diff options
author | Arnd Bergmann <[email protected]> | 2017-12-14 15:32:34 -0800 |
---|---|---|
committer | Linus Torvalds <[email protected]> | 2017-12-14 16:00:48 -0800 |
commit | 146734b091430c80d80bb96b1139a96fb4bc830e (patch) | |
tree | f115c5421f86214f39397efccdd12f1139a62f24 | |
parent | 13ab183d138f607d885e995d625e58d47678bf97 (diff) |
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.
[[email protected]: add comment]
Link: http://lkml.kernel.org/r/[email protected]
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 <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Daniel Micay <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Martin Wilck <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
-rw-r--r-- | include/linux/string.h | 5 |
1 files changed, 4 insertions, 1 deletions
diff --git a/include/linux/string.h b/include/linux/string.h index 410ecf17de3c..cfd83eb2f926 100644 --- a/include/linux/string.h +++ b/include/linux/string.h @@ -259,7 +259,10 @@ __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) + + /* 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) |