Message-ID: <1336966957.6727.15.camel@pasglop>
Subject: [PATCH] grub: Fix module trampoline for powerpc
From: Benjamin Herrenschmidt <benh@au1.ibm.com>
To: Brent Baude <baude@us.ibm.com>, pjones@fedoraproject.org
Cc: hamzy@us.ibm.com, Josh Boyer <jwboyer@gmail.com>
Date: Mon, 14 May 2012 13:42:37 +1000
Organization: IBM Australia
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[ --- snip --- ]

Not sure who to send that to, I picked Peter as he's in
the rpm log as author of the of the powerpc patches,
feel free to do whatever with that one, it should ultimately
go to upstream grub I suppose ...

Without this, grub doesn't work for me at all with fc17 beta

[ --- snip --- ]

The trampoline generated by grub powerpc's dl.c to call from
modules into the main grub code uses r0 as a scratch register.

However, nowadays, gcc can (and will) generate function calls
to spill registers to the stack (well, it's even stupid enough
to do it when there's only one register to save ! go figure....)

Those calls happen during the function prolog, before the
return address has been saved on the stack, typically it's held
in r0 at this stage. Since those calls will hit the trampoline
in grub, which clobbers r0, this will clobber the return address
and cause a crash.

This patch changes the trampolines to use r12 instead which
is safe to use in our case.

Note: It might be better to actually link those low level gcc
support functions statically into the modules but that's beyond
the level of grub hacking I'm prepared to do today.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---
diff --git a/grub-core/kern/powerpc/dl.c b/grub-core/kern/powerpc/dl.c
index b8a2b50..0a8ec85 100644
--- a/grub-core/kern/powerpc/dl.c
+++ b/grub-core/kern/powerpc/dl.c
@@ -89,9 +89,9 @@ struct trampoline
 
 static const struct trampoline trampoline_template = 
   {
-    0x3c000000,
-    0x60000000,
-    0x7c0903a6,
+    0x3d800000,
+    0x618c0000,
+    0x7d8903a6,
     0x4e800420,
   };