From e09d90de8fb1882fcfae2aa522d9275fc6ee7443 Mon Sep 17 00:00:00 2001 From: Rafael Espindola Date: Fri, 11 Dec 2015 22:52:32 +0000 Subject: [PATCH] Avoid buffered reads of /dev/urandom 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 kernel spinlock. Patch by Anton Blanchard! git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255386 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Support/Unix/Process.inc | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/lib/Support/Unix/Process.inc b/lib/Support/Unix/Process.inc index df13bd22173..27083eeb072 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; } -- 2.34.1