Date: Sat, 27 Dec 2003 09:56:36 -0500 (EST) From: Daniel Eischen <eischen@vigrid.com> To: Doug Rabson <dfr@nlsystems.com> Cc: arch@freebsd.org Subject: Re: sigaltstack with threads Message-ID: <Pine.GSO.4.10.10312270942550.26078-100000@pcnet5.pcnet.com> In-Reply-To: <1072527136.11617.17.camel@herring.nlsystems.com>
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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?
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