Compiler Attributes: add __alloc_size() for better bounds checking

Patch series "Add __alloc_size() for better bounds checking", v2.

GCC and Clang both use the "alloc_size" attribute to assist with bounds
checking around the use of allocation functions.  Add the attribute,
adjust the Makefile to silence needless warnings, and add the hints to the
allocators where possible.  These changes have been in use for a while now
in GrapheneOS.

This patch (of 2):

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 inform the results of
__builtin_dynamic_object_size() (for run-time values).

Because GCC sees the frequent use of struct_size() as an allocator size
argument, and notices it can return SIZE_MAX (the overflow indication), it
complains about these call sites may overflow (since SIZE_MAX is greater
than the default -Walloc-size-larger-than=PTRDIFF_MAX).  This isn't
helpful since we already know a SIZE_MAX will be caught at run-time (this
was an intentional design).  Instead, just disable this check as it is
both a false positive and redundant.  (Clang does not have this warning
option.)

Link: https://lkml.kernel.org/r/20210818214021.2476230-1-keescook@chromium.org
Link: https://lkml.kernel.org/r/20210818214021.2476230-2-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

diff --git a/Makefile b/Makefile
index 476e3fb9feb3f1b74e25123858f2b5b39381d021..56151c396958025a59c8d6035b3d75e728f992f8 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -1097,6 +1097,12 @@ ifdef CONFIG_CC_IS_GCC
 KBUILD_CFLAGS += -Wno-maybe-uninitialized
 endif
 
+ifdef CONFIG_CC_IS_GCC
+# 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
 KBUILD_CFLAGS  += -fno-strict-overflow
 

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 67c5667f804278556f5bb94b1b46957e59c54c19..203b0ac62d1545f77596242d44d8afc4140f5643 100644 (file)
--- 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