Windows device read failure through Python file interface with read buffering enabled

[therealergo]

Bug report

Bug description:

Attempting to read the last 16 sectors from a Windows device through the Python file interface fails unexpectedly when buffering is enabled. Those same sectors seem to be accessible without issue through the WIN32 file API itself.

I've included minimal reproduction code. To demonstrate this, configure DEVICE_SECTOR_CNT and DEVICE_PATH to match a drive attached to your system:

import win32file

DEVICE_PATH = "\\\\.\\PhysicalDrive3"
DEVICE_SECTOR_CNT = 62333952
DEVICE_SECTOR_SIZ = 512

for DEVICE_SECTOR_CHK in [DEVICE_SECTOR_CNT - 16, DEVICE_SECTOR_CNT - 15]:

	print("Read sector " + str(DEVICE_SECTOR_CHK) + " through win32file:")
	try:
		handle = win32file.CreateFile(
			DEVICE_PATH,
			win32file.GENERIC_READ | win32file.GENERIC_WRITE,
			win32file.FILE_SHARE_READ | win32file.FILE_SHARE_WRITE,
			None,
			win32file.OPEN_EXISTING,
			0,
			None
		)
		win32file.SetFilePointer(handle, DEVICE_SECTOR_CHK * DEVICE_SECTOR_SIZ, win32file.FILE_BEGIN)
		(_, result) = win32file.ReadFile(handle, DEVICE_SECTOR_SIZ)
		print(" ".join(["{:02X}".format(x) for x in result]))
	finally:
		win32file.CloseHandle(handle)

	print("Read sector " + str(DEVICE_SECTOR_CHK) + " through open:")
	with open(DEVICE_PATH, "rb") as device:
		device.seek(DEVICE_SECTOR_CHK * DEVICE_SECTOR_SIZ)
		result = device.read(DEVICE_SECTOR_SIZ)
		print(" ".join(["{:02X}".format(x) for x in result]))

Testing this with any drive attached to my system, I see a result like:

