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[PATCH] string.h: work around for increased stack usage

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The hardened strlen() function causes rather large stack usage
in at least one file in the kernel when CONFIG_KASAN is enabled:

drivers/media/usb/em28xx/em28xx-dvb.c: In function 'em28xx_dvb_init':
drivers/media/usb/em28xx/em28xx-dvb.c:2062:1: error: the frame size of 3256 bytes is larger than 204 bytes [-Werror=frame-larger-than=]

Analyzing this problem led to the discovery that gcc fails to
merge the stack slots for the i2c_board_info[] structures after
we strlcpy() into them, due to the 'noreturn' attribute on the
source string length check.

The compiler behavior should get fixed in gcc-8, but for users
of existing gcc versions, we can work around it using an empty
inline assembly statement before the call to fortify_panic().

The workaround is unfortunately very ugly, and I tried my best
to limit it being applied to affected versions of gcc when
KASAN is used. Alternative suggestions welcome.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 include/linux/string.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/string.h b/include/linux/string.h
index c7a1132cdc93..1bf5ecdf8e01 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -228,6 +228,16 @@ static inline const char *kbasename(const char *path)
 #define __RENAME(x) __asm__(#x)
 
 void fortify_panic(const char *name) __noreturn __cold;
+
+/* work around GCC PR82365 */
+#if defined(CONFIG_KASAN) && !defined(__clang__) && GCC_VERSION <= 80000
+#define fortify_panic(x) \
+	do { \
+		asm volatile(""); \
+		fortify_panic(x); \
+	} while (0)
+#endif
+
 void __read_overflow(void) __compiletime_error("detected read beyond size of object passed as 1st parameter");
 void __read_overflow2(void) __compiletime_error("detected read beyond size of object passed as 2nd parameter");
 void __read_overflow3(void) __compiletime_error("detected read beyond size of object passed as 3rd parameter");
-- 
2.9.0

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Re: [PATCH v4 4/9] em28xx: fix em28xx_dvb_init for KASAN Arnd Bergmann <arnd@arndb.de> - 2017-09-27 15:30 +0200
  Re: [PATCH v4 4/9] em28xx: fix em28xx_dvb_init for KASAN Arnd Bergmann <arnd@arndb.de> - 2017-09-28 16:40 +0200
    Re: [PATCH v4 4/9] em28xx: fix em28xx_dvb_init for KASAN Arnd Bergmann <arnd@arndb.de> - 2017-10-02 10:40 +0200
      [PATCH] string.h: work around for increased stack usage Arnd Bergmann <arnd@arndb.de> - 2017-10-02 10:50 +0200
        Re: [PATCH] string.h: work around for increased stack usage Arnd Bergmann <arnd@arndb.de> - 2017-10-02 11:10 +0200

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