fsdb_ufs - The UFS File System Debugger

Previously, I mentioned that I had run across fsdb and planned to spend some time playing around with it. I had some time last week, so here's a few simple notes on it.

There are 2 automagic variables in fsdb: ";" and ".". ";" represents the current i-node, and "." is the current location on disk. Between the two of them, you can do things like "go to i-node X and read the first 16 bytes." Then, since "." has been updated, you can readily read the next 16 bytes from the file, and so on until you hit the end of the file or disk.

All commands (except "shell escape") must begin with an i-node specification. ":" is the current i-node, but you can specify another i-node with "#:inode". The third part of the command tells fsdb how to update the value of ":". "inode mode" updates the value to the given inode, as long as it's already allocated. "dir mode" updates : to the given directory entry, ordered by inode (this is not the same order as the directory listing is in). This makes it much easier to modify directory entries than trying to fill over the correct bytes in the directory.

"?" is the "formatted output" command. It is similar in feel to gdb's "print" command, but there are file-system specific formats, such as "inode" and "dir." ("/" is unformatted output).

Some examples:

/dev/lofi/1 > 2:inode?ino
i#: 2              md: d---rwxr-xr-x  uid: 29b7          gid: 12c     
ln: 6              bs: 2              sz : c_flags : 0           200                 

db#0: 170          
        accessed: Mon Nov 30 16:31:46 2009
        modified: Mon Nov 30 16:26:25 2009
        created : Mon Nov 30 16:26:25 2009

inode 2 is special in UFS; it is always the root of the filesystem (inode 3 seems to always be lost+found, as well). The output format looks curiously like the output from stat, except that it doesn't actually stat() the file.

/dev/lofi/1 > 2:
/dev/lofi/1 > :ls -l
/:
i#: 2           ./
i#: 2           ../
i#: 4           bin/
i#: d           etc/
i#: 3           lost+found/
i#: 5           sbin/

:ls is a shorthand for listing the current directory or inode. There is also :cd which is easier than listing the current directory, finding the entry you want, and then updating the value of : to it.

Note that all the inodes are in hex. There is a :base command to change both the input and output bases (:base=0xa is decimal; :base=0x8 is octal, etc).

Updating a directory entry is done with N:dir=M. This sets directory slot N (starting from 0) to inode M. For example:

/dev/lofi/1 > :cd bin
/dev/lofi/1 > :ls -l
/bin:
i#: 4           ./
i#: 2           ../
i#: 9           alink?
i#: a           blink?
i#: b           clink@
i#: 6           cp*
i#: 8           ls*
i#: 7           mv*
/dev/lofi/1 > 6:dir:nm,10/c
   58454:       b   l   i   n   k   \0  \0  \0  \0  \0  \0  \?  \?  \?  \0  \?
/dev/lofi/1 > 6:dir=0x09
i#: 9           blink
/dev/lofi/1 > :ls -l
/bin:
i#: 4           ./
i#: 2           ../
i#: 9           alink?
i#: 9           blink?
i#: b           clink@
i#: 6           cp*
i#: 8           ls*
i#: 7           mv*

So, what have we done here? Since blink and alink now share the same inode number, they are hard links of the same symlink. Running 9:inode?ino reveals that fsdb updated the link count for the inode automatically, so we don't have to worry about file system corruption if we unlink one of these later.

N:dir:nm,10/c outputs the first 16 characters of the name of directory slot N (directory slot N's name to ". plus 0x10", unformatted output, characters). The man page seems to point to outputting the name of a file being possible by N:dir:nm,*/c, but that reads to the end of the block, not to the first terminating null.

Renaming a file is done by writing to the "nm" value. This won't allocate any extra space for the filename, so the filename is truncated if it is longer than the already allocated space. Here is the hard way to re-name those nasty files named "\008\008\008\008...".

