Re: [PATCH v3 2/2] Implement 'catch syscall' for gdbserver

[Josh Stone <jistone at redhat dot com>]

On 12/04/2015 06:16 PM, Josh Stone wrote:
> On 12/04/2015 05:18 AM, Pedro Alves wrote:
>> Quick question: What is supposed to happen to the QCatchSyscalls
>> when the process execs?  I'm thinking of 64-bit inferior execing
>> 32-bit inferior, etc.  The syscall numbers aren't necessarily shared
>> between the different architectures.  This implementation deletes discards
>> the previous QCatchSyscalls on exec, and I think that's what makes sense,
>> but, I think that this should be explicit in the packet's description.
> 
> Yes, I think it should be cleared to avoid any assumption about the
> architecture.  I'll add a note in the description codifying this.

After exploration, I'm having second thoughts about this point.  Yes,
the current implementation clears it, but only when PTRACE_O_TRACEEXEC
is enabled to actually get that event.  That's only if the client sent
"qSupports:exec-events+".  Otherwise, the server doesn't even know an
exec happened, so it can't really promise to reset the syscall table.

Since the server doesn't promise to always catch execs, I think we
should actually go the other way to stay consistent.  Let the syscall
list be carried over, and document that clients should probably send a
new list after execs in case the architecture changed.  Some clients may
just choose to live with the assumption that the arch is consistent in
their environment.

Thoughts?

>> I'm not sure gdb clears the inferior's "syscalls to the caught" VEC
>> on exec events, but it probably does (if it doesn't, I think it should.)
> 
> I'll see if I can find out.

AFAICT the only time anything is removed from syscall_catchpoint's VEC
syscalls_to_be_caught is in breakpoint_ops->remove_location and ->dtor,
respectively remove_catch_syscall and dtor_catch_syscall.  And since
this list isn't stored in the lwp structure itself, the exec doesn't
really affect anything.  Right?