linux-kernel - Re: [PATCH v5 2/3] random: introduce generic vDSO getrandom() implementation

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Date:   Mon, 21 Nov 2022 04:23:20 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>
Subject: Re: [PATCH v5 2/3] random: introduce generic vDSO getrandom()
 implementation

On Sun, Nov 20, 2022 at 02:04:49AM +0100, Jason A. Donenfeld wrote:
> On Sun, Nov 20, 2022 at 01:53:53AM +0100, Jason A. Donenfeld wrote:
> > I'm not quite sure what the best approach here is. One idea would be to
> > just note that libcs should wait until vgetrandom() has returned
> > everywhere before forking, using its atfork functionality. 
> 
> To elaborate on this idea a bit, the way this looks is:
> 
>   rwlock_t l;
>   pid_t fork(void)
>   {
>     pid_t pid;
>     write_lock(&l);
>     pid = syscall_fork();
>     write_unlock(&l);
>     return pid;
>   }
>   ssize_t getrandom(...)
>   {
>     ssize_t ret;
>     ...
>     if (!read_try_lock(&l))
>     	return syscall_getrandom(...);
>     ret = vdso_getrandom(...);
>     read_unlock(&l);
>     return ret;
>   }
> 
> So maybe that doesn't seem that bad, especially considering libc already
> has the kind of infrastructure in place to do that somewhat easily.
> Maybe there's a priority locking thing to get right here -- the writer
> should immediately starve out all future readers, so it's not unbound --
> but that seems par for the course.

Fortunately none of this was necessary, and I've got things implemented
without needing to resort to that, for v+1.

Jason