Comment 0 for bug 1785414

I applied several fixes to the seccomp sandbox in man-db 2.8.4, and I think they would all be worth backporting to 16.04. They're all corner cases, but at least the second and third of them turned up in an AskUbuntu post (https://askubuntu.com/questions/1039629/setting-up-man-db-crashes-system-with-bad-system-calls) and I had a fair amount of email responses to requests for details about it. Here are the details:

 * sandbox: Allow sched_setaffinity
   https://git.savannah.gnu.org/cgit/man-db.git/commit/?id=8fa6fb5eca612600b3a3d8da811f8345afec102e

   It's possible to run into this if reading xz-compressed manual pages with (e.g.) XZ_DEFAULTS=--threads=0 set in the environment.

 * sandbox: Allow some shared memory operations
   https://git.savannah.gnu.org/cgit/man-db.git/commit/?id=5e08ee9a4e563abedbdd2768c8bbfd96b57c1859

   Some unusual software that installs itself in /etc/ld.so.preload breaks man without this patch, such as the Astrill VPN.

 * sandbox: Improve ESET compatibility further
   https://git.savannah.gnu.org/cgit/man-db.git/commit/?id=7582fb9d69a126a53ee11223b12346d38c0f333a

   This is a refinement to some previous work I did to cope with ESET File Security (an antivirus program that installs itself in /etc/ld.so.preload).

[Test Case]
The first patch can be tested by recompressing a manual page using xz and setting XZ_DEFAULTS=--threads=0 before trying to read it. The other two require having Astrill or ESET installed; if this SRU is accepted I'll solicit feedback from people who do, although I think it would be sufficient for SRU purposes to just make sure that ordinary browsing of manual pages still works.

[Regression Potential]
This only adds more system calls to what the sandbox permits, so ensuring that man still works should be enough to catch all regressions.