mainline inclusion
from mainline-5.1
commit 5f074f3e192f
category: bugfix
bugzilla: 13649
CVE: NA

-------------------------------------------------

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.comSigned-off-by: NNick Desaulniers <ndesaulniers@google.com>
Reported-by: NNathan Chancellor <natechancellor@gmail.com>
Reported-by: NAdhemerval Zanella <adhemerval.zanella@linaro.org>
Suggested-by: NArnd Bergmann <arnd@arndb.de>
Suggested-by: NJames Y Knight <jyknight@google.com>
Suggested-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Suggested-by: NNathan Chancellor <natechancellor@gmail.com>
Suggested-by: NRasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: NNathan Chancellor <natechancellor@gmail.com>
Tested-by: NNathan Chancellor <natechancellor@gmail.com>
Reviewed-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NAndy 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: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 5f074f3e192f10c9fade898b9b3b8812e3d83342)
Signed-off-by: NXie XiuQi <xiexiuqi@huawei.com>
Reviewed-by: NYang Yingliang <yangyingliang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>