string.h: work around for increased stack usage
diff mbox

Message ID 20171002084119.3504771-1-arnd@arndb.de
State New
Headers show

Commit Message

Arnd Bergmann Oct. 2, 2017, 8:40 a.m. UTC
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(+)

Comments

Arnd Bergmann Oct. 2, 2017, 9:02 a.m. UTC | #1
On Mon, Oct 2, 2017 at 10:40 AM, Arnd Bergmann <arnd@arndb.de> wrote:
>
>  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

This broke the build for the fortify_panic() definition in lib/string.c which
clashes with the macro. I've fixed it locally by renaming it to __fortify_panic,
but won't post the fixed version until I get some feedback on the basic
approach.

      Arnd
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Andrey Ryabinin Oct. 2, 2017, 2:07 p.m. UTC | #2
On 10/02/2017 11:40 AM, Arnd Bergmann wrote:
> 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.
> 

I don't have a really strong preference, so this approach is fine by me,
but s/strlcpy/[strncpy|memcpy] approach seems a little better to me, because it's not ugly.

This ugly workaround would make more sense if we a had lot of cases like in em28xx_dvb_init().


> 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");
> 
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kbuild test robot Oct. 3, 2017, 6:10 p.m. UTC | #3
Hi Arnd,

[auto build test ERROR on linus/master]
[also build test ERROR on v4.14-rc3 next-20170929]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Arnd-Bergmann/string-h-work-around-for-increased-stack-usage/20171003-210611
config: x86_64-randconfig-ws0-10040032 (attached as .config)
compiler: gcc-4.8 (Debian 4.8.4-1) 4.8.4
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All error/warnings (new ones prefixed by >>):

   In file included from lib/string.c:23:0:
>> include/linux/string.h:235:2: error: expected identifier or '(' before 'do'
     do { \
     ^
>> lib/string.c:1048:6: note: in expansion of macro 'fortify_panic'
    void fortify_panic(const char *name)
         ^
>> include/linux/string.h:238:4: error: expected identifier or '(' before 'while'
     } while (0)
       ^
>> lib/string.c:1048:6: note: in expansion of macro 'fortify_panic'
    void fortify_panic(const char *name)
         ^

vim +235 include/linux/string.h

   231	
   232	/* work around GCC PR82365 */
   233	#if defined(CONFIG_KASAN) && !defined(__clang__) && GCC_VERSION <= 80000
   234	#define fortify_panic(x) \
 > 235		do { \
   236			asm volatile(""); \
   237			fortify_panic(x); \
 > 238		} while (0)
   239	#endif
   240	

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

Patch
diff mbox

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");