Re: [musl] Re: [PATCH] Make abort() AS-safe (Bug 26275).

[Rich Felker <dalias@libc.org>]

On Thu, Oct 01, 2020 at 05:11:19PM +0200, Florian Weimer wrote:
> * Rich Felker:
> 
> > On Thu, Oct 01, 2020 at 08:08:24AM +0200, Florian Weimer wrote:
> >> * Rich Felker:
> >> 
> >> > Even without fork, execve and posix_spawn can also see the SIGABRT
> >> > disposition change made by abort(), passing it on to a process that
> >> > should have started with a disposition of SIG_IGN if you hit exactly
> >> > the wrong spot in the race.
> >> 
> >> My feeling is that it's not worth bothering with this kind of leakage.
> >> We've had this bug forever in glibc, and no one has complained about
> >> it.
> >> 
> >> Carlos is investigating removal of the abort lock from glibc, I think.
> >
> > I don't think that's a good solution. The lock is really important in
> > that it protects against serious wrong behavior *within the process*
> > like an application-installed signal handler for SIGABRT getting
> > called more than once.
> 
> I think glibc currently has this bug.  We only avoid it for abort, but
> I'm not sure if it's a bug to handle the handler multiple times if abort
> is called more than once.

I don't see anything in the spec that allows for the signal handler to
be called multiple times. The signal is raised (thereby following
normal semantics for if/how signal handler runs), and if a handler
runs and returns, the process is then required to terminate abnormally
as if by SIGABRT. This isn't a license to execute the signal handler
again or do other random observable things.

> But even for the more general case (threads call sigaction to install a
> SIGABRT handler): Do we actually need a lock there?  We reach this state
> only after raise (SIGABRT) has returned.  At this point, we can set a
> flag (not a lock), and every other thread that calls signal or sigaction
> would instead perform the late-stage SIG_DFL-for-SIGABRT part of abort?
> It probably still needs some fiddling with sigprocmask.

There's a race between checking the flag and acting on it. If thread A
has already called signal(SIGABRT,foo) and gotten past the "are we
aborting?" check, then thread B calls abort(), thread A can reset the
disposition of SIGABRT to foo after thread B sets it to SIG_DFL, but
before thread B re-raises, unblocks, and acts on the signal.

Rich