[PATCH v2 2/2] microblaze: reserve the ABI argument-home area in the signal frame

[Ramin Moussavi]

The MicroBlaze procedure call standard reserves [r1+0] for the return
address and lets a callee store its incoming register arguments r5..r10
into the caller-provided home slots at [r1+4]..[r1+24].  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 a seven-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 | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/microblaze/kernel/signal.c b/arch/microblaze/kernel/signal.c
index c78a0ff48..9cca06d60 100644
--- a/arch/microblaze/kernel/signal.c
+++ b/arch/microblaze/kernel/signal.c
@@ -49,6 +49,14 @@ struct sigframe {
 };
 
 struct rt_sigframe {
+	/*
+	 * Home area for the handler's register arguments: the MicroBlaze
+	 * ABI reserves [r1+0] for the return address and lets the callee
+	 * store r5..r10 at [r1+4]..[r1+24], and r1 points at this frame
+	 * when the handler is entered.  Without the gap those stores
+	 * corrupt info/uc.
+	 */
+	unsigned long abi_gap[7];
 	struct siginfo info;
 	struct ucontext uc;
 	unsigned long tramp[2];	/* signal trampoline */
-- 
2.53.0