linux-kernel - [PATCH] lib/string.c: Disable tree-loop-distribute-patterns

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Message-Id: <20200818234307.3382306-1-nivedita@alum.mit.edu>
Date:   Tue, 18 Aug 2020 19:43:07 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH] lib/string.c: Disable tree-loop-distribute-patterns

gcc can transform the loop in a naive implementation of memset/memcpy
etc into a call to the function itself. This optimization is enabled by
-ftree-loop-distribute-patterns.

This has been the case for a while (see eg [0]), but gcc-10.x enables
this option at -O2 rather than -O3 as in previous versions.

Add -ffreestanding, which implicitly disables this optimization with
gcc. It is unclear whether clang performs such optimizations, but
hopefully it will also not do so in a freestanding environment.

This by itself is insufficient for gcc if the optimization was
explicitly enabled by CFLAGS, so also add a flag to explicitly disable
it.

[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56888

Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
---
 lib/Makefile | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/Makefile b/lib/Makefile
index e290fc5707ea..80edea49613f 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -15,11 +15,18 @@ KCOV_INSTRUMENT_debugobjects.o := n
 KCOV_INSTRUMENT_dynamic_debug.o := n
 KCOV_INSTRUMENT_fault-inject.o := n
 
+# string.o implements standard library functions like memset/memcpy etc.
+# Use -ffreestanding to ensure that the compiler does not try to "optimize"
+# them into calls to themselves.
+# The optimization pass that does such transformations in gcc is
+# tree-loop-distribute-patterns. Explicitly disable it just in case.
+CFLAGS_string.o := -ffreestanding $(call cc-option,-fno-tree-loop-distribute-patterns)
+
 # Early boot use of cmdline, don't instrument it
 ifdef CONFIG_AMD_MEM_ENCRYPT
 KASAN_SANITIZE_string.o := n
 
-CFLAGS_string.o := -fno-stack-protector
+CFLAGS_string.o += -fno-stack-protector
 endif
 
 # Used by KCSAN while enabled, avoid recursion.
-- 
2.26.2