Subject: 'mkdir' procedure is neither thread-safe nor safe (2.0.12)

From: [email-masked] (Ludovic Courtès)
Date: Mon, 10 Oct 2016 18:39:47 +0200

Hello!

When the optional ‘mode’ argument to the ‘mkdir’ procedure is omitted,
umask(2) is used to find out what the current mask is and to compute the
mode argument to mkdir(2):

--8<---------------cut here---------------start------------->8---
  if (SCM_UNBNDP (mode))
    {
      mask = umask (0);
      umask (mask);
      STRING_SYSCALL (path, c_path, rv = mkdir (c_path, 0777 ^ mask));
    }
--8<---------------cut here---------------end--------------->8---

The problem is that this changes the process’ umask globally for a short
duration, during which other threads will get 0 as the mask…

Furthermore, AFAICS, the above logic is redundant with what the kernel
does anyway.  That is, in a single-threaded program,

  mask = umask (0);
  umask (mask);
  mkdir (file, 0777 ^ mask);

is equivalent to:

  mkdir (file, 0777);

Am I right that we should just remove these two ‘umask’ calls?

Ludo’.

PS: This code has been there since 1996 or earlier.