RE: [PATCH v5 1/3] open: add close_range()

From: Szabolcs Nagy

Sent: 05 June 2020 15:56

...

currently there is no libc interface contract in place that
says which calls may use libc internal fds e.g. i've seen

  openlog(...) // opens libc internal syslog fd
  ...
  fork()
  closefrom(...) // close syslog fd
  open(...) // something that reuses the closed fd
  syslog(...) // unsafe: uses the wrong fd
  execve(...)

syslog uses a libc internal fd that the user trampled on and
this can go bad in many ways depending on what libc apis are
used between closefrom (or equivalent) and exec.

It is, of course, traditional that daemons only call
close(0); close(1); close(2);
Took us ages to discover that a misspelt fprintf()
was adding data to the stdout buffer and eventually
flushing 10k of ascii text into an inter-process pipe
that had a 32bit field for 'message extension length'.

FWIW isn't syslog() going to go badly wrong after fork()
anyway?
Unless libc's fork() calls closelog().

	David

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