On 8/17/2021 10:08 PM, Kees Cook wrote:
GCC and Clang can use the alloc_size attribute to better inform the
results of __builtin_object_size() (for compile-time constant values).
Clang can additionally use alloc_size to informt the results of
__builtin_dynamic_object_size() (for run-time values).
Additionally disables -Wno-alloc-size-larger-than since the allocators
already reject SIZE_MAX, and the compile-time warnings aren't helpful.
In addition to what Miguel said, it might be helpful to mention that
this warning is GCC specific, I was a little confused at first as to why
it was just being added in the GCC only block :)
Otherwise, the attribute addition looks good to me. I will add my tag on v2.
Cc: Miguel Ojeda <ojeda@xxxxxxxxxx>
Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: clang-built-linux@xxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Makefile | 6 +++++-
include/linux/compiler_attributes.h | 6 ++++++
2 files changed, 11 insertions(+), 1 deletion(-)
diff --git a/Makefile b/Makefile
index 1b238ce86ed4..3b6fb740584e 100644
--- a/Makefile
+++ b/Makefile
@@ -1076,9 +1076,13 @@ KBUILD_CFLAGS += $(call cc-disable-warning, stringop-overflow)
# Another good warning that we'll want to enable eventually
KBUILD_CFLAGS += $(call cc-disable-warning, restrict)
-# Enabled with W=2, disabled by default as noisy
ifdef CONFIG_CC_IS_GCC
+# Enabled with W=2, disabled by default as noisy
KBUILD_CFLAGS += -Wno-maybe-uninitialized
+
+# The allocators already balk at large sizes, so silence the compiler
+# warnings for bounds checks involving those possible values.
+KBUILD_CFLAGS += -Wno-alloc-size-larger-than
endif
# disable invalid "can't wrap" optimizations for signed / pointers
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 67c5667f8042..203b0ac62d15 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -54,6 +54,12 @@
#define __aligned(x) __attribute__((__aligned__(x)))
#define __aligned_largest __attribute__((__aligned__))
+/*
+ * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alloc_005fsize-function-attribute
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#alloc-size
+ */
+#define __alloc_size(x, ...) __attribute__((__alloc_size__(x, ## __VA_ARGS__)))
+
/*
* Note: users of __always_inline currently do not write "inline" themselves,
* which seems to be required by gcc to apply the attribute according
