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    Subject[PATCH v5 02/14] arm64: kprobes: Drop open-coded exception fixup
    Date
    From: Robin Murphy <robin.murphy@arm.com>

    The short-circuit call to fixup_exception() from kprobe_fault_handler()
    poses a problem now that the former wants to consume the fault address
    too, since the common kprobes API offers us no way to pass it through.
    Fortunately, however, it works out to be unnecessary:

    - uaccess instructions themselves are not probeable, so at most we
    should only ever expect to take a fixable fault from the pre or post
    handlers.
    - the pre and post handler run with preemption disabled, thus for any
    fault they may cause, an unhandled return from kprobe_page_fault()
    will proceed directly to __do_kernel_fault() thanks to the
    faulthandler_disabled() check.
    - __do_kernel_fault() will immediately call fixup_exception() unless
    we're in an EL1 instruction abort, and if we've somehow taken one of
    those on what we think is the middle of a uaccess routine, then the
    world is already very on fire.

    Thus we can reasonably drop the call from kprobe_fault_handler() and
    leave uaccess fixups to the regular flow.

    Signed-off-by: Robin Murphy <robin.murphy@arm.com>
    Signed-off-by: Oliver Swede <oli.swede@arm.com>
    ---
    arch/arm64/kernel/probes/kprobes.c | 7 -------
    1 file changed, 7 deletions(-)

    diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
    index 5290f17a4d80..c54c8252b32f 100644
    --- a/arch/arm64/kernel/probes/kprobes.c
    +++ b/arch/arm64/kernel/probes/kprobes.c
    @@ -328,13 +328,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
    */
    if (cur->fault_handler && cur->fault_handler(cur, regs, fsr))
    return 1;
    -
    - /*
    - * In case the user-specified fault handler returned
    - * zero, try to fix up.
    - */
    - if (fixup_exception(regs))
    - return 1;
    }
    return 0;
    }
    --
    2.17.1
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