Date: Tue, 3 Jul 2018 10:47:14 -0400 From: Rich Felker <dalias@...c.org> To: Florian Weimer <fweimer@...hat.com> Cc: musl@...ts.openwall.com Subject: Re: arc4random/csprng On Tue, Jul 03, 2018 at 03:36:59PM +0200, Florian Weimer wrote: > On 07/02/2018 10:39 PM, Rich Felker wrote: > >I haven't followed what's been happening with posix_random lately, but > >glibc has adding the arc4random interfaces and it seems reasonable > >that we should too, with the easy option to add the posix_random name > >for it and whatever interface details POSIX decides on. > > Note that it's probably not going to make it into glibc 2.28 at this point. Now the race is on, I guess. ;-) > >One topic I thought was a huge bikeshed was the whole fork-detection > >or fork-safety thing, but apparently it's not for glibc and perhaps > >other implementations because they've opted to make their csprng > >lock-free and incurred a lot of complexity with safely replacing > >pseudo-immutable state. I want to avoid most or all of this issue by > >just using a proper lock, but it might still be necessary to do some > >nasty hack for the case where fork is called from a signal handler > >interrupting the csprng. The only way to avoid that entirely is to > >block signals while the csprng runs, which is probably unjustifiably > >slow. > > The main lock (for non-current kernels) is needed for the fork > detection counters. Fork detection is required for compatibility > with applications which call clone/fork system calls directly, so How do you consider this supported usage at all? The tid in the TCB will be invalid after such a call, and other things may be broken too. IMO after syscall(SYS_fork or SYS_clone) the application is in an async-signal (or even more restricted) context and certainly can't use high level interfaces like arc4random. Rich
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