[PATCH 2/2] microblaze: reserve the ABI argument-home area in the signal frame

[Ramin Moussavi]

The MicroBlaze procedure call standard lets a callee store its incoming
register arguments r5..r10 into the caller-provided home slots at
[r1+4]..[r1+28].  When the kernel enters a signal handler it sets r1 to
point at struct rt_sigframe, whose leading members are the siginfo and
ucontext prepared for the handler.  A handler that homes its arguments -
which an unoptimised (-O0) build always does - therefore overwrites the
start of siginfo/ucontext, corrupting the signal state that the handler
and sys_rt_sigreturn() depend on.

Reserve the home area by making an eight-word gap the first member of
struct rt_sigframe, so the handler's argument stores land in scratch space
instead of clobbering siginfo.

Tested on qemu-system-microblazeel (petalogix-s3adsp1800) with the
uClibc-ng NPTL test suite: tst-timer4, tst-timer5, tst-mqueue5 and
tst-signal6 pass; before this change they failed because the handler
clobbered the signal frame.

Signed-off-by: Ramin Moussavi <ramin.moussavi@xxxxxxxxx>
---
 arch/microblaze/kernel/signal.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/microblaze/kernel/signal.c b/arch/microblaze/kernel/signal.c
index c78a0ff48..df9c07d77 100644
--- a/arch/microblaze/kernel/signal.c
+++ b/arch/microblaze/kernel/signal.c
@@ -49,6 +49,13 @@ struct sigframe {
 };
 
 struct rt_sigframe {
+	/*
+	 * Home area for the handler's register arguments: the MicroBlaze
+	 * ABI lets the callee store r5..r10 at [r1+4]..[r1+28], and r1
+	 * points at this frame when the handler is entered.  Without the
+	 * gap those stores corrupt info/uc.
+	 */
+	unsigned long abi_gap[8];
 	struct siginfo info;
 	struct ucontext uc;
 	unsigned long tramp[2];	/* signal trampoline */
-- 
2.53.0