[Bug 217763] [patch] [bsnmpd] MIBIF_ALIAS_SIZE is too low breaking ifXTable

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=217763

            Bug ID: 217763
           Summary: [patch] [bsnmpd] MIBIF_ALIAS_SIZE is too low breaking
                    ifXTable
           Product: Base System
           Version: 11.0-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Keywords: patch
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: eugen at freebsd.org
          Keywords: patch

Created attachment 180784
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=180784&action=edit
make ifAlias dynamic

FreeBSD allows to assign descriptions to network interfaces by means of
ifconfig(8) utility or kernel API utilized by daemons like net/mpd5.
sysctl net.ifdescr_maxlen limits maximum description length to 1024 bytes by
default, including terminating zero byte.

bsnmpd shows these descriptions with ifAlias OIDs in the ifXTable (MIB-II) but
has compiled-in limit MIBIF_ALIAS_SIZE equal to 65 bytes. If an interface has
longer description (say, 70 bytes), bsnmpd complains:

SIOCGIFDESCR (ng7): too long (70)

And no description is returned to interface's ifAlias OID.
Nevertheless, bsnmpd allocates a buffer of MIBIF_ALIAS_SIZE bytes to every
network interface, is it used or not.

Attached patch solves these problems and little more:

- bsnmpd's network interface buffers for description/ifAlias now have dynamic
size and allocate exact number of bytes; no extra memory allocated for
interfaces having no description;

- sysctl net.ifdescr_maxlen is used as limit instead of compiled-in
MIBIF_ALIAS_SIZE; changes to this sysctl updated without restart of bsnmpd;

- no more strlen() calls on every SNMP request for an ifAlias: we calculate
actual size of buffer once, store it and use when needed;

- corresponding data moved from struct mibif_private to struct mibif just as
suggested by a comment: /* this should be made public */.

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