[PATCH 2/2] start_kernel: omit prevent_tail_call_optimization for newer toolchains

[ndesaulniers at google.com]

prevent_tail_call_optimization was added in
commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")
to work around stack canaries getting inserted into functions that would
initialize the stack canary in the first place.

Now that we have no_stack_protector function attribute (gcc-11+,
clang-7+) and use it on start_kernel, remove the call to
prevent_tail_call_optimization such that we may one day remove it
outright.

Reviewed-by: Nathan Chancellor <nathan at kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Signed-off-by: Nick Desaulniers <ndesaulniers at google.com>
---
 init/main.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/init/main.c b/init/main.c
index 1265c8d11052..c6eef497c8c9 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1152,7 +1152,13 @@ void start_kernel(void)
 	/* Do the rest non-__init'ed, we're now alive */
 	arch_call_rest_init();
 
+	/*
+	 * Avoid stack canaries in callers of boot_init_stack_canary for gcc-10
+	 * and older.
+	 */
+#if !__has_attribute(__no_stack_protector__)
 	prevent_tail_call_optimization();
+#endif
 }
 
 /* Call all constructor functions linked into the kernel. */

-- 
2.40.0.634.g4ca3ef3211-goog