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    Subject[PATCH AUTOSEL for 4.14 92/97] signal/metag: Document a conflict with SI_USER with SIGFPE
    Date
    From: "Eric W. Biederman" <ebiederm@xmission.com>

    [ Upstream commit b80328be53c215346b153769267b38f531d89b4f ]

    Setting si_code to 0 results in a userspace seeing an si_code of 0.
    This is the same si_code as SI_USER. Posix and common sense requires
    that SI_USER not be a signal specific si_code. As such this use of 0
    for the si_code is a pretty horribly broken ABI.

    Further use of si_code == 0 guaranteed that copy_siginfo_to_user saw a
    value of __SI_KILL and now sees a value of SIL_KILL with the result
    hat uid and pid fields are copied and which might copying the si_addr
    field by accident but certainly not by design. Making this a very
    flakey implementation.

    Utilizing FPE_FIXME siginfo_layout will now return SIL_FAULT and the
    appropriate fields will reliably be copied.

    Possible ABI fixes includee:
    - Send the signal without siginfo
    - Don't generate a signal
    - Possibly assign and use an appropriate si_code
    - Don't handle cases which can't happen

    Cc: James Hogan <james.hogan@imgtec.com>
    Cc: linux-metag@vger.kernel.org
    Ref: ac919f0883e5 ("metag: Traps")
    Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    ---
    arch/metag/include/uapi/asm/siginfo.h | 7 +++++++
    arch/metag/kernel/traps.c | 2 +-
    2 files changed, 8 insertions(+), 1 deletion(-)

    diff --git a/arch/metag/include/uapi/asm/siginfo.h b/arch/metag/include/uapi/asm/siginfo.h
    index b54ef7186ca3..9a3f6cde9487 100644
    --- a/arch/metag/include/uapi/asm/siginfo.h
    +++ b/arch/metag/include/uapi/asm/siginfo.h
    @@ -6,4 +6,11 @@

    #include <asm-generic/siginfo.h>

    +/*
    + * SIGFPE si_codes
    + */
    +#ifdef __KERNEL__
    +#define FPE_FIXME 0 /* Broken dup of SI_USER */
    +#endif /* __KERNEL__ */
    +
    #endif
    diff --git a/arch/metag/kernel/traps.c b/arch/metag/kernel/traps.c
    index 444851e510d5..3b62b1b0c0b5 100644
    --- a/arch/metag/kernel/traps.c
    +++ b/arch/metag/kernel/traps.c
    @@ -735,7 +735,7 @@ TBIRES fpe_handler(TBIRES State, int SigNum, int Triggers, int Inst, PTBI pTBI)
    else if (error_state & TXSTAT_FPE_INEXACT_BIT)
    info.si_code = FPE_FLTRES;
    else
    - info.si_code = 0;
    + info.si_code = FPE_FIXME;
    info.si_errno = 0;
    info.si_addr = (__force void __user *)regs->ctx.CurrPC;
    force_sig_info(SIGFPE, &info, current);
    --
    2.14.1
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