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Date:   Wed, 30 Dec 2020 16:47:35 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marco Elver <elver@...gle.com>,
        George Popescu <georgepope@...roid.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH] ubsan: disable unsigned-integer-overflow sanitizer with clang

From: Arnd Bergmann <arnd@...db.de>

Building ubsan kernels even for compile-testing introduced these
warnings in my randconfig environment:

crypto/blake2b_generic.c:98:13: error: stack frame size of 9636 bytes in function 'blake2b_compress' [-Werror,-Wframe-larger-than=]
static void blake2b_compress(struct blake2b_state *S,
crypto/sha512_generic.c:151:13: error: stack frame size of 1292 bytes in function 'sha512_generic_block_fn' [-Werror,-Wframe-larger-than=]
static void sha512_generic_block_fn(struct sha512_state *sst, u8 const *src,
lib/crypto/curve25519-fiat32.c:312:22: error: stack frame size of 2180 bytes in function 'fe_mul_impl' [-Werror,-Wframe-larger-than=]
static noinline void fe_mul_impl(u32 out[10], const u32 in1[10], const u32 in2[10])
lib/crypto/curve25519-fiat32.c:444:22: error: stack frame size of 1588 bytes in function 'fe_sqr_impl' [-Werror,-Wframe-larger-than=]
static noinline void fe_sqr_impl(u32 out[10], const u32 in1[10])

Further testing showed that this is caused by
-fsanitize=unsigned-integer-overflow.

The one in blake2b immediately overflows the 8KB stack area on 32-bit
architectures, so better ensure this never happens by making this
option gcc-only.

Fixes: d0a3ac549f38 ("ubsan: enable for all*config builds")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 lib/Kconfig.ubsan | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index 8b635fd75fe4..e23873282ba7 100644
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -122,6 +122,8 @@ config UBSAN_SIGNED_OVERFLOW
 
 config UBSAN_UNSIGNED_OVERFLOW
 	bool "Perform checking for unsigned arithmetic overflow"
+	# clang hugely expands stack usage with -fsanitize=object-size
+	depends on !CC_IS_CLANG
 	depends on $(cc-option,-fsanitize=unsigned-integer-overflow)
 	help
 	  This option enables -fsanitize=unsigned-integer-overflow which checks
-- 
2.29.2

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