powerpc: do_notify_resume can be called with bad thread_info flags argument

Back in 7230c5644188 ("powerpc: Rework lazy-interrupt handling") we
added a call out to restore_interrupts() (written in c) before calling
do_notify_resume:

        bl      restore_interrupts
        addi    r3,r1,STACK_FRAME_OVERHEAD
        bl      do_notify_resume

Unfortunately do_notify_resume takes two arguments, the second one
being the thread_info flags:

void do_notify_resume(struct pt_regs *regs, unsigned long thread_info_flags)

We do populate r4 (the second argument) earlier, but
restore_interrupts() is free to muck it up all it wants. My guess is
the gcc compiler gods shone down on us and its register allocator
never used r4. Sometimes, rarely, luck is on our side.

LLVM on the other hand did trample r4.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: stable@vger.kernel.org
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 5bbd1bc8c3b0..0905c8da90f1 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -659,7 +659,13 @@ _GLOBAL(ret_from_except_lite)
 3:
 #endif
         bl      save_nvgprs
+        /*
+         * Use a non volatile GPR to save and restore our thread_info flags
+         * across the call to restore_interrupts.
+         */
+        mr      r30,r4
         bl      restore_interrupts
+        mr      r4,r30
         addi    r3,r1,STACK_FRAME_OVERHEAD
         bl      do_notify_resume
         b       ret_from_except