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Re: how to fix internal errors on connection to remote stub?


On 01/23/2015 11:07 AM, Paul Koning wrote:

If gdbserver is sending something that confuses gdb, the default
answer is that this is a gdb bug (it should not fall over) and
possibly in addition a gdbserver bug (it should obey the protocol
spec).   The reason I say “default answer” is because of the standard
distributed systems rule that it’s always your bug if a received
packet causes you to malfunction; the fact that the packet was
invalid is not an excuse.

You said that the stub is in an “inconsistent state”.  I’m not sure
about that.  The target is stopped by the initial connection, and at
that point you have a target thread, it’s stopped, it has registers,
so it’s in some state that can be reported.  Yes, that state has no
connection to the program GDB knows about, because it’s not in the
target yet.  So the target might be in some boot loader or other bit
of skeleton code, but it’s obviously executing something.  So I don’t
think “inconsistent” applies from the gdbserver point of view.

Hmmm, I'm not so sure about this. In the situations where we have been hitting this problem, a more exact description of what is going on in the stub is this: it previously completed normal execution of some other program in a different gdb instance and sent a 'W' packet. When a new gdb instance reconnects to the stub, the target is still sitting stopped at the semihosting breakpoint that triggered the 'W' packet. That's why I'm wondering whether the response it should be giving to the initial '?' packet on the new connection should be 'W' ("the program has exited and has no meaningful state any more") instead of 'S' ("the program is stopped"). But GDB only accepts a 'W' reply to '?' in extended-remote mode, which isn't supported by this stub.

Instead, it seems that gdb, when it queries gdbserver for the stopped
inferior state, gets back stuff that doesn’t fit in the program it’s
been told about.  But so what?  That can happen in other places for
other reasons, and gdb usually handles that just fine.  Consider the
“heuristic fencepost” machinery that protects from wild backtraces.
So it seems that we just have some gaps in gdb’s robustness, and
those are bugs that should be fixed.

New commands or new protocol mechanisms don’t seem like the right
answer; it’s not the user’s job to work around gdb bugs, nor is it
gdbserver’s job to know that it is out of sync with gdb.

It does seem like GDB could do a better job here of checking that the code in target memory (e.g., the instruction at the reported PC) matches what's expected from the program it's trying to debug. While fixing that might make these errors less likely, it wouldn't be as foolproof as the stub simply telling GDB that it definitely has no useful program state to report yet.

-Sandra


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