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    Subject[PATCH 4.19 71/91] x86/purgatory: Do not use __builtin_memcpy and __builtin_memset
    Date
    From: Nick Desaulniers <ndesaulniers@google.com>

    commit 4ce97317f41d38584fb93578e922fcd19e535f5b upstream.

    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 5f074f3e192f ("lib/string.c: implement a basic bcmp")

    Fixes: 8fc5b4d4121c ("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
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/boot/string.c | 8 ++++++++
    arch/x86/purgatory/Makefile | 3 +++
    arch/x86/purgatory/purgatory.c | 6 ++++++
    arch/x86/purgatory/string.c | 25 -------------------------
    4 files changed, 17 insertions(+), 25 deletions(-)

    --- a/arch/x86/boot/string.c
    +++ b/arch/x86/boot/string.c
    @@ -34,6 +34,14 @@ int memcmp(const void *s1, const void *s
    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;
    --- a/arch/x86/purgatory/Makefile
    +++ b/arch/x86/purgatory/Makefile
    @@ -6,6 +6,9 @@ purgatory-y := purgatory.o stack.o setup
    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)

    --- a/arch/x86/purgatory/purgatory.c
    +++ b/arch/x86/purgatory/purgatory.c
    @@ -70,3 +70,9 @@ 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) {}
    --- a/arch/x86/purgatory/string.c
    +++ /dev/null
    @@ -1,25 +0,0 @@
    -/*
    - * Simple string functions.
    - *
    - * Copyright (C) 2014 Red Hat Inc.
    - *
    - * Author:
    - * Vivek Goyal <vgoyal@redhat.com>
    - *
    - * This source code is licensed under the GNU General Public License,
    - * Version 2. See the file COPYING for more details.
    - */
    -
    -#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);
    -}

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