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From:   =?utf-8?q?Thomas_Wei=C3=9Fschuh?= <linux@weissschuh.net>
Date:   Sun, 21 May 2023 11:36:31 +0200
Subject: [PATCH 3/7] tools/nolibc: ensure stack protector guard is never
 zero
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Message-Id: <20230521-nolibc-automatic-stack-protector-v1-3-dad6c80c51c1@weissschuh.net>
References: <20230521-nolibc-automatic-stack-protector-v1-0-dad6c80c51c1@weissschuh.net>
In-Reply-To: <20230521-nolibc-automatic-stack-protector-v1-0-dad6c80c51c1@weissschuh.net>
To:     Willy Tarreau <w@1wt.eu>, "Paul E. McKenney" <paulmck@kernel.org>,
        Shuah Khan <shuah@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-riscv@lists.infradead.org, llvm@lists.linux.dev,
        =?utf-8?q?Thomas_Wei=C3=9Fschuh?= <linux@weissschuh.net>
Precedence: bulk

The all-zero pattern is one of the more probable out-of-bound writes so
add a special case to not accidentally accept it.

Also it enables the reliable detection of stack protector initialization
during testing.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
 tools/include/nolibc/stackprotector.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tools/include/nolibc/stackprotector.h b/tools/include/nolibc/stackprotector.h
index 77e5251c4490..b0156fc077a0 100644
--- a/tools/include/nolibc/stackprotector.h
+++ b/tools/include/nolibc/stackprotector.h
@@ -45,8 +45,9 @@ __attribute__((weak,no_stack_protector,section(".text.nolibc_stack_chk")))
 void __stack_chk_init(void)
 {
 	my_syscall3(__NR_getrandom, &__stack_chk_guard, sizeof(__stack_chk_guard), 0);
-	/* a bit more randomness in case getrandom() fails */
-	__stack_chk_guard ^= (uintptr_t) &__stack_chk_guard;
+	/* a bit more randomness in case getrandom() fails, ensure the guard is never 0 */
+	if (__stack_chk_guard != (uintptr_t) &__stack_chk_guard)
+		__stack_chk_guard ^= (uintptr_t) &__stack_chk_guard;
 }
 #endif /* defined(NOLIBC_STACKPROTECTOR) */
 

-- 
2.40.1