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    Subject[89/90] signal: Fix alternate signal stack check
    2.6.31-stable review patch.  If anyone has any objections, please let us know.

    ------------------
    From: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

    commit 2a855dd01bc1539111adb7233f587c5c468732ac upstream.

    All architectures in the kernel increment/decrement the stack pointer
    before storing values on the stack.

    On architectures which have the stack grow down sas_ss_sp == sp is not
    on the alternate signal stack while sas_ss_sp + sas_ss_size == sp is
    on the alternate signal stack.

    On architectures which have the stack grow up sas_ss_sp == sp is on
    the alternate signal stack while sas_ss_sp + sas_ss_size == sp is not
    on the alternate signal stack.

    The current implementation fails for architectures which have the
    stack grow down on the corner case where sas_ss_sp == sp.This was
    reported as Debian bug #544905 on AMD64.
    Simplified test case: http://download.breakpoint.cc/tc-sig-stack.c

    The test case creates the following stack scenario:
    0xn0300 stack top
    0xn0200 alt stack pointer top (when switching to alt stack)
    0xn01ff alt stack end
    0xn0100 alt stack start == stack pointer

    If the signal is sent the stack pointer is pointing to the base
    address of the alt stack and the kernel erroneously decides that it
    has already switched to the alternate stack because of the current
    check for "sp - sas_ss_sp < sas_ss_size"

    On parisc (stack grows up) the scenario would be:
    0xn0200 stack pointer
    0xn01ff alt stack end
    0xn0100 alt stack start = alt stack pointer base
    (when switching to alt stack)
    0xn0000 stack base

    This is handled correctly by the current implementation.

    [ tglx: Modified for archs which have the stack grow up (parisc) which
    would fail with the correct implementation for stack grows
    down. Added a check for sp >= current->sas_ss_sp which is
    strictly not necessary but makes the code symetric for both
    variants ]

    Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Roland McGrath <roland@redhat.com>
    Cc: Kyle McMartin <kyle@mcmartin.ca>
    LKML-Reference: <20091025143758.GA6653@Chamillionaire.breakpoint.cc>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    include/linux/sched.h | 13 ++++++++++---
    1 file changed, 10 insertions(+), 3 deletions(-)

    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -1999,11 +1999,18 @@ static inline int is_si_special(const st
    return info <= SEND_SIG_FORCED;
    }

    -/* True if we are on the alternate signal stack. */
    -
    +/*
    + * True if we are on the alternate signal stack.
    + */
    static inline int on_sig_stack(unsigned long sp)
    {
    - return (sp - current->sas_ss_sp < current->sas_ss_size);
    +#ifdef CONFIG_STACK_GROWSUP
    + return sp >= current->sas_ss_sp &&
    + sp - current->sas_ss_sp < current->sas_ss_size;
    +#else
    + return sp > current->sas_ss_sp &&
    + sp - current->sas_ss_sp <= current->sas_ss_size;
    +#endif
    }

    static inline int sas_ss_flags(unsigned long sp)



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