On Tue, 5 Dec 2017 22:51:19 +0100 Arnd Bergmann <arnd@xxxxxxxx> 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@xxxxxxxxxxxxxxx
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@xxxxxxxx>
>
> ...
>
> --- 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'))
> return __builtin_strlen(p);
> ret = strnlen(p, p_size);
> if (p_size <= ret)
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);
_