[PATCH 2/3] docs, parallelism: Do not leak blocking mode to writer

From: Kees Cook
Date: Wed Nov 20 2019 - 19:03:25 EST


Setting non-blocking via a local copy of the jobserver file descriptor
is safer than just assuming the writer on the original fd is prepared
for it to be non-blocking.

Suggested-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/44c01043-ab24-b4de-6544-e8efd153e27a@xxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
scripts/jobserver-count | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/scripts/jobserver-count b/scripts/jobserver-count
index 6e15b38df3d0..a68a04ad304f 100755
--- 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 +25,13 @@ 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 writer.
+ reader = os.open("/proc/self/fd/%d" % (reader), os.O_RDONLY)
+ flags = fcntl.fcntl(reader, fcntl.F_GETFL)
+ fcntl.fcntl(reader, fcntl.F_SETFL, flags | 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)
--
2.17.1