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660e899103e29aabcc05447ab20684811ea86107
linux/scripts/Makefile.warn
Vincent Mailhol 660e899103 kbuild: remove gcc's -Wtype-limits 
W=2 builds are heavily polluted by the -Wtype-limits warning.

Here are some W=12 statistics on Linux v6.19-rc1 for an x86_64
defconfig (with just CONFIG_WERROR set to "n") using gcc 14.3.1:

	 Warning name			count	percent
	-------------------------------------------------
	 -Wlogical-op			    2	  0.00 %
	 -Wmaybe-uninitialized		  138	  0.20 %
	 -Wunused-macros		  869	  1.24 %
	 -Wmissing-field-initializers	 1418	  2.02 %
	 -Wshadow			 2234	  3.19 %
	 -Wtype-limits			65378	 93.35 %
	-------------------------------------------------
	 Total				70039	100.00 %

As we can see, -Wtype-limits represents the vast majority of all
warnings. The reason behind this is that these warnings appear in
some common header files, meaning that some unique warnings are
repeated tens of thousands of times (once per header inclusion).

Add to this the fact that each warning is coupled with a dozen lines
detailing some macro expansion. The end result is that the W=2 output
is just too bloated and painful to use.

Three years ago, I proposed in [1] modifying one such header to
silence that noise. Because the code was not faulty, Linus rejected
the idea and instead suggested simply removing that warning.

At that time, I could not bring myself to send such a patch because,
despite its problems, -Wtype-limits would still catch the below bug:

	unsigned int ret;

	ret = check();
	if (ret < 0)
		error();

Meanwhile, based on another suggestion from Linus, I added a new check
to sparse [2] that would catch the above bug without the useless spam.

With this, remove gcc's -Wtype-limits. People who still want to catch
incorrect comparisons between unsigned integers and zero can now use
sparse instead.

On a side note, clang also has a -Wtype-limits warning but:

  * it is not enabled in the kernel at the moment because, contrary to
    gcc, clang did not include it under -Wextra.

  * it does not warn if the code results from a macro expansion. So,
    if activated, it would not cause as much spam as gcc does.

  * -Wtype-limits is split into four sub-warnings [3] meaning that if
    it were to be activated, we could select which one to keep.

So there is no present need to explicitly disable -Wtype-limits in
clang.

[1] linux/bits.h: GENMASK_INPUT_CHECK: reduce W=2 noise by 31% treewide
Link: https://lore.kernel.org/all/20220308141201.2343757-1-mailhol.vincent@wanadoo.fr/

[2] Warn about "unsigned value that used to be signed against zero"
Link: https://lore.kernel.org/all/20250921061337.3047616-1-mailhol@kernel.org/

[3] clang's -Wtype-limits
Link: https://clang.llvm.org/docs/DiagnosticsReference.html#wtype-limits

Reviewed-by: Nicolas Schier <nsc@kernel.org>
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20251220-remove_wtype-limits-v3-1-24b170af700e@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
2026-01-05 16:54:14 -07:00

