[BRLTTY] how to use brltty on mac

[Dave Mielke]

[quoted lines by rmgls on 2017/09/03 at 18:03 +0200]

>I changed the permissions at compile time of the screen executable that I 
>sent. Screen.c

That isn't necessary. If you start with pure screen source and apply our patch, 
it will work. If it doesn't then something else is wrong.

Tehre's another issue, too. Users have an implicit trust when getting source 
from official sources. I wouldn't dare run any executable that's been modified 
by anyone who doesn't have a well established reputation. They're either 
running this stuff as themselves or as root, and they simply, and rightly, 
don't want to take any chances.

>    shmid = shmget( key, size, IPC_CREAT | S_IRWXU | 660);

This modified line actually is a case in point. S_IRWXU is a constant that has 
the octal value 0700, i.e. user-only permissions. That's correct. Adding more 
bits makes the user's screen viewable by others, which is exactly what he 
doesn't want.

Now let's look at the 660 that's being ored to the correct value. While 660 may 
be correct chmod syntax, that's only because chmod is expecting an octal value. 
In C, 660 is being interpreted as a decimal number. 660 decimal works out to 
being 1224 octal. That's clearly a bad value for the permissions of a shared 
memory segment. Even if 660 were interpreted as octal, it'd be adding group 
permissions and that's not what a user would want anyway.

So please go back to pure screen source with just our patch applied. I'm going 
to guess regarding what the problem may be.

Screen needs to be started first (at least for the first time) to ensure that 
the shared memory segment is created before brltty tries to find it. Then 
brltty must be started, and it must be started by the very same user who 
started screen. This is nencessary for at least three reasons. First, that's 
the only way for user-only permissions to allow access. Second, it's the only 
way for both programs to be implicitly looking for the same shared memory 
segment. Third, shared memory segments are private by default which means that, 
without a bit of extra work, another user wouldn't be able to find it.

-- 
Dave Mielke           | 2213 Fox Crescent | http://Mielke.cc/
Phone: 1-613-726-0014 | Ottawa, Ontario   | http://Mielke.cc/bible/
EMail: Dave at Mielke.cc | Canada  K2A 1H7   | The Bible is the very Word of God.