[tip: objtool/core] start_kernel: Add __no_stack_protector function attribute

[tip-bot2 for ndesaulniers@xxxxxxxxxx]

The following commit has been merged into the objtool/core branch of tip:

Commit-ID:     514ca14ed5444b911de59ed3381dfd195d99fe4b
Gitweb:        https://git.kernel.org/tip/514ca14ed5444b911de59ed3381dfd195d99fe4b
Author:        ndesaulniers@xxxxxxxxxx <ndesaulniers@xxxxxxxxxx>
AuthorDate:    Mon, 17 Apr 2023 15:00:05 -07:00
Committer:     Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
CommitterDate: Tue, 16 May 2023 06:28:15 -07:00

start_kernel: Add __no_stack_protector function attribute

Back during the discussion of
commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")
we discussed the need for a function attribute to control the omission
of stack protectors on a per-function basis; at the time Clang had
support for no_stack_protector but GCC did not. This was fixed in
gcc-11. Now that the function attribute is available, let's start using
it.

Callers of boot_init_stack_canary need to use this function attribute
unless they're compiled with -fno-stack-protector, otherwise the canary
stored in the stack slot of the caller will differ upon the call to
boot_init_stack_canary. This will lead to a call to __stack_chk_fail()
then panic.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94722
Link: https://lore.kernel.org/all/20200316130414.GC12561@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> (powerpc)
Acked-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230412-no_stackp-v2-1-116f9fe4bbe7@xxxxxxxxxx
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

Signed-off-by: ndesaulniers@xxxxxxxxxx <ndesaulniers@xxxxxxxxxx>
---
 arch/powerpc/kernel/smp.c           |  1 +
 include/linux/compiler_attributes.h | 12 ++++++++++++
 init/main.c                         |  3 ++-
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 265801a..6903a72 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1605,6 +1605,7 @@ static void add_cpu_to_masks(int cpu)
 }
 
 /* Activate a secondary processor. */
+__no_stack_protector
 void start_secondary(void *unused)
 {
 	unsigned int cpu = raw_smp_processor_id();
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index e659cb6..8486476 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -256,6 +256,18 @@
 #define __noreturn                      __attribute__((__noreturn__))
 
 /*
+ * Optional: only supported since GCC >= 11.1, clang >= 7.0.
+ *
+ *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-no_005fstack_005fprotector-function-attribute
+ *   clang: https://clang.llvm.org/docs/AttributeReference.html#no-stack-protector-safebuffers
+ */
+#if __has_attribute(__no_stack_protector__)
+# define __no_stack_protector		__attribute__((__no_stack_protector__))
+#else
+# define __no_stack_protector
+#endif
+
+/*
  * Optional: not supported by gcc.
  *
  * clang: https://clang.llvm.org/docs/AttributeReference.html#overloadable
diff --git a/init/main.c b/init/main.c
index af50044..c445c1f 100644
--- a/init/main.c
+++ b/init/main.c
@@ -877,7 +877,8 @@ static void __init print_unknown_bootoptions(void)
 	memblock_free(unknown_options, len);
 }
 
-asmlinkage __visible void __init __no_sanitize_address __noreturn start_kernel(void)
+asmlinkage __visible __init __no_sanitize_address __noreturn __no_stack_protector
+void start_kernel(void)
 {
 	char *command_line;
 	char *after_dashes;