From: Arnd Bergmann <arnd@xxxxxxxx> Subject: string.h: workaround for increased stack usage The hardened strlen() function causes rather large stack usage in at least one file in the kernel, in particular 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. I reported this as a gcc bug, but it is unlikely to get fixed for gcc-8, since it is relatively easy to work around, and it gets triggered rarely. An earlier workaround I did added an empty inline assembly statement before the call to fortify_panic(), which works surprisingly well, but is really ugly and unintuitive. This is a new approach to the same problem, this time addressing it by not calling the 'extern __real_strnlen()' function for string constants where __builtin_strlen() is a compile-time constant and therefore known to be safe. We do this by checking if the last character in the string is a compile-time constant '\0'. If it is, we can assume that strlen() of the string is also constant. As a side-effect, this should also improve the object code output for any other call of strlen() on a string constant. [akpm@xxxxxxxxxxxxxxxxxxxx: add comment] Link: http://lkml.kernel.org/r/20171205215143.3085755-1-arnd@xxxxxxxx Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365 Link: https://patchwork.kernel.org/patch/9980413/ Link: https://patchwork.kernel.org/patch/9974047/ Fixes: 6974f0c4555 ("include/linux/string.h: add the option of fortified string.h functions") Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Cc: Alexander Potapenko <glider@xxxxxxxxxx> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> Cc: Daniel Micay <danielmicay@xxxxxxxxx> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Cc: Martin Wilck <mwilck@xxxxxxxx> Cc: Dan Williams <dan.j.williams@xxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/string.h | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff -puN include/linux/string.h~stringh-work-around-for-increased-stack-usage include/linux/string.h --- a/include/linux/string.h~stringh-work-around-for-increased-stack-usage +++ a/include/linux/string.h @@ -259,7 +259,10 @@ __FORTIFY_INLINE __kernel_size_t strlen( { __kernel_size_t ret; size_t p_size = __builtin_object_size(p, 0); - if (p_size == (size_t)-1) + + /* Work around gcc excess stack consumption issue */ + if (p_size == (size_t)-1 || + (__builtin_constant_p(p[p_size - 1]) && p[p_size - 1] == '\0')) return __builtin_strlen(p); ret = strnlen(p, p_size); if (p_size <= ret) _