Re: [PATCH] tools: jobserver: Add validation for jobserver tokens to ensure valid '+' characters

[Jonathan Corbet <corbet@lwn.net>]

Changbin Du <changbin.du@huawei.com> writes:

> Add validation for jobserver tokens to prevent infinite loops on invalid fds
> When using GNU Make's jobserver feature in kernel builds, a bug in MAKEFLAGS
> propagation caused "--jobserver-auth=3,4" to reference an unintended file
> descriptor (Here, fd 3 was inherited from a shell command that opened
> "/etc/passwd" instead of a valid pipe). This led to infinite loops in
> jobserver-exec's os.read() calls due to empty or corrupted tokens. (The
> version of my make is 4.3)
>
> $ 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
>
> The modified code now explicitly validates tokens:
> 1. Rejects empty reads (prevents infinite loops on EOF)
> 2. Checks all bytes are '+' characters (catches fd reuse issues)
> 3. Raises ValueError with clear diagnostics for debugging
> This ensures robustness against invalid jobserver configurations, even when
> external tools (like make) incorrectly pass non-pipe file descriptors.
>
> Signed-off-by: Changbin Du <changbin.du@huawei.com>
> ---
>  tools/lib/python/jobserver.py | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/tools/lib/python/jobserver.py b/tools/lib/python/jobserver.py
> index a24f30ef4fa8..88d005f96bed 100755
> --- a/tools/lib/python/jobserver.py
> +++ b/tools/lib/python/jobserver.py
> @@ -91,6 +91,8 @@ class JobserverExec:
>              while True:
>                  try:
>                      slot = os.read(self.reader, 8)
> +                    if not slot or any(c != b'+'[0] for c in slot):
> +                        raise ValueError("empty or unexpected token from jobserver")

So I had to stare at this for a while to figure out what it was doing; a
comment might help.

But if it finds something that's not b'+', it simply crashes the whole
thing?  Is that really what we want to do?  It would seem better to
proceed if we got any slots at all, and to emit a message telling the
poor user what they might want to do about the situation?

>                      self.jobs += slot
>                  except (OSError, IOError) as e:
>                      if e.errno == errno.EWOULDBLOCK:

Thanks,

jon