From: Kees Cook <keescook@xxxxxxxxxxxx>
Subject: Compiler Attributes: add __alloc_size() for better bounds checking
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 overflowing (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). To deal with this, we must disable this check
as it is both a false positive and redundant. (Clang does not have this
warning option.)
Unfortunately, just checking the -Wno-alloc-size-larger-than is not
sufficient to make the __alloc_size attribute behave correctly under older
GCC versions. The attribute itself must be disabled in those situations
too, as there appears to be no way to reliably silence the SIZE_MAX
constant expression cases for GCC versions less than 9.1:
In file included from ./include/linux/resource_ext.h:11,
from ./include/linux/pci.h:40,
from drivers/net/ethernet/intel/ixgbe/ixgbe.h:9,
from drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c:4:
In function 'kmalloc_node',
inlined from 'ixgbe_alloc_q_vector' at ./include/linux/slab.h:743:9:
./include/linux/slab.h:618:9: error: argument 1 value '18446744073709551615' exceeds maximum object size 9223372036854775807 [-Werror=alloc-size-larger-than=]
return __kmalloc_node(size, flags, node);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/slab.h: In function 'ixgbe_alloc_q_vector':
./include/linux/slab.h:455:7: note: in a call to allocation function '__kmalloc_node' declared here
void *__kmalloc_node(size_t size, gfp_t flags, int node) __assume_slab_alignment __malloc;
^~~~~~~~~~~~~~
Specifically:
-Wno-alloc-size-larger-than is not correctly handled by GCC < 9.1
https://godbolt.org/z/hqsfG7q84 (doesn't disable)
https://godbolt.org/z/P9jdrPTYh (doesn't admit to not knowing about option)
https://godbolt.org/z/465TPMWKb (only warns when other warnings appear)
-Walloc-size-larger-than=18446744073709551615 is not handled by GCC < 8.2
https://godbolt.org/z/73hh1EPxz (ignores numeric value)
Since anything marked with __alloc_size would also qualify for marking
with __malloc, just include __malloc along with it to avoid redundant
markings. (Suggested by Linus Torvalds.)
Finally, make sure checkpatch.pl doesn't get confused about finding the
__alloc_size attribute on functions. (Thanks to Joe Perches.)
Link: https://lkml.kernel.org/r/20210930222704.2631604-3-keescook@xxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Tested-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Andy Whitcroft <apw@xxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Daniel Micay <danielmicay@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Dennis Zhou <dennis@xxxxxxxxxx>
Cc: Dwaipayan Ray <dwaipayanray1@xxxxxxxxx>
Cc: Joe Perches <joe@xxxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Lukas Bulwahn <lukas.bulwahn@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Alexandre Bounine <alex.bou9@xxxxxxxxx>
Cc: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: Jing Xiangfeng <jingxiangfeng@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: kernel test robot <lkp@xxxxxxxxx>
Cc: Matt Porter <mporter@xxxxxxxxxxxxxxxxxxx>
Cc: Miguel Ojeda <ojeda@xxxxxxxxxx>
Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Souptick Joarder <jrdr.linux@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Makefile | 15 +++++++++++++++
include/linux/compiler-gcc.h | 8 ++++++++
include/linux/compiler_attributes.h | 10 ++++++++++
include/linux/compiler_types.h | 12 ++++++++++++
scripts/checkpatch.pl | 3 ++-
5 files changed, 47 insertions(+), 1 deletion(-)
--- a/include/linux/compiler_attributes.h~compiler-attributes-add-__alloc_size-for-better-bounds-checking
+++ a/include/linux/compiler_attributes.h
@@ -34,6 +34,15 @@
#define __aligned_largest __attribute__((__aligned__))
/*
+ * Note: do not use this directly. Instead, use __alloc_size() since it is conditionally
+ * available and includes other attributes.
+ *
+ * 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
* to its docs (and also "warning: always_inline function might not be
@@ -153,6 +162,7 @@
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-malloc-function-attribute
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#malloc
*/
#define __malloc __attribute__((__malloc__))
--- a/include/linux/compiler-gcc.h~compiler-attributes-add-__alloc_size-for-better-bounds-checking
+++ a/include/linux/compiler-gcc.h
@@ -144,3 +144,11 @@
#else
#define __diag_GCC_8(s)
#endif
+
+/*
+ * Prior to 9.1, -Wno-alloc-size-larger-than (and therefore the "alloc_size"
+ * attribute) do not work, and must be disabled.
+ */
+#if GCC_VERSION < 90100
+#undef __alloc_size__
+#endif
--- a/include/linux/compiler_types.h~compiler-attributes-add-__alloc_size-for-better-bounds-checking
+++ a/include/linux/compiler_types.h
@@ -250,6 +250,18 @@ struct ftrace_likely_data {
# define __cficanonical
#endif
+/*
+ * Any place that could be marked with the "alloc_size" attribute is also
+ * a place to be marked with the "malloc" attribute. Do this as part of the
+ * __alloc_size macro to avoid redundant attributes and to avoid missing a
+ * __malloc marking.
+ */
+#ifdef __alloc_size__
+# define __alloc_size(x, ...) __alloc_size__(x, ## __VA_ARGS__) __malloc
+#else
+# define __alloc_size(x, ...) __malloc
+#endif
+
#ifndef asm_volatile_goto
#define asm_volatile_goto(x...) asm goto(x)
#endif
--- a/Makefile~compiler-attributes-add-__alloc_size-for-better-bounds-checking
+++ a/Makefile
@@ -1008,6 +1008,21 @@ 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. 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@xxxxxxxxxxxxxxxx
+KBUILD_CFLAGS += $(call cc-ifversion, -ge, 0901, -Wno-alloc-size-larger-than)
+endif
+
# disable invalid "can't wrap" optimizations for signed / pointers
KBUILD_CFLAGS += -fno-strict-overflow
--- a/scripts/checkpatch.pl~compiler-attributes-add-__alloc_size-for-better-bounds-checking
+++ a/scripts/checkpatch.pl
@@ -489,7 +489,8 @@ our $Attribute = qr{
____cacheline_aligned|
____cacheline_aligned_in_smp|
____cacheline_internodealigned_in_smp|
- __weak
+ __weak|
+ __alloc_size\s*\(\s*\d+\s*(?:,\s*\d+\s*)?\)
}x;
our $Modifier;
our $Inline = qr{inline|__always_inline|noinline|__inline|__inline__};
_
