| From c5a0d4dcc5553fedc22c81449f687950cd301197 Mon Sep 17 00:00:00 2001 |
| From: Nick Desaulniers <ndesaulniers@google.com> |
| Date: Fri, 5 Apr 2019 18:38:45 -0700 |
| Subject: lib/string.c: implement a basic bcmp |
| |
| [ Upstream commit 5f074f3e192f10c9fade898b9b3b8812e3d83342 ] |
| |
| A recent optimization in Clang (r355672) lowers comparisons of the |
| return value of memcmp against zero to comparisons of the return value |
| of bcmp against zero. This helps some platforms that implement bcmp |
| more efficiently than memcmp. glibc simply aliases bcmp to memcmp, but |
| an optimized implementation is in the works. |
| |
| This results in linkage failures for all targets with Clang due to the |
| undefined symbol. For now, just implement bcmp as a tailcail to memcmp |
| to unbreak the build. This routine can be further optimized in the |
| future. |
| |
| Other ideas discussed: |
| |
| * A weak alias was discussed, but breaks for architectures that define |
| their own implementations of memcmp since aliases to declarations are |
| not permitted (only definitions). Arch-specific memcmp |
| implementations typically declare memcmp in C headers, but implement |
| them in assembly. |
| |
| * -ffreestanding also is used sporadically throughout the kernel. |
| |
| * -fno-builtin-bcmp doesn't work when doing LTO. |
| |
| Link: https://bugs.llvm.org/show_bug.cgi?id=41035 |
| Link: https://code.woboq.org/userspace/glibc/string/memcmp.c.html#bcmp |
| Link: https://github.com/llvm/llvm-project/commit/8e16d73346f8091461319a7dfc4ddd18eedcff13 |
| Link: https://github.com/ClangBuiltLinux/linux/issues/416 |
| Link: http://lkml.kernel.org/r/20190313211335.165605-1-ndesaulniers@google.com |
| Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> |
| Reported-by: Nathan Chancellor <natechancellor@gmail.com> |
| Reported-by: Adhemerval Zanella <adhemerval.zanella@linaro.org> |
| Suggested-by: Arnd Bergmann <arnd@arndb.de> |
| Suggested-by: James Y Knight <jyknight@google.com> |
| Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com> |
| Suggested-by: Nathan Chancellor <natechancellor@gmail.com> |
| Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> |
| Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org> |
| Reviewed-by: Nathan Chancellor <natechancellor@gmail.com> |
| Tested-by: Nathan Chancellor <natechancellor@gmail.com> |
| Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com> |
| Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com> |
| Cc: David Laight <David.Laight@ACULAB.COM> |
| Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk> |
| Cc: Namhyung Kim <namhyung@kernel.org> |
| Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> |
| Cc: Dan Williams <dan.j.williams@intel.com> |
| Cc: <stable@vger.kernel.org> |
| Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
| Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
| Signed-off-by: Sasha Levin <sashal@kernel.org> |
| --- |
| include/linux/string.h | 3 +++ |
| lib/string.c | 20 ++++++++++++++++++++ |
| 2 files changed, 23 insertions(+) |
| |
| diff --git a/include/linux/string.h b/include/linux/string.h |
| index 7927b875f80c..6ab0a6fa512e 100644 |
| --- a/include/linux/string.h |
| +++ b/include/linux/string.h |
| @@ -150,6 +150,9 @@ extern void * memscan(void *,int,__kernel_size_t); |
| #ifndef __HAVE_ARCH_MEMCMP |
| extern int memcmp(const void *,const void *,__kernel_size_t); |
| #endif |
| +#ifndef __HAVE_ARCH_BCMP |
| +extern int bcmp(const void *,const void *,__kernel_size_t); |
| +#endif |
| #ifndef __HAVE_ARCH_MEMCHR |
| extern void * memchr(const void *,int,__kernel_size_t); |
| #endif |
| diff --git a/lib/string.c b/lib/string.c |
| index 38e4ca08e757..3ab861c1a857 100644 |
| --- a/lib/string.c |
| +++ b/lib/string.c |
| @@ -866,6 +866,26 @@ __visible int memcmp(const void *cs, const void *ct, size_t count) |
| EXPORT_SYMBOL(memcmp); |
| #endif |
| |
| +#ifndef __HAVE_ARCH_BCMP |
| +/** |
| + * bcmp - returns 0 if and only if the buffers have identical contents. |
| + * @a: pointer to first buffer. |
| + * @b: pointer to second buffer. |
| + * @len: size of buffers. |
| + * |
| + * The sign or magnitude of a non-zero return value has no particular |
| + * meaning, and architectures may implement their own more efficient bcmp(). So |
| + * while this particular implementation is a simple (tail) call to memcmp, do |
| + * not rely on anything but whether the return value is zero or non-zero. |
| + */ |
| +#undef bcmp |
| +int bcmp(const void *a, const void *b, size_t len) |
| +{ |
| + return memcmp(a, b, len); |
| +} |
| +EXPORT_SYMBOL(bcmp); |
| +#endif |
| + |
| #ifndef __HAVE_ARCH_MEMSCAN |
| /** |
| * memscan - Find a character in an area of memory. |
| -- |
| 2.19.1 |
| |