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    Subject[PATCH 5.4 048/168] x86/xen: Distribute switch variables for initialization
    Date
    From: Kees Cook <keescook@chromium.org>

    [ Upstream commit 9038ec99ceb94fb8d93ade5e236b2928f0792c7c ]

    Variables declared in a switch statement before any case statements
    cannot be automatically initialized with compiler instrumentation (as
    they are not part of any execution flow). With GCC's proposed automatic
    stack variable initialization feature, this triggers a warning (and they
    don't get initialized). Clang's automatic stack variable initialization
    (via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
    doesn't initialize such variables[1]. Note that these warnings (or silent
    skipping) happen before the dead-store elimination optimization phase,
    so even when the automatic initializations are later elided in favor of
    direct initializations, the warnings remain.

    To avoid these problems, move such variables into the "case" where
    they're used or lift them up into the main function body.

    arch/x86/xen/enlighten_pv.c: In function ‘xen_write_msr_safe’:
    arch/x86/xen/enlighten_pv.c:904:12: warning: statement will never be executed [-Wswitch-unreachable]
    904 | unsigned which;
    | ^~~~~

    [1] https://bugs.llvm.org/show_bug.cgi?id=44916

    Signed-off-by: Kees Cook <keescook@chromium.org>
    Link: https://lore.kernel.org/r/20200220062318.69299-1-keescook@chromium.org
    Reviewed-by: Juergen Gross <jgross@suse.com>
    [boris: made @which an 'unsigned int']
    Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/x86/xen/enlighten_pv.c | 7 ++++---
    1 file changed, 4 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
    index 6ea215cdeadac..6d4d8a5700b71 100644
    --- a/arch/x86/xen/enlighten_pv.c
    +++ b/arch/x86/xen/enlighten_pv.c
    @@ -905,14 +905,15 @@ static u64 xen_read_msr_safe(unsigned int msr, int *err)
    static int xen_write_msr_safe(unsigned int msr, unsigned low, unsigned high)
    {
    int ret;
    +#ifdef CONFIG_X86_64
    + unsigned int which;
    + u64 base;
    +#endif

    ret = 0;

    switch (msr) {
    #ifdef CONFIG_X86_64
    - unsigned which;
    - u64 base;
    -
    case MSR_FS_BASE: which = SEGBASE_FS; goto set;
    case MSR_KERNEL_GS_BASE: which = SEGBASE_GS_USER; goto set;
    case MSR_GS_BASE: which = SEGBASE_GS_KERNEL; goto set;
    --
    2.20.1


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