238 lines
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Makefile
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# SPDX-License-Identifier: GPL-2.0
# ==========================================================================
# make W=... settings
#
# There are four warning groups enabled by W=1, W=2, W=3, and W=e
# They are independent, and can be combined like W=12 or W=123e.
# ==========================================================================
# Default set of warnings, always enabled
KBUILD_CFLAGS += -Wall
KBUILD_CFLAGS += -Wextra
KBUILD_CFLAGS += -Wundef
KBUILD_CFLAGS += -Werror=implicit-function-declaration
KBUILD_CFLAGS += -Werror=implicit-int
KBUILD_CFLAGS += -Werror=return-type
KBUILD_CFLAGS += -Werror=strict-prototypes
KBUILD_CFLAGS += -Wno-format-security
KBUILD_CFLAGS += -Wno-trigraphs
KBUILD_CFLAGS += $(call cc-option, -Wno-frame-address)
KBUILD_CFLAGS += $(call cc-option, -Wno-address-of-packed-member)
KBUILD_CFLAGS += -Wmissing-declarations
KBUILD_CFLAGS += -Wmissing-prototypes
ifneq ($(CONFIG_FRAME_WARN),0)
KBUILD_CFLAGS += -Wframe-larger-than=$(CONFIG_FRAME_WARN)
endif
KBUILD_CFLAGS-$(CONFIG_CC_NO_ARRAY_BOUNDS) += -Wno-array-bounds
ifdef CONFIG_CC_IS_CLANG
# The kernel builds with '-std=gnu11' and '-fms-extensions' so use of GNU and
# Microsoft extensions is acceptable.
KBUILD_CFLAGS += -Wno-gnu
KBUILD_CFLAGS += -Wno-microsoft-anon-tag
# Clang checks for overflow/truncation with '%p', while GCC does not:
# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
KBUILD_CFLAGS += $(call cc-option, -Wno-format-overflow-non-kprintf)
KBUILD_CFLAGS += $(call cc-option, -Wno-format-truncation-non-kprintf)
# Clang may emit a warning when a const variable, such as the dummy variables
# in typecheck(), or const member of an aggregate type are not initialized,
# which can result in unexpected behavior. However, in many audited cases of
# the "field" variant of the warning, this is intentional because the field is
# never used within a particular call path, the field is within a union with
# other non-const members, or the containing object is not const so the field
# can be modified via memcpy() / memset(). While the variable warning also gets
# disabled with this same switch, there should not be too much coverage lost
# because -Wuninitialized will still flag when an uninitialized const variable
# is used.
KBUILD_CFLAGS += $(call cc-option, -Wno-default-const-init-unsafe)
else
# gcc inanely warns about local variables called 'main'
KBUILD_CFLAGS += -Wno-main
endif
# Too noisy on range checks and in macros handling both signed and unsigned.
KBUILD_CFLAGS += -Wno-type-limits
# These result in bogus false positives
KBUILD_CFLAGS += $(call cc-option, -Wno-dangling-pointer)
# Stack Variable Length Arrays (VLAs) must not be used in the kernel.
# Function array parameters should, however, be usable, but -Wvla will
# warn for those. Clang has no way yet to distinguish between the VLA
# types, so depend on GCC for now to keep stack VLAs out of the tree.
# https://github.com/llvm/llvm-project/issues/57098
# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98217
KBUILD_CFLAGS += $(call cc-option,-Wvla-larger-than=1)
# disable pointer signed / unsigned warnings in gcc 4.0
KBUILD_CFLAGS += -Wno-pointer-sign
# In order to make sure new function cast mismatches are not introduced
# in the kernel (to avoid tripping CFI checking), the kernel should be
# globally built with -Wcast-function-type.
KBUILD_CFLAGS += $(call cc-option, -Wcast-function-type)
# Currently, disable -Wstringop-overflow for GCC 11, globally.
KBUILD_CFLAGS-$(CONFIG_CC_NO_STRINGOP_OVERFLOW) += $(call cc-option, -Wno-stringop-overflow)
KBUILD_CFLAGS-$(CONFIG_CC_STRINGOP_OVERFLOW) += $(call cc-option, -Wstringop-overflow)
# Currently, disable -Wunterminated-string-initialization as broken
KBUILD_CFLAGS += $(call cc-option, -Wno-unterminated-string-initialization)
# The allocators already balk at large sizes, so silence the compiler
# warnings for bounds checks involving those possible values. While
# -Wno-alloc-size-larger-than would normally be used here, earlier versions
# of gcc (<9.1) weirdly don't handle the option correctly when _other_
# warnings are produced (?!). Using -Walloc-size-larger-than=SIZE_MAX
# doesn't work (as it is documented to), silently resolving to "0" prior to
# version 9.1 (and producing an error more recently). Numeric values larger
# than PTRDIFF_MAX also don't work prior to version 9.1, which are silently
# ignored, continuing to default to PTRDIFF_MAX. So, left with no other
# choice, we must perform a versioned check to disable this warning.
# https://lore.kernel.org/lkml/20210824115859.187f272f@canb.auug.org.au
KBUILD_CFLAGS-$(call gcc-min-version, 90100) += -Wno-alloc-size-larger-than
