docs, parallelism: Do not leak blocking mode to other readers

Setting non-blocking via a local copy of the jobserver file descriptor
is safer than just assuming other reader processes with the same fd open
are prepared for it to be non-blocking.
Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/lkml/44c01043-ab24-b4de-6544-e8efd153e27a@rasmusvillemoes.dkSigned-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20191121205929.40371-3-keescook@chromium.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/jobserver-count

@@ -12,12 +12,6 @@ default="1"
 if len(sys.argv) > 1:
 	default=sys.argv[1]
 
-# Set non-blocking for a given file descriptor.
-def nonblock(fd):
-	flags = fcntl.fcntl(fd, fcntl.F_GETFL)
-	fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_NONBLOCK)
-	return fd
-
 # Extract and prepare jobserver file descriptors from envirnoment.
 try:
 	# Fetch the make environment options.
@@ -31,8 +25,12 @@ try:
 	# Parse out R,W file descriptor numbers and set them nonblocking.
 	fds = opts[0].split("=", 1)[1]
 	reader, writer = [int(x) for x in fds.split(",", 1)]
-	reader = nonblock(reader)
-except (KeyError, IndexError, ValueError, IOError):
+	# Open a private copy of reader to avoid setting nonblocking
+	# on an unexpecting process with the same reader fd.
+	reader = os.open("/proc/self/fd/%d" % (reader),
+			 os.O_RDONLY | os.O_NONBLOCK)
+except (KeyError, IndexError, ValueError, IOError, OSError) as e:
+	print(e, file=sys.stderr)
 	# Any missing environment strings or bad fds should result in just
 	# using the default specified parallelism.
 	print(default)