[Skiboot] [PATCH] core/cpu: Prevent clobbering of stack guard for boot-cpu

[Vaibhav Jain]

Commit 90d53934c2da ("core/cpu: discover stack region size before
initialising memory regions") introduced memzero for struct cpu_thread
in init_cpu_thread(). This has an unintended side effect of clobbering
the stack-guard cannery of the boot_cpu stack. This results in opal
failing to init with this failure message:

CPU: P9 generation processor (max 4 threads/core)
CPU: Boot CPU PIR is 0x0004 PVR is 0x004e1200
Guard skip = 0
Stack corruption detected !
Aborting!
CPU 0004 Backtrace:
 S: 0000000031c13ab0 R: 0000000030013b0c   .backtrace+0x5c
 S: 0000000031c13b50 R: 000000003001bd18   ._abort+0x60
 S: 0000000031c13be0 R: 0000000030013bbc   .__stack_chk_fail+0x54
 S: 0000000031c13c60 R: 00000000300c5b70   .memset+0x12c
 S: 0000000031c13d00 R: 0000000030019aa8   .init_cpu_thread+0x40
 S: 0000000031c13d90 R: 000000003001b520   .init_boot_cpu+0x188
 S: 0000000031c13e30 R: 0000000030015050   .main_cpu_entry+0xd0
 S: 0000000031c13f00 R: 0000000030002700   boot_entry+0x1c0

So the patch provides a fix by tweaking the memset() call in
init_cpu_thread() to skip over the stack-guard cannery.

Fixes:90d53934c2da("core/cpu: discover stack region size before initialising memory regions")
Signed-off-by: Vaibhav Jain <vaibhav at linux.vnet.ibm.com>
---
 core/cpu.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/core/cpu.c b/core/cpu.c
index 1a7ccb0b..6826fee0 100644
--- a/core/cpu.c
+++ b/core/cpu.c
@@ -826,7 +826,11 @@ static void init_cpu_thread(struct cpu_thread *t,
 			    enum cpu_thread_state state,
 			    unsigned int pir)
 {
-	memset(t, 0, sizeof(struct cpu_thread));
+	/* offset within cpu_thread to prevent stack_guard clobber */
+	const size_t guard_skip = container_off_var(t, stack_guard) +
+		sizeof(t->stack_guard);
+
+	memset(t + guard_skip, 0, sizeof(struct cpu_thread) - guard_skip);
 	init_lock(&t->dctl_lock);
 	init_lock(&t->job_lock);
 	list_head_init(&t->job_queue);
-- 
2.14.3