[tor-bugs] #15471 [Tor]: Tor should prctl(PR_SET_PDEATHSIG, SIGTERM) background processes.

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#15471: Tor should prctl(PR_SET_PDEATHSIG, SIGTERM) background processes.
----------------------------+------------------------------------
 Reporter:  yawning         |          Owner:
     Type:  enhancement     |         Status:  new
 Priority:  trivial         |      Milestone:  Tor: 0.2.7.x-final
Component:  Tor             |        Version:  Tor: unspecified
 Keywords:  pt-wants, easy  |  Actual Points:
Parent ID:                  |         Points:
----------------------------+------------------------------------
 From the man page:
 {{{
        PR_SET_PDEATHSIG (since Linux 2.1.57)
               Set  the  parent  process death signal of the calling
 process to
               arg2 (either a signal value in the  range  1..maxsig,  or  0
 to
               clear).   This  is  the signal that the calling process will
 get
               when its parent dies.  This value is cleared for the child
 of  a
               fork(2)  and (since Linux 2.4.36 / 2.6.23) when executing a
 set-
               user-ID or set-group-ID binary, or a binary that has
 associated
               capabilities  (see  capabilities(7)).   This  value is
 preserved
               across execve(2).
 }}}

 This will ensure at least on Linux that all background processes will get
 a `SIGTERM` if Tor dies, and can cleanup appropriately.

 I don't think this behavior would be particularly shocking to PT
 developers (who should be handling SIGTERM already), so this probably
 doesn't even need a spec patch since "tor dying" invoking the normal
 termination signaling is appropriate.

 The choice of `SIGTERM` over `SIGKILL` here is so that PTs have the option
 to trap it and terminate their own children as appropriate.

 Pros:

  * Easy to do.
  * The kernel does all of the heavy lifting for us, as a catchall.
  * Fixes certain nasty issues in unmodified pt code on the relevant
 platform automatically.

 Cons:

  * Non-portable (#15435 aka #10047 is a portable fix that requires pt
 modification).
  * We have pts (obfs4proxy in particular) that can be ran with elevated
 capabilities.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15471>
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