Re: [musl] More thoughts on wrapping signal handling

[Szabolcs Nagy <nsz@port70.net>]

* Rich Felker <dalias@libc.org> [2020-10-29 02:34:50 -0400]:
> In "Re: [musl] Re: [PATCH] Make abort() AS-safe (Bug 26275)."
> (20201010002612.GC17637@brightrain.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