KBUILD_CFLAGS += $(KBUILD_CFLAGS-y) $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
# Prohibit date/time macros, which would make the build non-deterministic
KBUILD_CFLAGS += -Werror=date-time
# enforce correct pointer usage
KBUILD_CFLAGS += $(call cc-option,-Werror=incompatible-pointer-types)
# Require designated initializers for all marked structures
KBUILD_CFLAGS += $(call cc-option,-Werror=designated-init)
# Warn if there is an enum types mismatch
KBUILD_CFLAGS += $(call cc-option,-Wenum-conversion)
KBUILD_CFLAGS += -Wunused
#
# W=1 - warnings which may be relevant and do not occur too often
#
ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),)
KBUILD_CFLAGS += -Wmissing-format-attribute
KBUILD_CFLAGS += -Wmissing-include-dirs
KBUILD_CFLAGS += $(call cc-option, -Wunused-const-variable)
KBUILD_CPPFLAGS += -Wundef
KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN1
else
# Some diagnostics enabled by default are noisy.
# Suppress them by using -Wno... except for W=1.
KBUILD_CFLAGS += $(call cc-option, -Wno-unused-but-set-variable)
KBUILD_CFLAGS += $(call cc-option, -Wno-unused-const-variable)
KBUILD_CFLAGS += $(call cc-option, -Wno-packed-not-aligned)
KBUILD_CFLAGS += $(call cc-option, -Wno-format-overflow)
ifdef CONFIG_CC_IS_GCC
KBUILD_CFLAGS += $(call cc-option, -Wno-format-truncation)
endif
KBUILD_CFLAGS += $(call cc-option, -Wno-stringop-truncation)
KBUILD_CFLAGS += -Wno-override-init # alias for -Wno-initializer-overrides in clang
ifdef CONFIG_CC_IS_CLANG
# Clang before clang-16 would warn on default argument promotions.
ifneq ($(call clang-min-version, 160000),y)
# Disable -Wformat
KBUILD_CFLAGS += -Wno-format
# Then re-enable flags that were part of the -Wformat group that aren't
# problematic.
KBUILD_CFLAGS += -Wformat-extra-args -Wformat-invalid-specifier
KBUILD_CFLAGS += -Wformat-zero-length -Wnonnull
# Requires clang-12+.
ifeq ($(call clang-min-version, 120000),y)
KBUILD_CFLAGS += -Wformat-insufficient-args
endif
endif
KBUILD_CFLAGS += $(call cc-option, -Wno-pointer-to-enum-cast)
KBUILD_CFLAGS += -Wno-tautological-constant-out-of-range-compare
KBUILD_CFLAGS += $(call cc-option, -Wno-unaligned-access)
KBUILD_CFLAGS += -Wno-enum-compare-conditional
endif
endif
#
# W=2 - warnings which occur quite often but may still be relevant
#
ifneq ($(findstring 2, $(KBUILD_EXTRA_WARN)),)
KBUILD_CFLAGS += -Wdisabled-optimization
KBUILD_CFLAGS += -Wshadow
KBUILD_CFLAGS += $(call cc-option, -Wlogical-op)
KBUILD_CFLAGS += $(call cc-option, -Wunused-macros)
KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN2
else
# The following turn off the warnings enabled by -Wextra
KBUILD_CFLAGS += -Wno-missing-field-initializers
KBUILD_CFLAGS += -Wno-shift-negative-value
ifdef CONFIG_CC_IS_CLANG
KBUILD_CFLAGS += -Wno-enum-enum-conversion
endif
ifdef CONFIG_CC_IS_GCC
KBUILD_CFLAGS += -Wno-maybe-uninitialized
endif
endif
#
# W=3 - more obscure warnings, can most likely be ignored
#
ifneq ($(findstring 3, $(KBUILD_EXTRA_WARN)),)
KBUILD_CFLAGS += -Wbad-function-cast
KBUILD_CFLAGS += -Wcast-align
KBUILD_CFLAGS += -Wcast-qual
KBUILD_CFLAGS += -Wconversion
KBUILD_CFLAGS += -Wpacked
KBUILD_CFLAGS += -Wpadded
KBUILD_CFLAGS += -Wpointer-arith
KBUILD_CFLAGS += -Wredundant-decls
KBUILD_CFLAGS += -Wsign-compare
KBUILD_CFLAGS += -Wswitch-default
KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN3
else
# The following turn off the warnings enabled by -Wextra
KBUILD_CFLAGS += -Wno-sign-compare
KBUILD_CFLAGS += -Wno-unused-parameter
endif
#
# W=e and CONFIG_WERROR - error out on warnings
#
ifneq ($(findstring e, $(KBUILD_EXTRA_WARN))$(CONFIG_WERROR),)
KBUILD_CPPFLAGS += -Werror
KBUILD_AFLAGS += -Wa,--fatal-warnings
KBUILD_LDFLAGS += --fatal-warnings
KBUILD_USERCFLAGS += -Werror
KBUILD_USERLDFLAGS += -Wl,--fatal-warnings
KBUILD_RUSTFLAGS += -Dwarnings
# While hostprog flags are used during build bootstrapping (thus should not
# depend on CONFIG_ symbols), -Werror is disruptive and should be opted into.
# Only apply -Werror to hostprogs built after the initial Kconfig stage.
KBUILD_HOSTCFLAGS += -Werror
KBUILD_HOSTLDFLAGS += -Wl,--fatal-warnings
KBUILD_HOSTRUSTFLAGS += -Dwarnings
endif
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