Date: Thu, 1 Oct 2020 11:28:48 -0400 From: Rich Felker <dalias@...c.org> To: Florian Weimer <fweimer@...hat.com> Cc: musl@...ts.openwall.com, Carlos O'Donell via Libc-alpha <libc-alpha@...rceware.org> Subject: Re: Re: [PATCH] Make abort() AS-safe (Bug 26275). 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
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