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

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Message-ID: <Y3l9MXCO02/rOpFx@zx2c4.com>
Date:   Sun, 20 Nov 2022 02:04:49 +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 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.

Jason