[v2,1/2] x86/purgatory: do not use __builtin_memcpy and __builtin_memset

Message ID 20190722213250.238685-1-ndesaulniers@google.com
State New
Headers show
Series
  • [v2,1/2] x86/purgatory: do not use __builtin_memcpy and __builtin_memset
Related show

Commit Message

Nick Desaulniers July 22, 2019, 9:32 p.m.
Implementing memcpy and memset in terms of __builtin_memcpy and
__builtin_memset is problematic.

GCC at -O2 will replace calls to the builtins with calls to memcpy and
memset (but will generate an inline implementation at -Os).  Clang will
replace the builtins with these calls regardless of optimization level.
$ llvm-objdump -dr arch/x86/purgatory/string.o | tail

0000000000000339 memcpy:
     339: 48 b8 00 00 00 00 00 00 00 00 movabsq $0, %rax
                000000000000033b:  R_X86_64_64  memcpy
     343: ff e0                         jmpq    *%rax

0000000000000345 memset:
     345: 48 b8 00 00 00 00 00 00 00 00 movabsq $0, %rax
                0000000000000347:  R_X86_64_64  memset
     34f: ff e0

Such code results in infinite recursion at runtime. This is observed
when doing kexec.

Instead, reuse an implementation from arch/x86/boot/compressed/string.c
if we define warn as a symbol.

Fixes: 8fc5b4d4121c ("purgatory: core purgatory functionality")
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=984056
Reported-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Tested-by: Vaibhav Rustagi <vaibhavrustagi@google.com>

Debugged-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Debugged-by: Manoj Gupta <manojgupta@google.com>
Suggested-by: Alistair Delva <adelva@google.com>
Signed-off-by: Vaibhav Rustagi <vaibhavrustagi@google.com>

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

---
Changes v1 -> v2:
* Add Fixes tag.
* Move this patch to first in the series.

 arch/x86/purgatory/Makefile    |  3 +++
 arch/x86/purgatory/purgatory.c |  6 ++++++
 arch/x86/purgatory/string.c    | 23 -----------------------
 3 files changed, 9 insertions(+), 23 deletions(-)
 delete mode 100644 arch/x86/purgatory/string.c

-- 
2.22.0.657.g960e92d24f-goog

Comments

Nick Desaulniers July 22, 2019, 9:41 p.m. | #1
Joe,
Is it possible to have scripts/get_maintainer.pl always cc
linux-kernel@vger.kernel.org?  I just sent out a series, and it seems
the cover letter didn't get sent to LKML.  I usually use this shell
function to send patches:
```
function kpatch () {
  patch=$1
  shift
  git send-email \
      --cc-cmd="./scripts/get_maintainer.pl --norolestats $patch" \
        $@ $patch
}
```

Invoked via:
```
$ mkdir purgatory
$ git format-patch HEAD~2 --cover-letter -o purgatory -v2
$ kpatch purgatory/v2-000* --cc peterz@infradead.org --cc
clang-built-linux@googlegroups.com
```
Maybe I should just add `--cc linux-kernel@vger.kernel.org` to my
shell function?

On Mon, Jul 22, 2019 at 2:33 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>

> 1. Reuse the implementation of memcpy and memset instead of relying on

> __builtin_memcpy and __builtin_memset as it causes infinite recursion

> in Clang (at any opt level) or GCC at -O2.

> 2. Don't reset KBUILD_CFLAGS, rather filter CONFIG_FUNCTION_TRACER flags

> via `CFLAGS_REMOVE_<file>.o = -pg`.

>

> The order of the patches are reversed; in case we ever need to bisect,

> the memcpy/memset infinite recursion would occur with just patch 2/2

> applied.

>

> V2 of: https://lkml.org/lkml/2019/7/17/755

>

> Nick Desaulniers (2):

>   x86/purgatory: do not use __builtin_memcpy and __builtin_memset

>   x86/purgatory: use CFLAGS_REMOVE rather than reset KBUILD_CFLAGS

>

>  arch/x86/purgatory/Makefile    | 15 ++++++++++-----

>  arch/x86/purgatory/purgatory.c |  6 ++++++

>  arch/x86/purgatory/string.c    | 23 -----------------------

>  3 files changed, 16 insertions(+), 28 deletions(-)

>  delete mode 100644 arch/x86/purgatory/string.c

>

> --

> 2.22.0.657.g960e92d24f-goog

>



-- 
Thanks,
~Nick Desaulniers

Patch

diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
index 3cf302b26332..91ef244026d2 100644
--- a/arch/x86/purgatory/Makefile
+++ b/arch/x86/purgatory/Makefile
@@ -6,6 +6,9 @@  purgatory-y := purgatory.o stack.o setup-x86_$(BITS).o sha256.o entry64.o string
 targets += $(purgatory-y)
 PURGATORY_OBJS = $(addprefix $(obj)/,$(purgatory-y))
 
+$(obj)/string.o: $(srctree)/arch/x86/boot/compressed/string.c FORCE
+	$(call if_changed_rule,cc_o_c)
+
 $(obj)/sha256.o: $(srctree)/lib/sha256.c FORCE
 	$(call if_changed_rule,cc_o_c)
 
diff --git a/arch/x86/purgatory/purgatory.c b/arch/x86/purgatory/purgatory.c
index 6d8d5a34c377..b607bda786f6 100644
--- a/arch/x86/purgatory/purgatory.c
+++ b/arch/x86/purgatory/purgatory.c
@@ -68,3 +68,9 @@  void purgatory(void)
 	}
 	copy_backup_region();
 }
+
+/*
+ * Defined in order to reuse memcpy() and memset() from
+ * arch/x86/boot/compressed/string.c
+ */
+void warn(const char *msg) {}
diff --git a/arch/x86/purgatory/string.c b/arch/x86/purgatory/string.c
deleted file mode 100644
index 01ad43873ad9..000000000000
--- a/arch/x86/purgatory/string.c
+++ /dev/null
@@ -1,23 +0,0 @@ 
-// SPDX-License-Identifier: GPL-2.0-only
-/*
- * Simple string functions.
- *
- * Copyright (C) 2014 Red Hat Inc.
- *
- * Author:
- *       Vivek Goyal <vgoyal@redhat.com>
- */
-
-#include <linux/types.h>
-
-#include "../boot/string.c"
-
-void *memcpy(void *dst, const void *src, size_t len)
-{
-	return __builtin_memcpy(dst, src, len);
-}
-
-void *memset(void *dst, int c, size_t len)
-{
-	return __builtin_memset(dst, c, len);
-}