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[screen-devel] [bug #55512] Fix blanker to work when screen is suid root

From: Scott Shambarger
Subject: [screen-devel] [bug #55512] Fix blanker to work when screen is suid root
Date: Fri, 18 Jan 2019 13:53:12 -0500 (EST)
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:64.0) Gecko/20100101 Firefox/64.0


                 Summary: Fix blanker to work when screen is suid root
                 Project: GNU Screen
            Submitted by: sshambar
            Submitted on: Fri 18 Jan 2019 06:53:11 PM UTC
                Category: Program Logic
                Severity: 3 - Normal
                Priority: 5 - Normal
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 4.6.2
           Fixed Release: None
         Planned Release: None
           Work Required: None



I found that the blankerprg setting wasn't working when screen was suid root. 
I tracked the problem down to the slave PTY for the blanker not having the
permissions correctly.

I've created a patch that changes RunBlanker to use OpenDevice
(rather than OpenPTY) so that the pty permissions are setup as in ForkWindow. 
I also updated the post-fork file-descriptor handling to mimic ForkWindow for
ptys (fixes some leaked descriptors).

To diagnose the problem, I modified RunBlanker to create screen.blanker
-DDEBUG files...(similar to screen.child files)

I also modified ReceiveMsg() to display MSG_ERROR when the screen is blocked
by the blanker (so errors launching the blanker are displayed).

Patch is attached.


File Attachments:

Date: Fri 18 Jan 2019 06:53:11 PM UTC  Name:
Fix-blanker-to-work-when-screen-is-suid-root.patch  Size: 5KiB   By: sshambar



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