PT_ATTACH resumes suspended process

Gary Jennejohn gljennjohn at
Sat May 8 11:50:41 UTC 2010

On Fri, 7 May 2010 13:52:15 -0700
Ben Widawsky <widawsky at> wrote:

> If a debugger attaches to a suspended process, the process will be
> resumed, and backgrounded. This seems like the incorrect behavior to me
> based what I read in the man page. "The tracing process will see the
> newly-traced process stop and may then control it as if it had been
> traced all along."
> The behavior exhibited in FreeBSD is that the process is resumed, and we
> will not reach ptracestop() until the next debugger command comes in.

Looking at the sendsig label in sys_process.c:kern_ptrace() makes it clear
what's happening - in your testing the process was already stopped so
the code sets td_xsig to SIGSTOP and wakes it up to send it the signal.

But td_xsig doesn't seem to be used anywhere to set pending signals.  Maybe
I missed the place where that happens.

The assumption seems to be that a process being traced will only be
stopped if the debugger is already attached and that any signals being
sent to it are coming from the debugger itself.

This assumption is wrong if the process being attached to was already

It seems to me that checking for req == PT_ATTACH when the process is
already stopped and doing a break; in that case might be a solution.

Gary Jennejohn

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