Re: [Patch v2] daemon: Set ownership of kept build directories to the calling user.

[Danny Milosavljevic <dannym@scratchpost.org>]

Hi Hartmut,

On Mon, 28 Nov 2016 22:31:44 +0100
Hartmut Goebel <h.goebel@crazy-compilers.com> wrote:
> I just need some C programming hint: uid_t is an unsigned int, so
> comparing with -1 raises a warning (which IMO is the same as en error).

The system call handler in the Linux kernel does this, among other things:

#define low2highuid(uid) ((uid) == (old_uid_t)-1 ? (uid_t)-1 : (uid_t)(uid))

So I'd say to compare with (uid_t)-1 would be fine

    if (settings.clientUid != (uid_t) -1) {
        ...
    }

... since they do something like it in Linux anyway - which is the one implementing the chown operation in the first place :)

> And casting uid_t to unisgned int might return the same uid as "nobody".

What do you mean? uid_t is __uid_t - which in turn is implementation-defined - but yes, it is unsigned. On Linux it changed size multiple time until now where it is 32 bit unsigned.

On GuixSD, user "nobody" has uid 65534. That's one less than the maximum value for 16 bit (!) uids (the maximum is hopefully invalid as uid since chown needs it as a flag).

(-1) in two-complement 16 bit integral encoding (which is technically not guaranteed to be used in C) would be 65535.