[PATCH] kbuild: treat char as always signed

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Recently, some compile-time checking I added to the clamp_t family of
functions triggered a build error when a poorly written driver was
compiled on ARM, because the driver assumed that the naked `char` type
is signed, but ARM treats it as unsigned, and the C standard says it's
architecture-dependent.

I doubt this particular driver is the only instance in which
unsuspecting authors assume that `char` with no `signed` or `unsigned`
designation is signed, because that's how the other types work. We were
lucky enough this time that that driver used `clamp_t(char,
negative_value, positive_value)`, so the new checking code found it, and
I've sent a patch to fix it, but there are likely other places lurking
that won't be so easily unearthed.

So let's just eliminate this particular variety of heisensigned bugs
entirely. Set `-fsigned-char` globally, so that gcc makes the type
signed on all architectures.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/lkml/202210190108.ESC3pc3D-lkp@intel.com/
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index f41ec8c8426b..f1abcaf7110e 100644
--- a/Makefile
+++ b/Makefile
@@ -562,7 +562,7 @@ KBUILD_AFLAGS   := -D__ASSEMBLY__ -fno-PIE
 KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
 		   -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE \
 		   -Werror=implicit-function-declaration -Werror=implicit-int \
-		   -Werror=return-type -Wno-format-security \
+		   -Werror=return-type -Wno-format-security -fsigned-char \
 		   -std=gnu11
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_RUSTFLAGS := $(rust_common_flags) \
-- 
2.38.1