tools: jobserver: Prevent deadlock caused by incorrect jobserver configuration and enhance error reporting

When using GNU Make's jobserver feature in kernel builds, a bug in MAKEFLAGS
propagation caused "--jobserver-auth=r,w" to reference an unintended file
descriptor. This led to infinite loops in jobserver-exec's os.read() calls
due to empty token.

My shell opened /etc/passwd for some reason without closing it, and as a
result, all child processes inherited this fd 3.

$ ls -l /proc/self/fd
total 0
lrwx------ 1 changbin changbin 64 Dec 25 13:03 0 -> /dev/pts/1
lrwx------ 1 changbin changbin 64 Dec 25 13:03 1 -> /dev/pts/1
lrwx------ 1 changbin changbin 64 Dec 25 13:03 2 -> /dev/pts/1
lr-x------ 1 changbin changbin 64 Dec 25 13:03 3 -> /etc/passwd
lr-x------ 1 changbin changbin 64 Dec 25 13:03 4 -> /proc/1421383/fd

In this case, the `make` should open a new file descriptor for jobserver
control, but clearly, it did not do so and instead still passed fd 3 as
"--jobserver-auth=3,4" in MAKEFLAGS. (The version of my gnu make is 4.3)

This update ensures robustness against invalid jobserver configurations,
even when `make` incorrectly pass non-pipe file descriptors.
 * Rejecting empty reads to prevent infinite loops on EOF.
 * Clearing `self.jobs` to avoid writing to incorrect files if invalid tokens
   are detected.
 * Printing detailed error messages to stderr to inform the user.

Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Changbin Du <changbin.du@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260108113836.2976527-1-changbin.du@huawei.com>

Author: Changbin Du

tools/lib/python/jobserver.py

@@ -91,6 +91,10 @@ def open(self):
             while True:
                 try:
                     slot = os.read(self.reader, 8)
+                    if not slot:
+                        # Clear self.jobs to prevent us from probably writing incorrect file.
+                        self.jobs = b""
+                        raise ValueError("unexpected empty token from jobserver fd, invalid '--jobserver-auth=' setting?")
                     self.jobs += slot
                 except (OSError, IOError) as e:
                     if e.errno == errno.EWOULDBLOCK:
@@ -105,7 +109,8 @@ def open(self):
             # to sit here blocked on our child.
             self.claim = len(self.jobs) + 1
 
-        except (KeyError, IndexError, ValueError, OSError, IOError):
+        except (KeyError, IndexError, ValueError, OSError, IOError) as e:
+            print(f"jobserver: warning: {repr(e)}", file=sys.stderr)
             # Any missing environment strings or bad fds should result in just
             # not being parallel.
             self.claim = None