string.h: workaround for increased stack usage

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]>