fortify: Explicitly disable Clang support

Clang has never correctly compiled the FORTIFY_SOURCE defenses due to
a couple bugs:

	Eliding inlines with matching __builtin_* names
	https://bugs.llvm.org/show_bug.cgi?id=50322

	Incorrect __builtin_constant_p() of some globals
	https://bugs.llvm.org/show_bug.cgi?id=41459

In the process of making improvements to the FORTIFY_SOURCE defenses, the
first (silent) bug (coincidentally) becomes worked around, but exposes
the latter which breaks the build. As such, Clang must not be used with
CONFIG_FORTIFY_SOURCE until at least latter bug is fixed (in Clang 13),
and the fortify routines have been rearranged.

Update the Kconfig to reflect the reality of the current situation.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/lkml/CAKwvOd=A+ueGV2ihdy5GtgR2fQbcXjjAtVxv3=cPjffpebZB7A@mail.gmail.com

diff --git a/security/Kconfig b/security/Kconfig
index 0ced7fd33e4d0d4bc5acf2570f95ff88c3562d17..fe6c0395fa025115c73f09b911ff0b876f150fbe 100644 (file)
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -191,6 +191,9 @@ config HARDENED_USERCOPY_PAGESPAN
 config FORTIFY_SOURCE
        bool "Harden common str/mem functions against buffer overflows"
        depends on ARCH_HAS_FORTIFY_SOURCE
+       # https://bugs.llvm.org/show_bug.cgi?id=50322
+       # https://bugs.llvm.org/show_bug.cgi?id=41459
+       depends on !CC_IS_CLANG
        help
          Detect overflows of buffers in common string and memory functions
          where the compiler can determine and validate the buffer sizes.