Commit 4ce97317 authored Aug 07, 2019 by Nick Desaulniers Committed by Thomas Gleixner Aug 08, 2019

x86/purgatory: Do not use __builtin_memcpy and __builtin_memset



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.
This requires to implement a stub function for warn(). Also, Clang may
lower memcmp's that compare against 0 to bcmp's, so add a small definition,
too. See also: commit 5f074f3e ("lib/string.c: implement a basic bcmp")

Fixes: 8fc5b4d4 ("purgatory: core purgatory functionality")
Reported-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: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Cc: stable@vger.kernel.org
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=984056
Link: https://lkml.kernel.org/r/20190807221539.94583-1-ndesaulniers@google.com

parent 7468a4ea

arch/x86/boot/string.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -37,6 +37,14 @@ int memcmp(const void s1, const void s2, size_t len)
		return diff;
		}

		/*
		* Clang may lower `memcmp == 0` to `bcmp == 0`.
		*/
		int bcmp(const void s1, const void s2, size_t len)
		{
		return memcmp(s1, s2, len);
		}

		int strcmp(const char str1, const char str2)
		{
		const unsigned char s1 = (const unsigned char )str1;

arch/x86/purgatory/Makefile

+3 −0

Original line number	Diff line number	Diff line
		@@ -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)

arch/x86/purgatory/purgatory.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -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) {}

arch/x86/purgatory/string.c

deleted100644 → 0

+0 −23

Original line number	Diff line number	Diff line
		// 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);
		}