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Date: Thu, 29 Oct 2020 15:21:17 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: More thoughts on wrapping signal handling
* Rich Felker <dalias@...c.org> [2020-10-29 02:34:50 -0400]:
> In "Re: [musl] Re: [PATCH] Make abort() AS-safe (Bug 26275)."
> (20201010002612.GC17637@...ghtrain.aerifal.cx,
> https://www.openwall.com/lists/musl/2020/10/10/1) I raised the
> longstanding thought of having libc wrap signal handling. This is a
> little bit of a big hammer for what it was proposed for -- fixing an
> extremely-rare race between abort and execve -- but today I had a
> thought about another use of it that's really compelling.
>
> What I noted before was that, by wrapping signal handlers, libc could
> implement a sort of "rollback" to restart a critical section that was
> interrupted. However this really only has any use when the critical
> section has no side effects aside from its final completion, and
> except for execve where replacement of the process gives the atomic
> cutoff for rollback, it requires __cp_end-like asm label of the end of
> the critical section. So it's of limited utility.
>
> However, what's more interesting than restarting the critical section
> when a signal is received is *allowing it to complete* before handling
> the signal. This can be implemented by having the wrapper, upon seeing
> that it interrupted a critical section, save the siginfo_t in TLS and
> immediately return, leaving signals blocked, without executing the
> application-installed signal handler. Then, when leaving the critical
> section, the unlock function can see the saved siginfo_t and call the
> application's signal handler. Effectively, it's as if the signal were
> just blocked until the end of the critical section.
this probably does not work with SIGSEGV and SIGBUS:
execution likely cannot continue to leave the critical
section, but the handlers must be invoked.
>
> What is the value in this?
>
> 1. It eliminates the need for syscalls to mask and unmask signals
> around all existing AS-safe locks and critical sections that can't
> safely be interrupted by application code.
>
> 2. It makes it so we can make almost any function that was AS-unsafe
> due to locking AS-safe, without any added cost. Even malloc can be
> AS-safe.
i guess this can introduce delay into signal handling,
depending on how long libc internal locks are held.
>
> 3. It makes it so a signal handler that fails to return promptly in
> one thread can't arbitrarily delay other threads waiting for
> libc-internal locks, because application code never interrupts our
> internal critical sections.
>
> This last property, #3, is the really exciting one -- it means that,
> short of swapping etc. (e.g. with mlockall and other realtime measures
> taken) most libc locks can be considered as held only for very small
> bounded time, rather than potentially-unbounded due to interruption by
> signal.
sounds interesting.
>
> I'm not sure if this is something worth pursuing, and certainly not in
> the immediate future, but it is sounding more appealing.
>
> Rich
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