When I am kernel debugging, I use the !list command a LOT. It basically walks a doubly-linked list and dumps the memory of each list entry. You can also have it dump a more readable type, or have it run commands for each list entry. Today I ran across a list that was extremely large. I wasn’t really interested in the data so much as understanding just how large it was. So I had to figure out how to count the number of list entries and I figured I would share.

r $t0 = 0; !list " -x \"dd @$extret L0; r $t0 = @$t0 + 1\" 0xffffffff'0a1bcdef "; ? @$t0

Gibberish? Let’s dig in and analyze the statement. For starters, it’s a multi-part command, separated by semi-colons. The first statement sets a pseudo-register to 0, the next part uses the !list command to hit every list entry, and the last part just evaluates the pseudo-registry and dumps it’s value afterward.

r $t0 = 0
!list " -x \"dd @$extret L0; r $t0 = @$t0 + 1\" 0xffffffff'0a1bcdef "
? @$t0

Now the guts of this is in the !list command, which runs a multi-part command for each list entry. The first part of the command is just there to use up the @$extret parameter, so windbg doesn’t try to tack it onto the end of our second command, which is simply incrementing the value in the pseudo-register.

Voila! Now I know that my list has 21,000 or so entries in it, and I now understand why there just might be a performance problem. ๐Ÿ˜‰

Happy debugging!

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