Implement DDB show vpath

[bz]

Summary

Sometimes it is helpful to get the path for a vnode.
Implement a ddb function to do this.

Based on: jhb gdb macro (updated)
Sponsored by: iXsystems, Inc.

[mjg]

I would add a message that there were not enough entries to reconstruct the full path.

sys/kern/vfs_cache.c

2547 ↗

(On Diff #40889)

this part makes no sense - you are back to what ncp was prior to the assignment

[bz]

sys/kern/vfs_cache.c

2547 ↗

(On Diff #40889)

@mjg Hmm, so we don't actually need any &TS checks anymore?

And looking at the data structures, I am not exactly clear how this is the same as before (as the pointers should be different) but I have to admit I started off from jhb's gdb macros and not from your rewrite.

[mjg]

sys/kern/vfs_cache.c

2547 ↗

(On Diff #40889)

The primitive has to accept ncp as found in the hash or vnode lists. This will be the very same pointer for 'regular' entries and one offset by a little bit for TS ones.

ncp_ts = __containerof(ncp, struct namecache_ts, nc_nc);

this grabs ncp and subtracts the offset of nc_nc, thus:

ncp = &ncp_ts->nc_nc;

this assigns ncp to what it already is

[bz]

Fix the _ts stuff by not doing it anymore.

[jhb]

There is also ddb.4 though many commands are not documented in it.

sys/kern/vfs_cache.c

2577 ↗

(On Diff #41290)

s/strcut/struct/

I thought we already had a DDB command that would show a path, but I am perhaps remembering wrong as I can't find one.