Re: [PATCH v4] Implement 'catch syscall' for gdbserver

[Josh Stone <jistone at redhat dot com>]

On 01/12/2016 11:22 AM, Pedro Alves wrote:
> On 01/12/2016 07:10 PM, Josh Stone wrote:
>> On 01/12/2016 04:05 AM, Pedro Alves wrote:
>>> On 01/09/2016 03:09 AM, Josh Stone wrote:
>>>
>>>>
>>>> 2016-01-08  Josh Stone  <jistone@redhat.com>
>>>> 	    Philippe Waroquiers  <philippe.waroquiers@skynet.be>
>>>>
>>>> 	* gdb.texinfo (Remote Configuration): List the QCatchSyscalls packet.
>>>> 	(Stop Reply Packets): List the syscall entry and return stop reasons.
>>>> 	(General Query Packets): Describe QCatchSyscalls, and add it to the
>>>> 	table and detailed list of stub features.
>>>>
>>>
>>> "table of detailed", I think.
>>
>> I'm referring to two hunks:
>> - the table: Feature Name / Value Required / Default / Probe Allowed
>> - the list below it, "currently defined stub features, in more detail"
>>
>> Maybe I just need another article, "to the table and the detailed list"
> 
> Ah.  Yes, that way I think wouldn't have been confused.
> 
>>
>>>> @@ -648,6 +658,12 @@ handle_extended_wait (struct lwp_info **orig_event_lwp, int wstat)
>>>>        event_thr->last_resume_kind = resume_continue;
>>>>        event_thr->last_status.kind = TARGET_WAITKIND_IGNORE;
>>>>  
>>>> +      /* Update syscall state in the new lwp, effectively mid-syscall too.
>>>> +	 The client really should send a new list to catch, in case the
>>>> +	 architecture changed, but for ANY_SYSCALL it doesn't matter.  */
>>>> +      event_lwp->syscall_state = TARGET_WAITKIND_SYSCALL_ENTRY;
>>>> +      proc->syscalls_to_catch = syscalls_to_catch;
>>>
>>> The tone of this comment sounds to me as if the client should always
>>> send a new list, just in case, but for some odd reason it sometimes doesn't.
>>>
>>> I think we want to convey the opposite, like:
>>>
>>>          /* Update syscall state in the new lwp, effectively mid-syscall too.  */
>>>          event_lwp->syscall_state = TARGET_WAITKIND_SYSCALL_ENTRY;
>>>
>>>          /* Restore the list to catch.  Don't rely on the client, which is free
>>>             to avoid sending a new list when the architecture doesn't change.
>>>             Also, for ANY_SYSCALL, the architecture doesn't really matter.  */
>>>          proc->syscalls_to_catch = syscalls_to_catch;
>>
>> Sure, I'll take your rewrite verbatim, if you don't mind.
> 
> Certainly don't mind.
> 
> 
>>
>>>>  static int
>>>> +linux_supports_catch_syscall (void)
>>>> +{
>>>> +  return (the_low_target.get_syscall_trapinfo != NULL
>>>> +	  && linux_supports_tracesysgood());
>>>
>>> Space: "linux_supports_tracesysgood ()"
>>
>> OK
>>
>>>> +proc test_catch_syscall_execve {} {
>>>> +    global gdb_prompt decimal
>>>> +
>>>> +    with_test_prefix "execve" {
>>>> +
>>>> +	# Tell the test program we want an execve.
>>>> +	gdb_test_no_output "set do_execve = 1"
>>>> +
>>>> +	# Check for entry/return across the execve, making sure that the
>>>> +	# syscall_state isn't lost when turning into a new process.
>>>> +	insert_catch_syscall_with_arg "execve"
>>>> +	check_continue "execve"
>>>> +
>>>> +	# Remotes that don't track exec may report the raw SIGTRAP for it.
>>>> +	# If we use stepi now, we'll get a consistent trap for all targets.
>>>> +	gdb_test "stepi" ".*" "step after execve"
>>>
>>> Why is it important to do this raw SIGTRAP handling?  What happens if you don't
>>> do this?  Won't those targets already FAIL the check_continue tests?
>>
>> Just in case, the context from Linux man ptrace:
>>
>>   If the PTRACE_O_TRACEEXEC option is not in effect for the execing
>>   tracee, and if the tracee was PTRACE_ATTACHed rather that
>>   PTRACE_SEIZEd, the kernel delivers an extra SIGTRAP to the tracee
>>   after execve(2) returns.  This is an ordinary signal (similar to
>>   one which can be generated by kill -TRAP), not a special kind of
>>   ptrace-stop.
>>
>> Since that's a signal-stop *after* execve returns, the check_continue
>> will have succeeded already.
> 
> Still can't see how that step would help -- check_continue does two
> "continue"s.  So one would stop at the random SIGTRAP, and FAIL,
> and another would lose control of the inferior, probably running
> to end.

The first continue hits syscall_entry:execve.  The second continue hits
syscall_return:execve.  The second is the bug I fixed -- if it forgot
syscall_state across exec, the return was reported like another entry.

After that, there might be an impending SIGTRAP stop, depending on
whether PTRACE_O_TRACEEXEC is active.  The step was a way of just
effecting a SIGTRAP for everyone, so it was easy to match.

>> The check_continue is really the only bit I care about for this test
>> anyway.  The rest is just trying to finish the target process cleanly.
>> I was having trouble matching consistent output since plain remote was
>> getting that SIGTRAP, but extended-remote would use exec events and not
>> report anything extra.  Adding the stepi made both stop the same way.
>>
>> This is moot now, since plain remotes are now tracking exec events too.
>>  I developed this test just before that went in last month. :)
>> I just tried with that stepi commented out, and the test still passes on
>> local, remote, and extended-remote, so I'll remove it.
> 
> OK, yes, let's drop it then.  :-)
> 
> Patch is OK with these changes, BTW.

Great!  I'll make these final tweaks and push it out.