lguest: fix 'unhandled trap 13' with CONFIG_CC_STACKPROTECTOR

We don't set up the canary; let's disable stack protector on boot.c so
we can get into lguest_init, then set it up.  As a side effect,
switch_to_new_gdt() sets up %fs for us properly too.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 14 insertions, 4 deletions

diff --git a/arch/x86/lguest/Makefile b/arch/x86/lguest/Makefile
index 27f0c9ed7f60..94e0e54056a9 100644
--- a/arch/x86/lguest/Makefile
+++ b/arch/x86/lguest/Makefile
@@ -1 +1,2 @@
 obj-y           := i386_head.o boot.o
+CFLAGS_boot.o   := $(call cc-option, -fno-stack-protector)
diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index ca7ec44bafc3..33a93b417396 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -67,6 +67,7 @@
 #include <asm/mce.h>
 #include <asm/io.h>
 #include <asm/i387.h>
+#include <asm/stackprotector.h>
 #include <asm/reboot.h>         /* for struct machine_ops */
 
 /*G:010 Welcome to the Guest!
@@ -1088,13 +1089,21 @@ __init void lguest_init(void)
          * lguest_init() where the rest of the fairly chaotic boot setup
          * occurs. */
 
+        /* The stack protector is a weird thing where gcc places a canary
+         * value on the stack and then checks it on return.  This file is
+         * compiled with -fno-stack-protector it, so we got this far without
+         * problems.  The value of the canary is kept at offset 20 from the
+         * %gs register, so we need to set that up before calling C functions
+         * in other files. */
+        setup_stack_canary_segment(0);
+        /* We could just call load_stack_canary_segment(), but we might as
+         * call switch_to_new_gdt() which loads the whole table and sets up
+         * the per-cpu segment descriptor register %fs as well. */
+        switch_to_new_gdt(0);
+
         /* As described in head_32.S, we map the first 128M of memory. */
         max_pfn_mapped = (128*1024*1024) >> PAGE_SHIFT;
 
-        /* Load the %fs segment register (the per-cpu segment register) with
-         * the normal data segment to get through booting. */
-        asm volatile ("mov %0, %%fs" : : "r" (__KERNEL_DS) : "memory");
-
         /* The Host<->Guest Switcher lives at the top of our address space, and
          * the Host told us how big it is when we made LGUEST_INIT hypercall:
          * it put the answer in lguest_data.reserve_mem  */