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

[Andrey Ryabinin]

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@xxxxxxxx>
> ---
>  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");
>