Allow pppd to start as non-root.

[micah-morton]

This patch adds #ifndef macros in 2 spots in order to allow pppd to be
spawned as a non-root user with only runtime capabilities (e.g.
CAP_NET_{RAW/ADMIN}) instead of giving pppd full root privileges. This
is helpful if pppd is itself spawned by a non-root user and the use of
file permissions (e.g. setuid-root) on the pppd binary is not a
desirable solution.

[paulusmack]

The basic idea seems fine, and the commit message is OK, though lacking a signed-off-by line.

With the ifndef in pppd/main.c, I'd prefer to see an explicit check that we have the capabilities we need if we're not running with euid=0, rather than simply not checking.

The ifndef in pppd/options.c doesn't look to be necessary. That code is about preventing a non-root user from overriding options set by the system administrator, it has basically nothing to do with the euid.

[micah-morton]

Any idea regarding the set of runtime capabilities that pppd uses (and hence which capabilities we should check for in pppd/main.c)? The only capability I know for sure pppd needs is CAP_NET_ADMIN, but we are currently giving it CAP_NET_RAW and CAP_NET_BIND_SERVICE as well as part of a tree of processes. Then again I'm not familiar with the different functionalities of pppd to know if there are any other capabilities it may need in other use cases that I haven't mentioned here.

[Neustradamus]

@micah-morton: Have you looked the PR from @a-andreyev?

• pppd: Check CAP_NET_RAW capability on Linux rather than requiring euid 0 #136

What do you think?

@paulusmack: Can you look the update from today too?