lkml.org 
[lkml]   [2022]   [Jun]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH 5.17 703/772] Kconfig: Add option for asm goto w/ tied outputs to workaround clang-13 bug
    Date
    From: Sean Christopherson <seanjc@google.com>

    commit 1aa0e8b144b6474c4914439d232d15bfe883636b upstream.

    Add a config option to guard (future) usage of asm_volatile_goto() that
    includes "tied outputs", i.e. "+" constraints that specify both an input
    and output parameter. clang-13 has a bug[1] that causes compilation of
    such inline asm to fail, and KVM wants to use a "+m" constraint to
    implement a uaccess form of CMPXCHG[2]. E.g. the test code fails with

    <stdin>:1:29: error: invalid operand in inline asm: '.long (${1:l}) - .'
    int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }
    ^
    <stdin>:1:29: error: unknown token in expression
    <inline asm>:1:9: note: instantiated into assembly here
    .long () - .
    ^
    2 errors generated.

    on clang-13, but passes on gcc (with appropriate asm goto support). The
    bug is fixed in clang-14, but won't be backported to clang-13 as the
    changes are too invasive/risky.

    gcc also had a similar bug[3], fixed in gcc-11, where gcc failed to
    account for its behavior of assigning two numbers to tied outputs (one
    for input, one for output) when evaluating symbolic references.

    [1] https://github.com/ClangBuiltLinux/linux/issues/1512
    [2] https://lore.kernel.org/all/YfMruK8%2F1izZ2VHS@google.com
    [3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98096

    Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
    Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: Sean Christopherson <seanjc@google.com>
    Message-Id: <20220202004945.2540433-2-seanjc@google.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    init/Kconfig | 5 +++++
    1 file changed, 5 insertions(+)

    --- a/init/Kconfig
    +++ b/init/Kconfig
    @@ -77,6 +77,11 @@ config CC_HAS_ASM_GOTO_OUTPUT
    depends on CC_HAS_ASM_GOTO
    def_bool $(success,echo 'int foo(int x) { asm goto ("": "=r"(x) ::: bar); return x; bar: return 0; }' | $(CC) -x c - -c -o /dev/null)

    +config CC_HAS_ASM_GOTO_TIED_OUTPUT
    + depends on CC_HAS_ASM_GOTO_OUTPUT
    + # Detect buggy gcc and clang, fixed in gcc-11 clang-14.
    + def_bool $(success,echo 'int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }' | $CC -x c - -c -o /dev/null)
    +
    config TOOLS_SUPPORT_RELR
    def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "NM=$(NM)" "OBJCOPY=$(OBJCOPY)" $(srctree)/scripts/tools-support-relr.sh)


    \
     
     \ /
      Last update: 2022-06-07 23:51    [W:2.402 / U:0.128 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site