/dev/lofi/1 > :ls -l
/bin:
i#: 4           ./
i#: 2           ../
i#: 9           alink?
i#: 9           blink?
i#: b           clink@
i#: 6           cp*
i#: 8           ls*
i#: 7           mv*
/dev/lofi/1 > 7:dir:nm,10/c
   58464:       c   l   i   n   k   \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
/dev/lofi/1 > 7:dir:nm="dlink"
i#: b           dlink
/dev/lofi/1 > :ls -l
/bin:
i#: 4           ./
i#: 2           ../
i#: 9           alink?
i#: 9           blink?
i#: 6           cp*
i#: b           dlink@
i#: 8           ls*
i#: 7           mv*

Now you can impress all your sysadmin friends by creating hard links and renaming files.

But what about something you can't do with normal shell tools?

/dev/lofi/1 > :cd
/dev/lofi/1 > :cd etc
/dev/lofi/1 > :ls -l
/etc:
i#: d           ./
i#: 2           ../
i#: e           etc@
/dev/lofi/1 > 2:dir:nm,10/c
  178c20:       e   t   c   \0  t   \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
/dev/lofi/1 > 2:dir=0xd
i#: d           etc
/dev/lofi/1 > :ls -l
/etc:
i#: d           ./
i#: 2           ../
i#: d           etc/

Exit fsdb and remount the file system to see changes (yes, this is required; I suspected caching, but this occurs even with files that haven't been looked at yet).

$ ls -lain etc
total 6
        13 drwxr-xr-x   2 10679    300          512 Nov 30 16:26 .
         2 drwxr-xr-x   6 10679    300          512 Nov 30 16:26 ..
        13 drwxr-xr-x   2 10679    300          512 Nov 30 16:26 etc

And there you have it, a hard-linked directory. ln has been lying to you this entire time.

:wq

GraphViz for Fun and Profit

A long time ago, in a data center far, far away, we had a NIS server. It was a good little NIS server. It knew of usernames, and passwords, and automounter maps, and netgroups. Ah, the joys of NIS netgroups. There is one basic form of netgroup that we used:

([computer],[user],-)

Specifies a computer, user, or computer/user combination ("-" indicates "any user"). In addition, netgroups can contain other netgroups, so you get a nice graph of which netgroups are part of other netgroups.

Along with this wonderful little NIS server, there was an old Python script which would run through and generate a set of webpages to graphically display netgroup relations. If netgroup A contains netgroups B and C, and C contains D, we could check the generated graph to see:

    A
   / \
  B   C
      |
      D

Fast forward a decade or so to an insanely large NIS tree, with netgroups for things we've long since thrown away (e.g. line-printer-access). When we upgraded from NIS to LDAP as the backend directory service, we kept the netgroups around, though. Still in their NIS format. Of course, our wonderful little Python app broke because of the transition to LDAP, but nobody wanted to admit to knowing enough Python to fix it.

So in working on the replacement for our Python app, I got to learn a whole new set of tools. Graphviz is a suite of tools for displaying graphs and trees. A friend of mine showed me his Perl program to generate "dot" diagrams for Tic-tac-toe (turns out the only winning move really is not to play), and I realized that just redoing the generator in ruby or Perl would be much easier than trying to tackle Python's LDAP bindings (not to mention debugging Python).

The "dot" language spec, while exhaustive, isn't very helpful for learning the language. The User Manual, though slightly dated, serves as a much gentler introduction. The syntax allows for a lot of room for error, once you get the basics down: semi-colons are (nearly) optional; whitespace isn't counted for or against you; and nodes that aren't pre-declared are created without complaint.

The most basic directed graph is something like:

digraph G {
    foo -> bar;
    bar -> blort;
    foo -> quux;
}

Generating that from a list of netgroups in ruby is also quite simple:

puts "digraph G {"

netgroups.each do |netgroup|
    netgroup.children.each do |child|
        puts "\"#{netgroup.name}\" -> \"#{child.name}\";"
    end
end

puts "}"

Which can then be used to generate a nice image by:

./generator.rb | dot -Tpng -o image.png

This is exactly what we used to have (except in dia) for our Python/NIS version, except that that had a page for the subgraph at every node. So, clicking on C (using dot-generated HTML image maps) should pop up a page with:

    C
    |
    D

The original script had all the busy work to figure out which nodes to include on subgraphs done in native Python, and tons of generated dia files that got passed on to generate the images and image maps. Graphviz, however, includes "gvpr," an awk-like tool for graphs written in "dot."

