Date: Sat, 27 Dec 2003 15:05:53 +0000 From: Doug Rabson <dfr@nlsystems.com> To: Daniel Eischen <eischen@vigrid.com> Cc: arch@freebsd.org Subject: Re: sigaltstack with threads Message-ID: <1072537553.1143.19.camel@herring.nlsystems.com> In-Reply-To: <Pine.GSO.4.10.10312270942550.26078-100000@pcnet5.pcnet.com> References: <Pine.GSO.4.10.10312270942550.26078-100000@pcnet5.pcnet.com>
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On Sat, 2003-12-27 at 14:56, Daniel Eischen wrote: > On Sat, 27 Dec 2003, Doug Rabson wrote: > > > I've been using my festive spare time in porting the excellent valgrind > > error checker to FreeBSD. While working through the complex way that > > valgrind deals with signals, I noticed a tricky problem. > > > > When a signal stack is installed in FreeBSD, the stack location is > > recorded in the per-process state (p_sigstk to be exact). This stack is > > used in sendsig() for signals handlers which asked for SA_ONSTACK. This > > is a problem for valgrind since it uses two threads (I use the thr_* > > syscalls for this purpose). > > > > The main thread does most of the work and wants to take a selection of > > signals (SIGSEGV, SIGBUS etc) on a signal stack allocated for the > > purpose - all other signals are blocked. > > > > The other thread is used for executing blocking syscalls (actually there > > can be more than one of these for pthread programs but I don't have that > > working yet). This syscall thread runs with something like the debugged > > program's normal signal mask so most of those signals get routed to the > > syscall thread. The main problem is that the syscall thread wants to > > take the signals on its normal stack but it can't use a different > > setting for sigaltstack. > > > > It seems to me that sigaltstack needs to be per-thread state, not > > per-process state otherwise it is essentially useless for threaded > > programs. As far as I can make out, this is the behaviour for Linux > > threads created using their clone syscall. > > In the '96 POSIX spec, using sigaltstack() in threaded applications > was undefined. The new (SUSv3 200[123]?) POSIX spec allows this > but says "Use of this function by library threads that are not bound > to kernel-scheduled entities results in undefined behavior". I think > that allows you to make it per-thread, but what behavior do we > want when there are multiple threads in a KSE/KSEG? Should it be > per KSE or KSEG? I think that if its supported at all in threaded programs, it must be per-thread state otherwise you can't prevent two different threads colliding on the same signal stack. I can't quite see how this maps to KSE/KSEG since I only have the most hazy model of that stuff in my head right now. Anyway, I've worked around things by not setting SA_ONSTACK for the handlers that I want to run on the thread stack.
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