NFS EACCES regression since 6.15.4

[Alkis Georgopoulos]

Hi, up to the 6.15.2 mainline kernel, these commands correctly returned EROFS (Read-only file system):

/usr/lib/klibc/bin/nfsmount -o ro 192.168.67.1:/srv/ltsp /mnt
true <>/mnt/some-existing-file
bash: /mnt/some-existing-file: Read-only file system

Strace says the respective call and result is:
openat(AT_FDCWD, "/mnt/some-existing-file", O_RDWR|O_CREAT, 0666) = -1 EROFS (Read-only file system)

Since 6.15.4 mainline kernel, they started incorrectly returning EACCES instead:

bash: /mnt/some-existing-file: Permission denied
openat(AT_FDCWD, "/mnt/some-existing-file", O_RDWR|O_CREAT, 0666) = -1 EACCES (Permission denied)

This breaks for example the klibc losetup command, which in turn breaks e.g. netbooting in certain cases:

/usr/lib/klibc/bin/losetup -f /mnt/some-existing-file
Permission denied

Looking at https://kernel.ubuntu.com/mainline/v6.15.4/CHANGES, I wonder if it's related to this change:
nfs: clear SB_RDONLY before getting superblock

[Juerg Haefliger (juergh)]

You're running an Ubuntu mainline kernel which is not supported. Please file a new report if you encounter this issue with a supported Ubuntu kernel on a supported release.

[Alkis Georgopoulos (alkisg)]

@juergh the problem happens on the Ubuntu 25.10 kernel, that's where I first saw it.
Then I took the extra time to bisect it using the mainline kernels.

Anyway I reported this upstream and the offending commit was reverted,
you'll get the fix in Ubutu 26.04.