RE: [ping] [PATCH 1/2] gdbserver: catch fetch registers error

["Metzger, Markus T" <markus dot t dot metzger at intel dot com>]

> -----Original Message-----
> From: Luis Machado [mailto:lgustavo@codesourcery.com]
> Sent: Tuesday, January 3, 2017 7:29 PM
> To: Metzger, Markus T <markus.t.metzger@intel.com>; gdb-
> patches@sourceware.org
> Cc: Daniel Jacobowitz <drow@false.org>; Pedro Alves (palves@redhat.com)
> <palves@redhat.com>
> Subject: Re: [ping] [PATCH 1/2] gdbserver: catch fetch registers error

Hi Luis,

Thanks for your feedback.

> >> When the PTRACE_PEEKUSER ptrace request to read registers fails, gdbserer
> >> throws
> >> an error that is caught in captured_main, where it causes a E01 error packet to
> >> be sent and gdbserer to quit (if --once was specified) or the event loop to be
> >> re-started (otherwise).
> >>
> >> We may get such ptrace errors when trying to fetch registers for an exited or
> >> running thread.  There are checks in GDB that check those conditions and
> throw
> >> meaningful error messages before we could run into the above ptrace error,
> >> e.g. thread.c:validate_registers_access.
> >>
> >> I ran into a new case and, rather than adding another call to
> >> validate_registers_access in GDB, I propose to catch the error already when
> >> handling the 'g' packet in gdbserver and reply with an error packet - assuming
> >> that gdbserver's internal state is still intact.
> >>
> >> To not replace a meaningful error message with E01, I'm trying to generate a
> >> useful error message when the error is detected and the exception is thrown.
> >>
> >> It would look like this ...
> >>
> >> gdb) PASS: gdb.btrace/enable-running.exp: continue to breakpoint: cont to 44
> >> cont&
> >> Continuing.
> >> (gdb) PASS: gdb.btrace/enable-running.exp: cont&
> >> record btrace
> >> warning: Remote failure reply: E.Selected thread is running.
> >> warning: Remote failure reply: E.Selected thread is running.
> >>
> >> ... although in this particular case, I'm going to suppress the warning.
> >>
> >> To make this look a bit nicer, we could consider stripping the "E." or the
> >> entire "Remote failure reply: E." when (re-)throwing the error inside GDB in
> >> remote.c.
[...] 
> Is this a guaranteed recoverable scenario? I've seen GDB get confused
> and mess up its internal state multiple times when it can't fetch
> something essential like memory or registers.
> 
> So, even if we handle things gracefully in gdbserver, does GDB handle
> that gracefully enough to carry on with a debugging session?

At the moment, GDBserver quits or starts over.  This is not recoverable as
GDB doesn't have a clue that GDBserver just started over.

This patch improves the situation by making GDBserver send an appropriate error
message to GDB and resume normally.  It suffices in one concrete case (see next
patch in series).  If in other cases GDB does not handle the error, I'd say we fix GDB
for those cases.

Regards,
Markus.
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