As a process really changes credentials at the moment proc_set_cred() or
proc_unset_cred() is called, these functions are the proper locations to
perform the update of the new and old real users' process count (using
chgproccnt()).
Before this change, change_ruid() instead would perform that update,
although it operates only on a passed credential which is a priori not
tied to the calling process (or not to any process at all). This was
arguably a flaw of commit b1fc0ec1a7a49ded, r77183, based on its commit
message, and in particular the portion "(...) In each case, the call now
acts on a credential not a process (...)".
Fixing this makes using change_ruid() more natural when building
candidate credentials that in the end are not applied to a process,
e.g., because of some intervening privilege check. Also, it removes
a hack around this unwanted process count change in unionfs.
We also introduce the new proc_set_cred_enforce_proc_lim() so that
callers can respect the per-user process limit, and will use it for the
upcoming setcred(). We plan to change all callers of proc_set_cred() to
call this new function instead at some point. In the meantime, both
proc_set_cred() and the new function will coexist.
As detailed in some proc_set_cred_enforce_proc_lim()'s comment, checking
against the process limit is currently flawed as the kernel doesn't
really maintain the number of processes per UID (besides RLIMIT_NPROC,
this in fact also applies to RLIMIT_KQUEUES, RLIMIT_NPTS, RLIMIT_SBSIZE
and RLIMIT_SWAP). The applied limit is currently that of the old real
UID. Root (or a process granted with PRIV_PROC_LIMIT) is not subject to
this limit.
Fixes: b1fc0ec1a7a49ded