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Date:   Fri, 10 Sep 2021 13:11:32 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>, Miguel Ojeda <ojeda@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Marco Elver <elver@...gle.com>, Will Deacon <will@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ardb@...nel.org>, llvm@...ts.linux.dev,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v2] Compiler Attributes: Check GCC version for __alloc_size attribute

Unfortunately, just version checking the use of -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)

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Miguel Ojeda <ojeda@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: Marco Elver <elver@...gle.com>
Cc: Will Deacon <will@...nel.org>
Cc: Arvind Sankar <nivedita@...m.mit.edu>
Cc: Masahiro Yamada <masahiroy@...nel.org>
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
Cc: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Ard Biesheuvel <ardb@...nel.org>
Cc: llvm@...ts.linux.dev
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
And now I've added GCC 4.9, 5, and 6 to my build configs... 4.9 is so
old I have to turn off RETPOLIN in defconfig. :P

v2:
- fix lack of VARARGS in fall-back macro
- add reviewed-by
---
 include/linux/compiler-gcc.h        | 9 +++++++++
 include/linux/compiler_attributes.h | 6 ------
 include/linux/compiler_types.h      | 5 +++++
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 01985821944b..3f3efbb651e0 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -151,3 +151,12 @@
 #else
 #define __diag_GCC_8(s)
 #endif
+
+/*
+ * https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alloc_005fsize-function-attribute
+ * However, prior to 9.1, -Wno-alloc-size-larger-than does not work,
+ * making this attribute unusable.
+ */
+#if GCC_VERSION < 90100
+#define __alloc_size(x, ...)	/**/
+#endif
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 19f178e20e61..2487be0e7199 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -53,12 +53,6 @@
 #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
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index c43308b0a9a9..91de9bead40d 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -254,6 +254,11 @@ struct ftrace_likely_data {
 #define asm_volatile_goto(x...) asm goto(x)
 #endif
 
+/* If not specifically disabled, allow the use of __alloc_size attribute. */
+#ifndef __alloc_size
+# define __alloc_size(x, ...)		__attribute__((__alloc_size__(x, ## __VA_ARGS__)))
+#endif
+
 #ifdef CONFIG_CC_HAS_ASM_INLINE
 #define asm_inline asm __inline
 #else
-- 
2.30.2

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