x86: conditionalize stack canary handling in hot path

Impact: no unnecessary stack canary swapping during context switch

There's no point in moving stack_canary around during context switch
if it's not enabled.  Conditionalize it.

Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 16 insertions, 6 deletions

diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
index 8cadfe9b1194..b77bd8bd3cc2 100644
--- a/arch/x86/include/asm/system.h
+++ b/arch/x86/include/asm/system.h
@@ -86,17 +86,28 @@ do {									\
         , "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \
           "r12", "r13", "r14", "r15"
 
+#ifdef CONFIG_CC_STACKPROTECTOR
+#define __switch_canary                                                   \
+        "movq %P[task_canary](%%rsi),%%r8\n\t"                            \
+        "movq %%r8,%%gs:%P[pda_canary]\n\t"
+#define __switch_canary_param                                             \
+        , [task_canary] "i" (offsetof(struct task_struct, stack_canary))  \
+        , [pda_canary] "i" (offsetof(struct x8664_pda, stack_canary))
+#else   /* CC_STACKPROTECTOR */
+#define __switch_canary
+#define __switch_canary_param
+#endif  /* CC_STACKPROTECTOR */
+
 /* Save restore flags to clear handle leaking NT */
 #define switch_to(prev, next, last) \
-        asm volatile(SAVE_CONTEXT                                                   \
+        asm volatile(SAVE_CONTEXT                                         \
              "movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */       \
              "movq %P[threadrsp](%[next]),%%rsp\n\t" /* restore RSP */    \
              "call __switch_to\n\t"                                       \
              ".globl thread_return\n"                                     \
              "thread_return:\n\t"                                         \
              "movq "__percpu_arg([current_task])",%%rsi\n\t"              \
-             "movq %P[task_canary](%%rsi),%%r8\n\t"                       \
-             "movq %%r8,%%gs:%P[pda_canary]\n\t"                          \
+             __switch_canary                                              \
              "movq %P[thread_info](%%rsi),%%r8\n\t"                       \
              LOCK_PREFIX "btr  %[tif_fork],%P[ti_flags](%%r8)\n\t"        \
              "movq %%rax,%%rdi\n\t"                                       \
@@ -108,9 +119,8 @@ do {									\
                [ti_flags] "i" (offsetof(struct thread_info, flags)),      \
                [tif_fork] "i" (TIF_FORK),                                 \
                [thread_info] "i" (offsetof(struct task_struct, stack)),   \
-               [task_canary] "i" (offsetof(struct task_struct, stack_canary)),\
-               [current_task] "m" (per_cpu_var(current_task)),            \
-               [pda_canary] "i" (offsetof(struct x8664_pda, stack_canary))\
+               [current_task] "m" (per_cpu_var(current_task))             \
+               __switch_canary_param                                      \
              : "memory", "cc" __EXTRA_CLOBBER)
 #endif