The patch titled
Subject: string.h: workaround for increased stack usage
has been added to the -mm tree. Its filename is
stringh-work-around-for-increased-stack-usage.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/stringh-work-around-for-increased-stack-usage.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/stringh-work-around-for-increased-stack-usage.patch
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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.
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 | 3 ++-
1 file changed, 2 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 +259,8 @@ __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)
+ 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)
_
Patches currently in -mm which might be from arnd@xxxxxxxx are
stringh-work-around-for-increased-stack-usage.patch
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