Re: ptrace bugs and related problems

[Daniel Jacobowitz <dan@debian.org>]

On Thu, Jul 27, 2006 at 09:17:48PM -0400, Albert Cahalan wrote:
> Minor correction: the message is sent with bad data.
> Here at home I happen to have 2.6.17-rc5, so
> looking in the kernel/fork.c file there:
> 
> The fork_traceflag function looks only at the flags
> used to follow processes, including PT_TRACE_VFORK.
> 
> In do_fork, the result of fork_traceflag is assigned
> to the "trace" variable. Note that PT_TRACE_VFORK_DONE
> does not cause "trace" to be non-zero.
> 
> Then we hit this code:
> 
>                if (unlikely (trace)) {
>                        current->ptrace_message = nr;
>                        ptrace_notify ((trace << 8) | SIGTRAP);
>                }
> 
> That doesn't run. The ptrace_message is thus not set when
> ptrace_notify is called to send the PTRACE_EVENT_VFORK_DONE
> message. You get random stale data from a previous message.

Why do you want the message data anyway?

FORK/VFORK/CLONE events have a message: it says what the new process's
PID is.  VFORK_DONE doesn't have a message, because it only indicates
that the current process is about to resume; it's an event that only
has one process associated with it.

I really don't think this is a bug.

> The forced exits show up, oddly. I see one for each task,
> except for the task which called execve(). The task calling
> execve() will silently go away. The leader task, despite
> being reported as dead, returns from execve. Ouch. It would
> be much more friendly to have the task calling execve()
> send a (new) PTRACE_EVENT_TID_CHANGE message with the new ID
> as the ptrace_message. If this is the very last message sent
> by the task doing execve and is made to arrive in proper order,
> the debugger can renumber the structures it uses to track tasks.

Or just present things as if the leader task did the execve, which is
effectively what happens, and what I thought would happen for ptrace
too.

> Note that the new unshare() system call will need to send
> ptrace events for all tasks affected. Sending the event from
> one task is no good because the event might arrive after the
> debugger has responded to some other task. Consider breakpoints
> in a shared mm, with the mm suddenly becoming unshared.

The interface was never designed to handle unsharing.  I don't really
think it should be extended to; whoever needs this functionality should
design something cleaner for utrace.

> There is also no way to find all the tasks which share an mm.
> This is needed so that tasks don't die if the debugger attaches
> to a pre-existing task and sets a breakpoint.

Ditto.  In practice, thread groups or LinuxThreads libthread_db suffice
for daily use.

> The /proc/*/auxv files don't work immediately after starting
> a process via the usual fork,PTRACE_TRACEME,exec method.
> One has to wait some undetermined amount of time.

I have no idea what this refers to, sorry.

> PTRACE_GETSIGINFO has 0x0605 as si_code when a process exits.
> This is not defined anywhere.

It's garbage.  PTRACE_GETSIGINFO is only valid after the process stops
with a signal.

-- 
Daniel Jacobowitz
CodeSourcery
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