• Heiko Carstens's avatar
    s390/ftrace,kprobes: allow to patch first instruction · c933146a
    Heiko Carstens authored
    If the function tracer is enabled, allow to set kprobes on the first
    instruction of a function (which is the function trace caller):
    
    If no kprobe is set handling of enabling and disabling function tracing
    of a function simply patches the first instruction. Either it is a nop
    (right now it's an unconditional branch, which skips the mcount block),
    or it's a branch to the ftrace_caller() function.
    
    If a kprobe is being placed on a function tracer calling instruction
    we encode if we actually have a nop or branch in the remaining bytes
    after the breakpoint instruction (illegal opcode).
    This is possible, since the size of the instruction used for the nop
    and branch is six bytes, while the size of the breakpoint is only
    two bytes.
    Therefore the first two bytes contain the illegal opcode and the last
    four bytes contain either "0" for nop or "1" for branch. The kprobes
    code will then execute/simulate the correct instruction.
    
    Instruction patching for kprobes and function tracer is always done
    with stop_machine(). Therefore we don't have any races where an
    instruction is patched concurrently on a different cpu.
    Besides that also the program check handler which executes the function
    trace caller instruction won't be executed concurrently to any
    stop_machine() execution.
    
    This allows to keep full fault based kprobes handling which generates
    correct pt_regs contents automatically.
    Signed-off-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
    Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
    c933146a
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