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.dk
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20191121205929.40371-3-keescook@chromium.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff --git a/scripts/jobserver-count b/scripts/jobserver-count
index 6e15b38..7807bfa 100755 (executable)
--- a/scripts/jobserver-count
+++ b/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)