Dear stable team,
I'm asking that
commit 3f61631d47f1 ("take care to handle NULL ->proc_lseek()")
gets backported to the stable and LTS kernels down to 5.10.
Background: We are in the process of upgrading our kernels. One target kernel is based on 5.15 LTS.
Here we found that, if proc file drivers do not implement proc_lseek, user space crashes easily, because various library routines internally perform lseek(2). The crash happens in proc_reg_llseek, where it wants to jump to a NULL pointer.
We could, arguably, fix these drivers to use ".proc_lseek = no_llseek". But this doesn't seem like a worthwhile path forward, considering that latest Linux kernels (including 6.1 LTS) allow proc_lseek == NULL again and *remove* no_lseek. Essentially, on HEAD, it's best practice to leave proc_lseek == NULL. Therefore, I ask that the above procfs fix gets backported so that our drivers can work across all kernel versions, including latest 6.x.
I checked that this commit applies and works as expected on a board that runs Linux 5.15, and the observed crash goes away.
Furthermore, I investigated that the fix applies to older LTS kernels, down to 5.10. The lseek(2) path uses vfs_llseek() which checks for FMODE_LSEEK. This has been like that forever since the initial git import. However, 5.4 LTS and older kernels do not have "struct proc_ops".
Thank you in advance.
Best regards, Thomas Martitz