Re: statmount: requesting more information: statfs, devname, label

[Christian Brauner <brauner@kernel.org>]

On Wed, May 29, 2024 at 03:36:39PM -0300, Thiago Macieira wrote:
> Hello Miklos & others
> 
> Thank you for the listmount() & statmount() API. I found it very easy to use 
> and definitely easier than parsing of /proc/self/mountinfo. I am missing three 
> pieces of information from statmount(), two of which I can get from elsewhere 
> but the third is a showstopper.
> 
> The showstopper is the lack of mnt_devname anywhere in the output. It is 
> present in mountinfo and even in the older /etc/mtab and I couldn't find a way 
> to convert back from the device's major/minor to a string form. Scanning /dev 
> will not work because the process in question may not have a populated /dev 
> and even if it does, it would be wasteful to scan /dev for all devices. 
> Moreover, given symlinks for device-mapper, /dev/dm-1 isn't as descriptive as 
> /dev/mapper/system-root or /dev/system/root.
> 
> Is there a chance of getting this in a new version of the kernel?
> 
> Of the two others, we can get via other system calls, but would be nice if 
> statmount() also provided it.
> 
> First, the information provided by statfs(), which is the workaround. It's 
> easy to call statfs() with the returned mount point path, though that causes a 
> minor race.
> 
> The second is the filesystem label. The workaround for this is opening the 
> mount point and issuing ioctl(FS_IOC_GETFSLABEL), but that again introduces a 
> minor race and also requires that the ability to open() the path in question. 
> The second fallback to that is to scan /dev/disks/by-label, which is populated 
> by udev/udisks/systemd.

I think that mnt_devname makes sense!
I don't like the other additions because they further blur the
distinction between mount and filesystem information.