Messages in this thread | | | Date | Mon, 28 Feb 2011 14:29:41 +0100 | From | Tejun Heo <> | Subject | Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE_ATTACH |
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On Mon, Feb 28, 2011 at 02:16:48PM +0100, Denys Vlasenko wrote: > (gdb) print getpid() > > gdb modifies IP, sets breakpoint on return address, and issues PTRACE_CONT(0). > Kernel has to put the tracee into group-stop, right? > Becuase if it doesn't, if it makes tracee run, then the kernel is > still broken. For example, > stracing a program and sending SIGSTOP on it won't work: the sequence > of events will be > got SIGSTOP because SIGSTOP was delivered > PTRACE_SYSCALL(SIGSTOP) - "inject it" > got SIGSTOP because tracee is in group-stop now > PTRACE_SYSCALL(SIGSTOP) - equivalent to PTRACE_SYSCALL(0) > because we aren't in signal delivery ptrace-stop > and tracee continues. > > That's why I think gdb's "print getpid()" today depends on the bug. > If we simply fix the bug (by making PTRACE_CONT/SYSCALL(0) > re-enter group-stop), then "print getpid()" will stop working > for stopped tracees.
There's no reason to make the tracee re-enter group stop after pulling it out to execute 'print getpid()'. The only thing necessary is a way for the debugger to find out that group stop has been lifted. The debugger then can resume the tracee if it wishes so. ie. group stop becomes a trap point + a state which the debugger can monitor. If the debugger wants the tracee to follow the jctl behavior, it can do so by resuming the tracee as it sees fit.
Thanks.
-- tejun
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