Dumping a hash table with external chaining from the debugger

Raymond Chen

I was doing some crash dump debugging, as I am often called upon to do, and one of the data structure I had to grovel through was something that operated basically like an atom table, so that’s what I’ll call it. Like an atom table, it manages a collection of strings. You can add a string to the table (getting a unique value back, which we will call an atom), and later you can hand it the atom and it will give you the string back. It looked something like this:

struct ENTRY
  ENTRY *next;
  UINT   atom;
  PCWSTR value;

(It didn’t actually look like this; I’ve reduced it to the smallest example that still illustrates my point.)

As part of my debugging, I had an atom and needed to look it up in the table. A program would do this by simply calling the “here is an atom, please give me the string” function, but since this was a crash dump, there’s nothing around to execute anything. (Not that having a live machine would’ve made things much easier, because this was a kernel-mode crash, so you don’t get any of this edit-and-continue froofy stuff. This is real debugging™.)

But even though the crashed system can’t execute anything, the debugger can execute stuff, and the debugger engine used by kd comes with its own mini-programming language. Here’s how I dumped the atom table:

// note: this was entered all on one line
// broken into two lines for readability
0: kd&gt .for (r $t0=0; @$t0 < 0n19; r $t0=@$t0+1)
         { dl poi (fffff8a0`06b69930+@$t0*8) 99 2 }
fffff8a0`06ad3b90  fffff8a0`037a3fc0 fffff8a0`0000000c \
fffff8a0`037a3fc0  fffff8a0`037a4ae0 00000000`00000025 | $t0=0
fffff8a0`037a4ae0  fffff8a0`02257580 00000000`00000036 |
fffff8a0`02257580  00000000`00000000 00000000`00000056 /
fffff8a0`06ad3b30  fffff8a0`06ad3ad0 a9e8a9d8`0000000d \
fffff8a0`06ad3ad0  fffff8a0`037a4700 000007fc`0000000e |
fffff8a0`037a4700  fffff8a0`01f96fb0 00000000`0000003f | $t0=1
fffff8a0`01f96fb0  fffff8a0`06cfa5d0 fffff8a0`00000044 |
fffff8a0`06cfa5d0  00000000`00000000 00181000`00000060 /
fffff8a0`033e7a70  fffff8a0`037a4770 00000020`00000023 \
fffff8a0`037a4770  fffff8a0`023b52f0 00000000`0000003e | $t0=2
fffff8a0`023b52f0  fffff8a0`03b6e020 006f0063`00000059 |
fffff8a0`03b6e020  00000000`00000000 00000000`00000075 /
fffff8a0`037a0670  fffff8a0`02b08870 fffff8a0`00000026 \ $t0=3
fffff8a0`03b9e390  00000000`00000000 00240000`00000071 /

Let’s take that weirdo command apart one piece at a time.

The overall command is

.for (a; b; c) { d }

This operates the same as the C programming language. (Sorry, Delphi programmers, for yet another C-centric example.) In our case, we use the $t0 pseudo-register as our loop control.

  • r $t0=0 — this sets $t0 to zero

  • @$t0 < 0n19 — this stops once $t0 reaches 19.

  • r $t0=@$t0+1 — this increments $t0.

Note that here as well as in the loop body, I prefix the register name with the @ character when I want to obtain its value, in order to force it to be interpreted as a register. (Otherwise, the debugger will look for a symbol called $t0.)

The command being executed at each iteration is { dl poi (fffff8a0`06b69930+@$t0*8) 99 2 }. Let’s break this down, too:

  • dl — this command dumps a singly-linked list.

  • poi (fffff8a0`06b69930+@$t0*8) — this is the head of the linked list. In this example, 0xfffff8a0`06b69930 is the address of the buckets array, so we add the loop counter times the size of a pointer (8, in this case) to get the address of the $t0‘th entry, and then dereference it (poi) to get the address of the head of the linked list.

  • 99 — This is the maximum length of the linked list. I picked an arbitrary large-enough number. I like using 9’s because it carries the most value per keypress. Other people like to use nice round numbers like 1000, but 999 saves you a whole keypress and is just one less. (On the other hand, I don’t use fff because that runs the risk of being misinterpreted as a symbol.)

  • 2 — This is the number of pointer-sized objects to dump at the start of each node. For 32-bit code, I use 4, whereas for 64-bit code, I use 2. Why those values? Because those are the maximum number of pointer-sized objects that the debugger will print on one line.

Okay, so now I have that linked list dump. The value I’m looking for is the atom whose value is 0x3F, so I skim down the last column looking for 0000003f, and hey there it is.

fffff8a0`037a4700  fffff8a0`01f96fb0 00000000`0000003f

Now I have my ENTRY, and I can dump it to see what the corresponding value is.

0: kd> dt contoso!ENTRY fffff8a0`037a4700
    +0x000 next: 0xfffff8a0`01f96fb0
    +0x008 atom: 0x0000003f
    +0x010 value: 0xffff8a0`01f97e20 -> "foo"


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