BEGIN{} and END{} blocks function exactly the same as in awk.
BEG_G{} and END_G{} blocks are called at the beginning and end of each graph to be processed.
N{} blocks are called for each node, and
E{} blocks are called for each edge.

Just like awk, there is an optional test parameter (inside [], not //) to specify which "things" to operate on. For example:

N [ color == "blue" ] { color = "red" }
E [ color == "red" ] { color = "blue" }

turns all the blue nodes red and all the red edges blue. There is a whole API built in to work with graphs, nodes, and edges within gvpr, which includes a rich set of tools for "subgraphs."

Say we want to generate a graph from a couple nodes in the original graph we were given (known as "root" in gvpr).

graph_t g = subg( root, "SubGraph" ); /* create a new subgraph */
subnode( g, node( root, "C" ) );  /* add node "C" to the subgraph */
subnode( g, node( root, "D" ) );  /* add node "D" to the subgraph */

Now, this is just the nodes. To copy the edges, we could loop through all the edges from "C" (there are for and while in gvpr), or, we can "induce" the edge relations:

induce( g );

Which the man page states "extends g to its node-induced subgraph extension in its root graph." Roughly translated to English, this means "draw all the edges as they are in the original graph."

Add in some writeG() calls to output to files, and several Python files are now around 15 lines of gvpr, 40 lines of ruby, and a shell script to fire it all off.

:wq

Let's Do the Time Warp Again...

I'm reposting the work related stuff from my old blog, in reverse order. Now I can use the personal blog for personal stuff and this one for sysadmin/work stuff, and the two don't have to intermix.

This means that rather than posts getting pushed onto the stack, they're being enqueued, but the all-powerful google will still index them and make them searchable. Also, I'm making sure that the "new" old posts are tagged.

:wq

fsdb, or Thank God for Snapshots

Scanning through the logs as they went flying by today, two of the scariest lines I've seen in a long time went flying past:

fileserver: 2009-Nov-25 17:29:08 ufs:
        [ID 879645 kern.notice] NOTICE: /usr: unexpected free inode 1147, run fsck(1M) -o f
fileserver: 2009-Nov-25 17:29:13 ufs:
        [ID 879645 kern.notice] NOTICE: /usr: unexpected free inode 1145, run fsck(1M) -o f

Now, /usr is important for any system, but a primary file server isn't something we can just take down for an hour to run fsck a couple times. Especially the day before a long weekend. So, how to fix this? Sun helpfully recommends rebooting into single user mode from alternate media (actually, `boot net -s` from the openboot prom, but good luck getting that to happen on a thumper), so the Sun docs are out.

Or are they? Squirreled away in the See Also section of the fsck(1M) man page is clri(1M). This little utility should clear out the data with zeroes and release the inode back into the free list. This is (almost) exactly right, as it will just delete the offending files. But...there are important things in /usr, so what are those inodes supposed to be?

There's an app a utility for that. ncheck(1M) will look at the disk and generate a list of pathnames from inode numbers or inode numbers from pathnames. I had already generated a list of suspects from the errors running `find` on /usr, but it's good to have confirmation.

fileserver# ncheck -i 1145,1147 /dev/md/rdsk/d20
/dev/md/rdsk/d20:
1145    /bin/hd
1147    /bin/hdadm

`ls -i` confirms that these indeed do map to these inode numbers, but ncheck is more complete in that it shows all the hard links to each file. So, now that I know the names of what I'm missing...what am I missing? These are symlinks into /opt/SUNWhd/hd/bin for the thumper and thor hard drive utilities hd and hdadm. The SUNWhd package installs them, so whenever triage is done, it's probably best if they are put back. Thankfully, they're just symlinks.

So, I don't need the data. The only problem with clri is that it won't muck with the directory listing for the file. That's not so much a failing of clri as a bonus (do one thing: clear a file), but if the directory isn't cleaned up, then fsck will fail just as hard. It may even try to undo all my hard work, and we can't have that. In a fit of insanity, I tried pulling up the directory in ed to see if I could do anything with it (it's just a file like everything else, right?), but that way lies even more madness. There must be a tool somewhere to do this (and be less prone to hunan error).

