Date: Thu, 29 Oct 2020 14:45:34 +0300 From: Alexey Izbyshev <izbyshev@...ras.ru> To: musl@...ts.openwall.com Cc: Rich Felker <dalias@...c.org> Subject: Re: More thoughts on wrapping signal handling On 2020-10-29 09:34, Rich Felker wrote: > 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. > As described, that would call the application's signal handler on the wrong stack in case SA_ONSTACK was used. And what happens if the application wants to modify ucontext via the third argument of the signal handler?
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