Re: [Info-mtools] operation not permitted while setting disk parameters in find_device

[Frank Loeffler]

Hi,

please allow me to ping this issue again. I could easily and happily 
would provide a patch. I just wanted to discuss it before actually 
suggesting it. It is a pretty small change...

On Tue, Sep 08, 2020 at 10:43:28PM +0200, Frank Loeffler wrote:
> I am asking for help understanding parts of the code. Some necessary 
> back story: I am trying to get floppy auto configuration to work, i.e.: 
> mtools should set up the drive according to the geometry it finds when 
> accessing the boot sector (number of sectors, cylinders, ect). This 
> worked fine on my (old) machine, using the stock kernel for Debian 4.0 
> (yes, that old). Because it was so old, I decided to install Debian 8.0 
> (the last that supports the CPU). Now, mtools does not work anymore, at 
> least not without specifying the exact format of the disk before using 
> it. This is especially a problem because I do have 5.25 inch floppies 
> which are a mix between the (usual) 1.2MB and 1.44MB-formatted kind 
> (and several smaller ones). Guessing the correct format each time isn't 
> exactly fun, especially when you know that mtools should have the 
> necessary logic already built-in.
>
> On the surface, where before
>
> mdir b:
>
> gave me a directory listing, using the same disk, I now get:
>
> Can't set disk parameters for B: Operation not permitted
> Cannot initialize 'B:'
>
> This is, however, not a regression in mtools itself: the old binary is 
> still executable within the new Linux installation, but shows the same 
> problem, regardless of mtools version 3.9.10 or 4.0.18.
>
> After reading the source of mtools and the kernel source of the floppy 
> driver and especially fs/block_dev.c, I see that ioctl-access is only 
> granted if mode & O_ACCMODE == 3. "3" should be O_RDWR.
>
> The device is opened in minfo.c within find_device(), but the mode is 
> one of its arguments. One usage of this is within minfo.c, but there 
> O_RDONLY is given as mode. Changing this to O_RDWR makes minfo work 
> again: the ioctl call succeeds because the FD was opened with write 
> permissions.
>
> Since both the old mtools (binary) and a much newer version show the 
> same problem: are you aware of any changes in the linux driver 
> concerning permissions necessary for ioctls on block devices? It does 
> make sense to only allow them for rw-access, but I would have expected 
> this problem in mtools to surface a lot earlier than 2020. Is there 
> another, better way to have mtools work in case they have to reset 
> drive parameters depending on the disk that is inserted? Could it pose 
> a problem to open the FD with rw-permissions instead or ro?

thanks

Frank Löffler

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