On Tue, 5 Dec 2017 22:51:19 +0100 Arnd Bergmann arnd@arndb.de wrote:
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.
Cc: stable@vger.kernel.org
I'll add
Fixes: 6974f0c4555 ("include/linux/string.h: add the option of fortified string.h functions")
to simplify stable@'s life.
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/ Signed-off-by: Arnd Bergmann arnd@arndb.de
...
--- a/include/linux/string.h +++ b/include/linux/string.h @@ -259,7 +259,8 @@ __FORTIFY_INLINE __kernel_size_t strlen(const char *p) { __kernel_size_t ret; size_t p_size = __builtin_object_size(p, 0);
- if (p_size == (size_t)-1)
- if (p_size == (size_t)-1 ||
(__builtin_constant_p(p[p_size - 1]) && p[p_size - 1] == '\0'))
Let's have some sympathy for our poor readers?
--- a/include/linux/string.h~stringh-work-around-for-increased-stack-usage-fix +++ a/include/linux/string.h @@ -259,6 +259,8 @@ __FORTIFY_INLINE __kernel_size_t strlen( { __kernel_size_t ret; size_t p_size = __builtin_object_size(p, 0); + + /* 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); _