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f0907827a8
This adds wrappers for the __builtin overflow checkers present in gcc
5.1+ as well as fallback implementations for earlier compilers. It's not
that easy to implement the fully generic __builtin_X_overflow(T1 a, T2
b, T3 *d) in macros, so the fallback code assumes that T1, T2 and T3 are
the same. We obviously don't want the wrappers to have different
semantics depending on $GCC_VERSION, so we also insist on that even when
using the builtins.
There are a few problems with the 'a+b < a' idiom for checking for
overflow: For signed types, it relies on undefined behaviour and is
not actually complete (it doesn't check underflow;
e.g. INT_MIN+INT_MIN == 0 isn't caught). Due to type promotion it
is wrong for all types (signed and unsigned) narrower than
int. Similarly, when a and b does not have the same type, there are
subtle cases like
u32 a;
if (a + sizeof(foo) < a)
return -EOVERFLOW;
a += sizeof(foo);
where the test is always false on 64 bit platforms. Add to that that it
is not always possible to determine the types involved at a glance.
The new overflow.h is somewhat bulky, but that's mostly a result of
trying to be type-generic, complete (e.g. catching not only overflow
but also signed underflow) and not relying on undefined behaviour.
Linus is of course right [1] that for unsigned subtraction a-b, the
right way to check for overflow (underflow) is "b > a" and not
"__builtin_sub_overflow(a, b, &d)", but that's just one out of six cases
covered here, and included mostly for completeness.
So is it worth it? I think it is, if nothing else for the documentation
value of seeing
if (check_add_overflow(a, b, &d))
return -EGOAWAY;
do_stuff_with(d);
instead of the open-coded (and possibly wrong and/or incomplete and/or
UBsan-tickling)
if (a+b < a)
return -EGOAWAY;
do_stuff_with(a+b);
While gcc does recognize the 'a+b < a' idiom for testing unsigned add
overflow, it doesn't do nearly as good for unsigned multiplication
(there's also no single well-established idiom). So using
check_mul_overflow in kcalloc and friends may also make gcc generate
slightly better code.
[1] https://lkml.org/lkml/2015/11/2/658
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Kees Cook <keescook@chromium.org>
51 lines
1.3 KiB
C
51 lines
1.3 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __LINUX_COMPILER_TYPES_H
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#error "Please don't include <linux/compiler-intel.h> directly, include <linux/compiler.h> instead."
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#endif
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#ifdef __ECC
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/* Some compiler specific definitions are overwritten here
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* for Intel ECC compiler
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*/
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#include <asm/intrinsics.h>
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/* Intel ECC compiler doesn't support gcc specific asm stmts.
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* It uses intrinsics to do the equivalent things.
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*/
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#undef barrier
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#undef barrier_data
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#undef RELOC_HIDE
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#undef OPTIMIZER_HIDE_VAR
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#define barrier() __memory_barrier()
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#define barrier_data(ptr) barrier()
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#define RELOC_HIDE(ptr, off) \
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({ unsigned long __ptr; \
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__ptr = (unsigned long) (ptr); \
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(typeof(ptr)) (__ptr + (off)); })
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/* This should act as an optimization barrier on var.
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* Given that this compiler does not have inline assembly, a compiler barrier
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* is the best we can do.
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*/
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#define OPTIMIZER_HIDE_VAR(var) barrier()
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/* Intel ECC compiler doesn't support __builtin_types_compatible_p() */
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#define __must_be_array(a) 0
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#endif
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#ifndef __HAVE_BUILTIN_BSWAP16__
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/* icc has this, but it's called _bswap16 */
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#define __HAVE_BUILTIN_BSWAP16__
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#define __builtin_bswap16 _bswap16
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#endif
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/*
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* icc defines __GNUC__, but does not implement the builtin overflow checkers.
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*/
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#undef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
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