Read sector 62333936 through win32file:
FD FF F3 FF FB FF F3 FF F9 FF F8 FF FF FF FB FF 01 00 01 00 03 00 02 00 07 00 05 00 09 00 02 00 06 00 03 00 04 00 02 00 08 00 06 00 0C 00 08 00 0D 00 07 00 0B 00 07 00 0B 00 04 00 0E 00 05 00 0D 00 04 00 0C 00 06 00 0B 00 07 00 0C 00 08 00 0D 00 07 00 0D 00 07 00 0D 00 08 00 0B 00 05 00 0B 00 04 00 0F 00 06 00 13 00 0C 00 15 00 10 00 16 00 0F 00 14 00 0E 00 14 00 0F 00 16 00 0E 00 16 00 0F 00 17 00 10 00 15 00 10 00 11 00 0F 00 13 00 0A 00 14 00 06 00 13 00 09 00 12 00 0C 00 15 00 08 00 11 00 06 00 13 00 06 00 14 00 0A 00 12 00 0D 00 14 00 0A 00 0E 00 07 00 0F 00 04 00 11 00 06 00 0F 00 08 00 0D 00 0C 00 0D 00 0A 00 0F 00 03 00 0D 00 FF FF 0B 00 FD FF 09 00 FF FF 08 00 04 00 07 00 01 00 0A 00 00 00 0C 00 FD FF 0B 00 FC FF 06 00 FE FF 02 00 FD FF 01 00 FC FF 06 00 FD FF 09 00 FA FF 06 00 FD FF 04 00 FF FF 01 00 F9 FF FE FF FC FF FF FF F8 FF 02 00 F7 FF 01 00 F7 FF FE FF FA FF FE FF F6 FF FD FF F5 FF FD FF F5 FF FB FF F5 FF FC FF F4 FF FC FF FA FF FC FF FB FF FF FF FD FF FE FF FA FF 03 00 FE FF 04 00 FD FF FD FF FD FF 01 00 FC FF 02 00 FE FF 01 00 FC FF 02 00 F8 FF 00 00 F7 FF FF FF FB FF 00 00 FB FF 01 00 F5 FF FE FF F7 FF FD FF FA FF FE FF F9 FF 00 00 F7 FF FF FF F8 FF FF FF F9 FF 01 00 F3 FF 00 00 F5 FF FE FF F7 FF FF FF F5 FF FF FF F7 FF 01 00 FD FF 00 00 FA FF FC FF F5 FF F9 FF F3 FF FB FF F2 FF FC FF F3 FF FE FF F9 FF 01 00 FD FF FE FF FE FF FF FF F4 FF FF FF F0 FF FC FF F1 FF FF FF F8 FF 03 00 04 00 04 00 03 00 06 00 FD FF 04 00 F6 FF FC FF F5 FF F8 FF F5 FF F9 FF F9 FF FC FF FD FF 02 00 FB FF 05 00 FA FF
Read sector 62333936 through open:
FD FF F3 FF FB FF F3 FF F9 FF F8 FF FF FF FB FF 01 00 01 00 03 00 02 00 07 00 05 00 09 00 02 00 06 00 03 00 04 00 02 00 08 00 06 00 0C 00 08 00 0D 00 07 00 0B 00 07 00 0B 00 04 00 0E 00 05 00 0D 00 04 00 0C 00 06 00 0B 00 07 00 0C 00 08 00 0D 00 07 00 0D 00 07 00 0D 00 08 00 0B 00 05 00 0B 00 04 00 0F 00 06 00 13 00 0C 00 15 00 10 00 16 00 0F 00 14 00 0E 00 14 00 0F 00 16 00 0E 00 16 00 0F 00 17 00 10 00 15 00 10 00 11 00 0F 00 13 00 0A 00 14 00 06 00 13 00 09 00 12 00 0C 00 15 00 08 00 11 00 06 00 13 00 06 00 14 00 0A 00 12 00 0D 00 14 00 0A 00 0E 00 07 00 0F 00 04 00 11 00 06 00 0F 00 08 00 0D 00 0C 00 0D 00 0A 00 0F 00 03 00 0D 00 FF FF 0B 00 FD FF 09 00 FF FF 08 00 04 00 07 00 01 00 0A 00 00 00 0C 00 FD FF 0B 00 FC FF 06 00 FE FF 02 00 FD FF 01 00 FC FF 06 00 FD FF 09 00 FA FF 06 00 FD FF 04 00 FF FF 01 00 F9 FF FE FF FC FF FF FF F8 FF 02 00 F7 FF 01 00 F7 FF FE FF FA FF FE FF F6 FF FD FF F5 FF FD FF F5 FF FB FF F5 FF FC FF F4 FF FC FF FA FF FC FF FB FF FF FF FD FF FE FF FA FF 03 00 FE FF 04 00 FD FF FD FF FD FF 01 00 FC FF 02 00 FE FF 01 00 FC FF 02 00 F8 FF 00 00 F7 FF FF FF FB FF 00 00 FB FF 01 00 F5 FF FE FF F7 FF FD FF FA FF FE FF F9 FF 00 00 F7 FF FF FF F8 FF FF FF F9 FF 01 00 F3 FF 00 00 F5 FF FE FF F7 FF FF FF F5 FF FF FF F7 FF 01 00 FD FF 00 00 FA FF FC FF F5 FF F9 FF F3 FF FB FF F2 FF FC FF F3 FF FE FF F9 FF 01 00 FD FF FE FF FE FF FF FF F4 FF FF FF F0 FF FC FF F1 FF FF FF F8 FF 03 00 04 00 04 00 03 00 06 00 FD FF 04 00 F6 FF FC FF F5 FF F8 FF F5 FF F9 FF F9 FF FC FF FD FF 02 00 FB FF 05 00 FA FF
