linux-kernel - [PATCH 2/2] start_kernel: omit prevent_tail_call

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Message-ID: <20230412-no_stackp-v1-2-46a69b507a4b@google.com>
Date:   Wed, 12 Apr 2023 11:32:13 -0700
From:   ndesaulniers@...gle.com
To:     "Borislav Petkov (AMD)" <bp@...en8.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Miguel Ojeda <ojeda@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH 2/2] start_kernel: omit prevent_tail_call_optimization for
 newer toolchains

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.

Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
 init/main.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/init/main.c b/init/main.c
index 213baf7b8cb1..c8503d02dfa6 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1152,7 +1152,12 @@ 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. */

-- 
2.40.0.577.gac1e443424-goog