More windbg tricks with Delphi – how to ignore specific exceptions

This is a really quick post, just noting more flexible ways of handling exceptions in WinDBG, for example ignoring EIdConnClosedGracefully or EAbort by default.

There are two parts to this:

  1. Use script files to build complex breakpoint or exception statements. While you could technically embed it all in one line, it becomes increasingly unwieldy. It’s still painful in a script file but slightly less so.
  2. Use as to create aliases for specific memory addresses in the script, making string comparisons much simpler.

To tell WinDbg to use a script file when a Delphi exception occurs:

sxe -c "$$><C:\\scripts\\windbg_delphi_exception.txt" 0EEDFADE

Then, the script file itself:

$$
$$ Report on Delphi exceptions: setup aliases
$$

.block {
	as ${/v:exception_cleanup} ad /q ${/v:exception_message}; ad /q ${/v:exception_name}; ad /q ${/v:exception_address}
}
.block {
	exception_cleanup
}
.block {
	aS /mu ${/v:exception_message} poi(poi(ebp+1c)+4)
	aS /ma ${/v:exception_name} poi(poi(poi(ebp+1c))-38)+1
	aS /x ${/v:exception_address} poi(ebp+4)
}

$$
$$ Do everything in this block, no matter what the exception is. This way we 
$$ get reports on exceptions without interrupting program execution. You may want to
$$ add extra reporting, e.g. stack traces, other variables.
$$

.block {
	.echo Delphi exception exception_name at ${exception_address}: exception_message
}

$$
$$ In this block, we add conditionals for exception types we want to ignore, or even
$$ specific addresses or other conditions as you need.
$$
$$ Don't include anything below this block, not even comments, because that breaks the 
$$ "gc" command
$$

.block {
	.if ($scmp( "${exception_name}", "EIdConnClosedGracefully") == 0) { exception_cleanup; gc }
	.if ($scmp( "${exception_name}", "EAbort") == 0) { exception_cleanup; gc }
	.echo
	exception_cleanup
}

Yes, this file has a bit of complexity in it. The WinDbg script interpreter is excessively pernickity. Important things to note are:

  • aliases are not expanded until the block is started (hence usage of the aliases is in a separate block to the definition)
  • you can’t have any additional statements after a block containing a gc or t or similar command, otherwise the script interpreter has a little hissy fit
  • the exception_cleanup alias is pretty essential to avoid leaving aliases about that then get expanded at the wrong time (leading to premature expansion of aliases from a previous exception).
  • the exception_name alias is not 100% reliable. This is because it is referencing a Delphi short string, which is not null-terminated, leading to garbage characters displayed at the end in some cases. Sadface. There is probably a way to work around this but I haven’t found it yet.
  • You’ll see sometimes I use the ${alias_name} expansion and other times I use just alias_name. Use ${alias_name} where non-whitespace characters may be immediately after the alias, as they will be interpreted as part of the alias.
  • The ${/v:alias_name} version prevents expansion of the alias, which is useful for referring to the name of the alias if it is already defined, for example to delete it.

Useful knowledge on WinDbg scripts from other sources:

Leave a Reply

Your email address will not be published. Required fields are marked *