[PATCH] Support: Avoid buffered reads of /dev/urandom

[Anton Blanchard via llvm-commits]

> LGTM.
> 
> Switching to getrandom might be possible, but can be done as a
> followup patch.

Thanks Rafael!

> Do you have commit access?

No, I don't have access.

Anton

> On 25 November 2015 at 21:15, Anton Blanchard <anton at samba.org> wrote:
> > Hi,
> >
> >> Would it be best to just use open/read/close instead of
> >> fopen/fread/fclose?
> >
> > I just realised I forgot to send a reworked version out. How does
> > this look?
> >
> > Anton
> > --
> >
> > I am seeing disappointing clang performance on a large PowerPC64
> > Linux box. GetRandomNumberSeed() does a buffered read from
> > /dev/urandom to seed its PRNG. As a result we read an entire page
> > even though we only need 4 bytes.
> >
> > With every clang task reading a page worth of /dev/urandom we
> > end up spending a large amount of time stuck on a kernel spinlock.
> >
> > diff --git a/lib/Support/Unix/Process.inc
> > b/lib/Support/Unix/Process.inc index df13bd2..27083ee 100644
> > --- a/lib/Support/Unix/Process.inc
> > +++ b/lib/Support/Unix/Process.inc
> > @@ -430,13 +430,18 @@ const char *Process::ResetColor() {
> >  #if !defined(HAVE_DECL_ARC4RANDOM) || !HAVE_DECL_ARC4RANDOM
> >  static unsigned GetRandomNumberSeed() {
> >    // Attempt to get the initial seed from /dev/urandom, if
> > possible.
> > -  if (FILE *RandomSource = ::fopen("/dev/urandom", "r")) {
> > +  int urandomFD = open("/dev/urandom", O_RDONLY);
> > +
> > +  if (urandomFD != -1) {
> >      unsigned seed;
> > -    int count = ::fread((void *)&seed, sizeof(seed), 1,
> > RandomSource);
> > -    ::fclose(RandomSource);
> > +    // Don't use a buffered read to avoid reading more data
> > +    // from /dev/urandom than we need.
> > +    int count = read(urandomFD, (void *)&seed, sizeof(seed));
> > +
> > +    close(urandomFD);
> >
> >      // Return the seed if the read was successful.
> > -    if (count == 1)
> > +    if (count == sizeof(seed))
> >        return seed;
> >    }
> >