Re: [PATCH v2] docs: document file-posix locking protocol

[Vladimir Sementsov-Ogievskiy]

03.07.2021 16:50, Vladimir Sementsov-Ogievskiy wrote:
> +Permission bytes. If permission byte is rd-locked, it means that some process
> +uses corresponding permission on that file.
> +
> +Byte    Operation
> +100     read
> +          Lock holder can read
> +101     write
> +          Lock holder can write
> +102     write-unchanged
> +          Lock holder can write same data if it sure, that this write doesn't
> +          break concurrent readers. This is mostly used internally in Qemu
> +          and it wouldn't be good idea to exploit it somehow.

Let's make it more strict:

New software should never lock this byte and interpret this byte locked by 
other process like write permission (same as 101).

> +103     resize
> +          Lock holder can resize the file. "write" permission is also required
> +          for resizing, so lock byte 103 only if you also lock byte 101.
> +104     graph-mod
> +          Undefined. QEMU may sometimes locks this byte, but external programs
> +          should not. QEMU will stop locking this byte in future
> +
> +Unshare bytes. If permission byte is rd-locked, it means that some process
> +does not allow the others use corresponding options on that file.
> +
> +Byte    Operation
> +200     read
> +          Lock holder don't allow read operation to other processes.
> +201     write
> +          Lock holder don't allow write operation to other processes. This
> +          still allows others to do write-uncahnged operations. Better not
> +          exploit outside of Qemu.
> +202     write-unchanged
> +          Lock holder don't allow write-unchanged operation to other processes.

And here, correspondingly:

New software should never lock this byte and interpret this byte locked by 
other process like write permission unshared (same as 201).

> +203     resize
> +          Lock holder don't allow resizing the file by other processes.
> +204     graph-mod
> +          Undefined. QEMU may sometimes locks this byte, but external programs
> +          should not. QEMU will stop locking this byte in future
> +
> +Handling the permissions works as follows: assume we want to open the file to 
> do
> +some operations and in the same time want to disallow some operation to other
> +processes. So, we want to lock some of the bytes described above. We operate as
> +follows:
> +


--
Best regards,
Vladimir