Back to the man pages, and I ran across fsdb(1M). It looks fairly useful, as it lets you debug the file system. If this is anything like gdb (and it slightly resembles it), this includes displaying various values, as well as editing them. But, the man page is very vague and doesn't give any indication of how to actually use it.

Enter the google. fsdb is a wrapper around the file system specific debugger, so the actual relevant man page is fsdb_ufs(1M). The syntax is based on something called adb, which looks like a horrible thing to program in. And I've written brainfuck before (editor's note: don't ever write anything in brainfuck).

So, the next step is to grab a test box and learn how to use this debugging tool. I hear tales on the intarblag that there is a zfs version, too. But for that, I could just have snapshots instead of a tarball in case I explode the real file system. And given some of the things you can do ("fill an area of disk with pattern p"), I fully expect to blow away a few test filesystems learning it.

:wq

Test Blog Post with BlogWriter Lite

This is a test blog post from BlogWriter Lite.

Though I haven't yet used it, BlogPress' free version has a camera button built in; I assume this works in conjunction with the integrated Picaso client to allow posting of images stored on the iPhone or taking images with the webcam and posting them immediately.

BlogWriter doesn't have any image support in the free version, but supposedly the full version does offer HTML and image support.

Neither program allows for turning the phone sideways and using the landscape keyboard, which would be really nice.

:wq

Blogging From the iPhone

I just downloaded a couple mobile blogging apps for iPhone that support the Blogger API. BlogWriter and BlogPress both have free trial versions, so I figured my first attempt to get things working would be with them. If I can't connect at all to blogspot, then it's no money out of my pocket for the experiment.

A word from the not-so-wise: look up your username and password when setting up something on the iPhone. I could have posted this 3 hours earlier if I hadn't been entering the wrong password to my gmail account. Thank you, Firefox, for letting me forget.

After the 3 hours of learning about XML-RPC, the nasty guts of the original Blogger API, and how to integrate with Drupal/Joomla, I finally have two working mobile blogging apps.

BlogWriter seems to be aimed at a larger community of bloggers. It supports Blogger, WordPress, and anything that can speak the Blogger API at a custom URI.

BlogPress, despite being a portmanteau of Blogger and WordPress, only supports Blogger. The configuration is simple (provided you know your password), but it does require both Blogger/BlogSpot and Picaso credentials. They're the same now, though, and thankfully the app pre-filled my Picaso login information.

BlogPress also appears to be able to work with drafts; I don't yet know I'd they're only local to the iPhone or if it's integrated into the main Blogger web interface.

This just in: BlogPress doesn't turn sideways. Really a bummer because the iPhone keyboard is much easier to use in landscape orientation. Spellcheck and autocorrect can only do so much.

That's really all I have to compare until I actually use BlogWriter. So far, though, it looks like blogging on the iPhone is definitely doable, even without Google's SMS and e-mail hacks.

:wq

Making uzbl a little more vim-like

In the process of tweaking my uzbl config to be a little more vi(m)-like, I've made several modifications:

<<, >> (scroll to begin, end) are replaced with the more correct gg and G
b, m (next, previous page) are replaced with vimperator's H and L
 
t opens a new tab in uzbl-tabbed, so I removed it from the config
 
B (add bookmark) replaced with the more correct m
u (load bookmark) replaced with the more correct `
 
y2url is just long; y is the original vi(m) for it anyway
:q should also exit; unfortunately, this happens immediately and I haven't figured out how to require pressing as part of the command

Swapped case on F, fl, and FL to match vimperator better

And finally, since google.com automagically moves the focus to its input area, I commented out the "home page." gh still goes to google, though.

Uzbl-tabbed modifications:

I used to have ^W,h and ^W,l switch between tabs, but haven't gotten this to work correctly in the latest version of uzbl yet. Instead, Ctrl+Tab and Ctrl+Shift+Tab move between tabs.

And, of course, the colors have been modified. I find the color scheme I've got helps improve readability. It also color codes based on SSL/non-SSL, even with tabs that aren't selected.

My current complete config file can be viewed here.

:wq