Read sector 62333937 through win32file:
FF FF FD FF 01 00 FC FF 01 00 FA FF FE FF F6 FF 02 00 F5 FF 06 00 F7 FF 03 00 FD FF 00 00 03 00 00 00 00 00 04 00 FA FF 07 00 F9 FF 05 00 F7 FF 00 00 FB FF FC FF FD FF 00 00 FC FF 04 00 FB FF 04 00 FB FF 02 00 FF FF 01 00 F9 FF FE FF F4 FF BF 75 03 00 EB B2 11 00 C0 31 2D 67 00 06 00 00 FF FF F2 FF 02 00 F2 FF 03 00 F7 FF 01 00 FB FF FB FF F4 FF FA FF F4 FF F8 FF ED FF F7 FF E8 FF F5 FF F0 FF F6 FF F4 FF F7 FF F9 FF F9 FF F6 FF FF FF F6 FF FF FF F7 FF FC FF F5 FF FA FF F5 FF F7 FF F6 FF FC FF F4 FF FF FF FA FF FE FF FC FF FC FF FD FF FC FF F8 FF F8 FF F3 FF F6 FF EE FF F3 FF EE FF F6 FF F4 FF F6 FF F6 FF FB FF F7 FF FF FF F8 FF FF FF F6 FF FC FF F5 FF F9 FF F5 FF F8 FF F3 FF F6 FF F4 FF FA FF F7 FF FC FF F8 FF FA FF FC FF FE FF FB FF FC FF F6 FF FA FF F4 FF FB FF F2 FF FA FF F3 FF FE FF F5 FF F9 FF FC FF F8 FF FB FF FA FF F4 FF FC FF F7 FF FF FF F5 FF FB FF F8 FF F9 FF F6 FF F7 FF F6 FF F9 FF F3 FF F9 FF F1 FF FA FF F4 FF FA FF F8 FF F6 FF FA FF F6 FF F4 FF F5 FF F1 FF F9 FF F3 FF FB FF F6 FF F6 FF F6 FF F5 FF F6 FF FC FF F9 FF 00 00 F4 FF FC FF F4 FF FD FF F7 FF FD FF F9 FF 01 00 FA FF FF FF FE FF 00 00 FE FF 01 00 02 00 03 00 03 00 03 00 FE FF 05 00 FF FF 05 00 04 00 09 00 0A 00 0C 00 0F 00 0B 00 10 00 0F 00 0D 00 12 00 09 00 11 00 0A 00 11 00 0B 00 0D 00 12 00 11 00 16 00 15 00 15 00 18 00 19 00 19 00 18 00 14 00 18 00 11 00 16 00 12 00 14 00 13 00 15 00 17 00 1C 00 17 00 20 00 1A 00 1F 00 1D 00 1C 00 1A 00 16 00 16 00 14 00 11 00 11 00 11 00 15 00 14 00 1F 00 19 00 1E 00 1B 00 1C 00 15 00 17 00 17 00 16 00
Read sector 62333937 through open:
Traceback (most recent call last):
  File "...", line 29, in <module>
    result = device.read(DEVICE_SECTOR_SIZ)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PermissionError: [Errno 13] Permission denied

Note that when I see this, the generated Errno is different for different devices. This could possibly be a bug in some Windows API that Python is relying on, but I'm not the one to determine that. If buffering is disabled in the Python file interface (by adding , 0 to the end of the open(...) call) then this issue does not occur, so my immediate suspicion is that the buffer implementation is reading past the end of the device for some reason.

This has been tested with Python 3.12.3 on Windows 11 10.0.22631

CPython versions tested on:

3.12

Operating systems tested on:

Windows

[ZeroIntensity]

Generally, OSErrors are friends aren't anything that we can fix. Does this work e.g. when Python is run as an admin?

cc @zooba

[zooba]

Most likely the buffering is assuming that the result will be a partial read when it reaches the end of the file/device.

I doubt there's anything sensible we can change here. We don't know that it's a device, or that it's going to behave strangely. Unless there's a flag we can pass for all types of files to make Windows do a partial read rather than return some error code (or if this is a unique error code - print out winerror on that exception to see what the OS returned), all we can really do is suggest disabling buffering.