Comment 0 for bug 1942991

Reading /proc/sys/kernel/spl/hostid no longer works in 5.7+ kernels upwards.

The Fix:

commit 7de4c88b39473f358add601e8c227ca9002b1bee
Author: наб <email address hidden>
Date: Sun Apr 11 01:18:40 2021 +0200

    linux/spl: base proc_dohostid() on proc_dostring()

    This fixes /proc/sys/kernel/spl/hostid on kernels with mainline commit
    32927393dc1ccd60fb2bdc05b9e8e88753761469 ("sysctl: pass kernel pointers
    to ->proc_handler") ‒ 5.7-rc1 and up

    The access_ok() check in copy_to_user() in proc_copyout_string() would
    always fail, so all userspace reads and writes would fail with EINVAL

    proc_dostring() strips only the final new-line,
    but simple_strtoul() doesn't actually need a back-trimmed string ‒
    writing "012345678 \n" is still allowed, as is "012345678zupsko", &c.

    This alters what happens when an invalid value is written ‒
    previously it'd get set to what-ever simple_strtoul() returned
    (probably 0, thereby resetting it to default), now it does nothing

    Reviewed-by: Brian Behlendorf <email address hidden>
    Signed-off-by: Ahelenia Ziemiańska <email address hidden>
    Closes #11878
    Closes #11879