sites, I feel like we don't need to be super paranoid
here. The only time I've heard of this code (the current version) tripping up
is when folks are hacking on ftrace.

quoted

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Reviewed-by: Torsten Duwe <redacted>

[for reference:]

quoted

+#define STUB_MAGIC 0x73747562 /* stub */

Which is one of:

ori     r21,r19,29811
andi.   r20,r27,30050

Both of which are pretty improbable. They don't appear in any kernel I have
around here.

I have more plans for this code, which would hopefully mean we can get rid of
the magic checking entirely. But I think this is OK for now.

cheers