See this command's documentation:
https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/-dbgprint
and the section "DbgPrint buffer and the debugger"
https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/reading-and-filtering-debugging-messages#dbgprint-buffer-and-the-debugger
for more details.
- Loosely implement the function, based on our existing circular printout
buffers in kdio.c.
- Enable its usage in the KdpPrint() and KdpPrompt() functions.
Notice that this function will *only* capture the strings being sent **to**
the debugger, and not the strings the debugger itself produce. (This means
that we cannot use the KdPrintCircularBuffer as a replacement for our
KDBG dmesg one, for example...)
How to test:
Run ReactOS under WinDbg, and use the !dbgprint command to view the
buffer. You can also use the Memory Window, place yourself at the
address pointed by KdPrintCircularBuffer and KdPrintWritePointer, and
read its contents.
What you should observe:
Prior notice: The circular buffer in debug builds of ReactOS and Windows
is 0x8000 bytes large. In release builds, its size is down to 0x1000.
1- When you start e.g. the 2nd-stage GUI installation of ReactOS, going
past the initial "devices installation" and letting it stabilize on
the Welcome page, break into WinDbg and run the !dbgprint command. You
should notice that the end of its output is weirdly truncated, compared
to what has been actually emitted to the debug output. Comparing this
with the actual contents of the circular buffer (via Memory Window),
shows that the buffer contents is actually correct.
2- Copy all the text that has been output by the !dbgprint command and
paste it in an editor; count the number of all characters appearing +
newlines (only CR or LF), and observe that this number is "mysteriously"
equal to 16384 == 0x4000.
3- Continue running ReactOS installation for a little while, breaking back
back into WinDbg and looking at !dbgprint again. Its output seems to be
still stopping at the same place as before (but the actual buffer memory
contents shows otherwise). Continue running ROS installation, and break
into the debugger when ROS is about to restart. You should now observe
that the dbgprint buffer rolled over:
dd nt!KdPrintRolloverCount shows 1.
Carefully analysing the output of !dbgprint, however, you will notice
that it looks a bit garbage-y: the first part of the output is actually
truncated after 16384 characters, then you get a second part of the
buffer showing what ReactOS was printing while shutting down. Then
you get again what was shown at the top of the !dbgprint output.
(Of course, comparing with the actual contents of the circular buffer
in memory shows that its contents are fine...)
The reason of these strange observations, is because there is an intrinsic
bug in the !dbgprint command implementation (in kdexts.dll). Essentially,
it displays the contents of the circular buffer in two single dprintf()
calls: one for the "older" (bottom) part of the buffer:
[WritePointer, EndOfBuffer]
and one for the "newer" (upper) part of the buffer:
[CircularBuffer, WritePointer[ .
The first aspect of the bug (causing observation 3), is that those two
parts are not necessarily NULL-terminated strings (especially after
rollover), so for example, displaying the upper part of the buffer, will
potentially also display part of the buffer's bottom part.
The second aspect of the bug (explaining observations 1 and 2), is due
to the implementation of the dprintf() function (callback in dbgenv.dll).
There, it uses a fixed-sized buffer of size 0x4000 == 16384 characters.
Since the output of the circular buffer is not done by little chunks,
but by the two large parts, if any of those are larger than 0x4000 they
get truncated on display.
(This last observation is confirmed in a completely different context by
https://community.osr.com/discussion/112439/dprintf-s-max-string-length .)
KD64: Raise to HIGH_LEVEL when entering trap
KDBG: lower to DISPATCH_LEVEL when applying IRQL hack & use a worker thread to load symbols
KD&KDBG: Actually unload symbols when required
Our legacy KD module is slowly being phased out for the more recent KD64
Kernel Debugger that supports WinDbg, but at the same time we must retain
support for GCC debugging and the KDBG interface.
For the time being few #ifdef _WINKD_ have been introduced in KD64 so that
some of its code/data does not completely get shared yet with the legacy KD,
until the latter becomes phased out.
KD Modifications:
=================
- Remove the implementation of NtQueryDebugFilterState() /
NtSetDebugFilterState() that now comes entirely from KD64.
- Remove KD variables that are now shared with KD64.
- Share common code with KD64: KdpMoveMemory(), KdpZeroMemory(),
KdpCopyMemoryChunks(), KdpPrint(), KdpPrompt().
- KDBG: Remove the duplicated KdpCopyMemoryChunks() function.
- In KdpServiceDispatcher() and KdpEnterDebuggerException(), call the
KdpPrint() worker function that correctly probes and captures its arguments.
- Temporarily stub out KdEnterDebugger() and KdExitDebugger() that is used
by the shared code, until KD is removed and only the KD64 version of these
functions remain.
- Re-implement the KD/KDBG KdpPrompt() function using a custom KdpPromptString()
helper compatible with KD64, that is called by the KD64 implementation of
KdpPrompt(). This KdpPromptString() helper now issues the prompt on all
the KD loggers: e.g. if you use both at the same time COM-port and SCREEN
debugging, the prompt will appear on both. Before that the prompt was always
being displayed on COM port even if e.g. a SCREEN-only debug session was used...
- ppc_irq.c: Fix the prototype of KdpServiceDispatcher().
KD64 Fixes:
===========
- Initialize the MaximumLength member of the counted STRING variables
before using them elsewhere.
- Get rid of alloca() within SEH block in KdpPrint() (addendum to 7b95fcf9).
- Add the ROS-specific handy dump commands in KdSystemDebugControl().
- KD64: Update the list of supported Debug Filter Masks (KdComponentTable)
with the more up-to-date one from KDBG, that includes some components
that have been added in Vista+, but some of which we also use in ReactOS.
- NtQueryDebugFilterState(), NtSetDebugFilterState() and KdpPrint():
Add the Vista+ behaviour or falling back to the DEFAULT component ID
settings for unknown Components (compiled in only wheen NTDDI_VERSION >= NTDDI_VISTA).
+ Remove redundant comments and update these functions with SAL2 annotations.
- KDBG: Add extra documentation for the debug filter components list.
- CONFIG: Load all the supported Debug Filter Masks settings from the
registry.
