[lldb-dev] Linux/Posix spawning, signal masking

[Todd Fiala]

> It is indeed the right fix.

Ok - I'll take care of that part.  Thanks!


On Wed, Jan 22, 2014 at 9:52 AM, Greg Clayton <gclayton at apple.com> wrote:

> It is indeed the right fix. Host.mm has a mac specific version which does
> this:
>
>     sigset_t no_signals;
>     sigset_t all_signals;
>     sigemptyset (&no_signals);
>     sigfillset (&all_signals);
>     ::posix_spawnattr_setsigmask(&attr, &no_signals);
>     ::posix_spawnattr_setsigdefault(&attr, &all_signals);
>
> Compared to the incorrect version you noticed in Host.cpp:
>
>     sigset_t no_signals;
>     sigset_t all_signals;
>     sigemptyset (&no_signals);
>     sigfillset (&all_signals);
>     ::posix_spawnattr_setsigmask(&attr, &all_signals);
>     ::posix_spawnattr_setsigdefault(&attr, &no_signals);
>
> (notice all_signals and no_signals are reversed in the last two function
> calls in Host.cpp.
>
> You might compare the "LaunchProcessPosixSpawn" in Host.mm and try and
> just copy it to Host.cpp and see if it works. I can't find anything darwin
> specific in the code after a quick glance and we have used the Host.mm
> LaunchProcessPosixSpawn() for a few years now, so it is definitely tested...
>
> So the fix would seem to be:
> 1 - Remove LaunchProcessPosixSpawn() from Host.mm
> 2 - Copy it over into Host.cpp
> 3 - Make sure LaunchProcessPosixSpawn is compiled for Apple builds as well
> in Host.cpp
>
>
> On Jan 21, 2014, at 10:49 PM, Todd Fiala <tfiala at google.com> wrote:
>
> > Hi all,
> >
> > In source/Host/common/Host.cpp, there is a posix process spawning method
> called LaunchProcessPosixSpawn.  On my Linux system, it is used to start
> the target process in lldb-gdbserver.  It looks like FreeBSD uses it as
> well.  Most of the Platform launchers appear to funnel to it as well (via
> Host::LaunchProcess ()).
> >
> > There is a section of code in LaunchProcessPosixSpawn that masks all
> signals for the child process that is started up:
> >
> > ::posix_spawnattr_setsigmask(&attr, &all_signals)
> >
> > When that is set, it seems to be preventing the child from receiving
> everything except the non-blockable signals.  This has the effect of (at
> the very least) blocking SIGINT (i.e. ^C from the keyboard) and SIGTERM
> (standard unadorned "kill pid").  This appears to be the cause of a few
> bugs as I try to get lldb-gdbserver working on Linux.  For example:
> >
> > 1. hitting ^C on an lldb-gdbserver that spawned a debuggee target
> process would kill the lldb-gdbserver, but not the debuggee target, which
> continues to run.
> >
> > 2. sending a SIGTERM (kill {debuggee pid}) while it is getting debugged
> and running is ignored.
> >
> > 3. sending a SIGTERM to the debuggee after issue #1 (where
> lldb-gdbserver is no longer running) is ignored.
> >
> > 4. killing lldb-gdbserver from an attached lldb that shuts down and
> kills the remote does indeed kill lldb-gebserver, but does not kill the
> target process.
> >
> > (Of course sending a 'kill -9 {debuggee pid}' works fine to kill the
> target process).
> >
> >
> > I've modified this method locally to not mask any signals:
> >
> > ::posix_spawnattr_setsigmask (&attr, &no_signals)
> >
> > This seems to address all the problems I had above for lldb-gdbserver as
> signals propagate properly, and the target responds correctly to signals
> sent when the parent dies, etc.
> >
> > I've also run all the existing tests (on my system, that amounts to 275
> tests that really run), and those are all passing.
> >
> > However, given that so much code flows through here (or at least appears
> to) that is not directly related to the lldb-gdbserver --- i.e. local
> linux/FreeBSD debugging --- I'm highly skeptical that this is the right
> fix.  I did try using local debugging with lldb with my change in place,
> and that seemed to work fine.  But I'm thinking that perhaps the signal
> blocking was intended and that behavior is needed in some cases that
> (perhaps) are not covered by tests that run on Linux.
> >
> > Any thoughts on why all the signals were getting masked on process
> spawning?  Does that change look okay as is?
> >
> > Thanks!
> >
> > Sincerely,
> > Todd Fiala
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>
>


-- 
Todd Fiala | Software Engineer | tfiala at google.com | 650-943-3180
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