linux-kernel - [PATCH v2] kbuild: treat char as always unsigned

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Date:   Wed, 19 Oct 2022 14:30:34 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-toolchains@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH v2] kbuild: treat char as always unsigned

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 make assumptions about `char` with no `signed` or
`unsigned` specifier. 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 heisensign bugs
entirely. Set `-funsigned-char` globally, so that gcc makes the type
unsigned on all architectures.

This will break things in some places and fix things in others, so this
will likely cause a bit of churn while reconciling the type misuse.

Cc: Masahiro Yamada <masahiroy@...nel.org>
Cc: Kees Cook <keescook@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Link: https://lore.kernel.org/lkml/202210190108.ESC3pc3D-lkp@intel.com/
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index f41ec8c8426b..bbf376931899 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 -funsigned-char \
 		   -std=gnu11
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_RUSTFLAGS := $(rust_common_flags) \
-- 
2.38.1