[Bug 1854878] [NEW] Physical USB thumbdrive treated as read-only

[Ben321]

Public bug reported:

So I have installed FreeDOS on my USB thumbdrive, by using Rufus.
Everything goes as expected so far. That's good.

When I run QEMU with this command line:
qemu-system-x86_64.exe -drive file=\\.\PhysicalDrive1

it of course is read-only, just like the resulting console message says:
WARNING: Image format was not specified for '\\.\PhysicalDrive1' and probing 
guessed raw.
         Automatically detecting the format is dangerous for raw images, write 
operations on block 0 will be restricted.
         Specify the 'raw' format explicitly to remove the restrictions.


So what I then did, was I ran QEMU with this command line:
qemu-system-x86_64.exe -drive file=\\.\PhysicalDrive1,format=raw

As expected, the above mentioned console message no longer appears.
However, beyond that, QEMU doesn't behave as it should regarding read-only 
status. When I try any operation that involves writing to the drive, it becomes 
quite clear that the drive is still read-only. Any writing operations to the 
drive result in FreeDOS giving me the error message:
Error writing to drive C: DOS area: sector not found.

The above situation is clearly a bug. QEMU should not be treating it as
read-only once I specify format=raw.

Note that drive C is how the guest OS refers to the USB thumbdrive (it's
drive E in my host OS, and drive C in my host OS is the actual system
drive).

And yes, it is a QEMU bug. It's not a FreeDOS bug I tested it with this command 
line, so that all changes would be written to a temporary snapshot file:
qemu-system-x86_64.exe -drive file=\\.\PhysicalDrive1,format=raw,snapshot
That last drive option "snapshot" tells QEMU to create a temporary snapshot 
file, and to write all changes to that. When I do that, all write operations 
are successful. So it seems that there is a bug in QEMU where it keeps 
read-only mode in place for a physical drive, even when format=raw is 
specified. Please fix this bug. Thanks in advance.

Here's my current setup.
Host OS: Windows 10 (64bit)
Guest OS: FreeDOS
QEMU version: 4.1.0

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1854878

Title:
  Physical USB thumbdrive treated as read-only

Status in QEMU:
  New

Bug description:
  So I have installed FreeDOS on my USB thumbdrive, by using Rufus.
  Everything goes as expected so far. That's good.

  When I run QEMU with this command line:
  qemu-system-x86_64.exe -drive file=\\.\PhysicalDrive1

  it of course is read-only, just like the resulting console message says:
  WARNING: Image format was not specified for '\\.\PhysicalDrive1' and probing 
guessed raw.
           Automatically detecting the format is dangerous for raw images, 
write operations on block 0 will be restricted.
           Specify the 'raw' format explicitly to remove the restrictions.

  
  So what I then did, was I ran QEMU with this command line:
  qemu-system-x86_64.exe -drive file=\\.\PhysicalDrive1,format=raw

  As expected, the above mentioned console message no longer appears.
  However, beyond that, QEMU doesn't behave as it should regarding read-only 
status. When I try any operation that involves writing to the drive, it becomes 
quite clear that the drive is still read-only. Any writing operations to the 
drive result in FreeDOS giving me the error message:
  Error writing to drive C: DOS area: sector not found.

  The above situation is clearly a bug. QEMU should not be treating it
  as read-only once I specify format=raw.

  Note that drive C is how the guest OS refers to the USB thumbdrive
  (it's drive E in my host OS, and drive C in my host OS is the actual
  system drive).

  And yes, it is a QEMU bug. It's not a FreeDOS bug I tested it with this 
command line, so that all changes would be written to a temporary snapshot file:
  qemu-system-x86_64.exe -drive file=\\.\PhysicalDrive1,format=raw,snapshot
  That last drive option "snapshot" tells QEMU to create a temporary snapshot 
file, and to write all changes to that. When I do that, all write operations 
are successful. So it seems that there is a bug in QEMU where it keeps 
read-only mode in place for a physical drive, even when format=raw is 
specified. Please fix this bug. Thanks in advance.

  Here's my current setup.
  Host OS: Windows 10 (64bit)
  Guest OS: FreeDOS
  QEMU version: 4.1.0

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