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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 .) |
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| .. | ||
| amd64 | ||
| arm | ||
| i386 | ||
| kdapi.c | ||
| kdbreak.c | ||
| kddata.c | ||
| kdinit.c | ||
| kdlock.c | ||
| kdprint.c | ||
| kdtrap.c | ||