Add emulation for pledge with seccomp

[ghost]

... for #269 ... it's horrible but compiles and seems to work ... needs some more testing ...

[codecov-io]

Codecov Report

Merging #271 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #271   +/-   ##
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  Coverage   90.53%   90.53%           
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  Files           1        1           
  Lines         507      507           
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  Hits          459      459           
  Misses         48       48

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[mptre]

Thanks a great start @DBOTW! See #273, I wonder if we instead of
mimicking pledge on different OSs should come up with a different
abstraction and reason about the privilegies related to the state of
pick. Would really appreciated your thoughts.

/cc @DBOTW @mike-burns @calleerlandsson

[mike-burns]

Oh wow this is cool as heck! And I do agree that #273 is probably the right direction for this project but I wonder whether something like this pledge implementation belongs in libbsd.

Also it was illuminating to me to see pledge defined in terms of seccomp.

I'm not qualified to have detected non-obvious bugs in this code.

[ghost]

Thanks @mptre and @mike-burns, but this was just a quick hack based on
https://github.com/aggsol/linux-pledge/ and modified for the needs of pick ...

strace was used to determine the syscalls that terminate the execution of
pick and these were added to the "rules" (marked with /* Necessary */).

This is probably not the right approach when it comes to "security": by adding
more exceptions to a packet filter, after a certain time it's useless anyway ...

Even worse: I'm using seccomp for the first time, so chances are high that the
added exceptions "undermine" the whole "security" (RULE(prctl);) ...

So I'm concerned about the patch (that said I'm also not qualified for this) ...

[ghost]

OK, not that bad: RULE(prctl); is necessary because pledge is called a second time (of course without prctl no rules can be defined) ...

I have removed rpath wpath cpath from the first call and it still works here, so with #273 there is a better way to "fine tune" privilegies depending on the OS ...

The new patch contains only the minimum of necessary rules to run pick ...

Patch for the new sandbox-layer is in progress ...

[ghost]

Closing this in favor of #274 ...