Description of problem:

When umount() fails with EACCES, libmount returns the error string "block devices are not permitted on filesystem", regardless of the reason for the EACCES (i.e. the filesystem to be unmounted may have nothing to do with block devices).

This is misleading in cases where umount() returns EACCES for a different reason.

In this particular case, unmounting an nfs mount which is no longer exported to the client system may return a permission error from the server, resulting in umount() returning EACCES.



Version-Release number of selected component (if applicable):

util-linux-2.32.1-39.el8_7
(really, all versions of util-linux, including upstream)


How reproducible:

with certain nfs servers, easy
RHEL nfs server does not return the error which leads to the EACCES


Steps to Reproduce:

using a particular nfs server (customer reported the issue with a NetApp nfs server; RHEL will not reproduce the error):

on nfs server:
    /etc/exports:
	/exports *(rw,no_root_squash)

on nfs client:
    # mount server:/exports /mnt -overs=4.2,sec=sys

on nfs server, change /etc/exports and re-export:
    /etc/exports:
	#/exports *(rw,no_root_squash)
	/exports some_other_hostname(rw,no_root_squash)

    (filesystem is still exported, but not to this nfs client)

on nfs client:
    # umount /mnt
    /mnt: block devices are not permitted on filesystem.



Actual results:

error message is "block devices are not permitted on filesystem."



Expected results:

the error message is more accurate with respect to the actual problem being experienced



Additional info:

an strace indicates that umount() failed with EACCES:

	15:40:18.941193 umount2("/mnt", 0) = -1 EACCES (Permission denied) <0.000656>

and the error message is determined here:

libmount/src/context_umount.c
int mnt_context_get_umount_excode(
                        struct libmnt_context *cxt,
                        int rc,
                        char *buf,
                        size_t bufsz)
{
...
        /*
         * umount(2) errors
         */
        if (buf) {
                int syserr = mnt_context_get_syscall_errno(cxt);

                switch (syserr) {
...
                case EACCES:
                        snprintf(buf, bufsz, _("block devices are not permitted on filesystem"));
                        break;


a packet capture indicates that during the umount(), the nfs client performs a GETATTR call to the server with the filehandle of the directory/filesystem to be unmounted, and the server responds with NFS4ERR_PERM:

16:46:00.846472 IP client.949 > server.nfs: Flags [P.], seq 408:628, ack 365, win 9798, options [nop,nop,TS val 296853323 ecr 1813851030], length 220: NFS request xid 2973536849 216 getattr fh 0,1/53
16:46:00.847151 IP server.nfs > client.949: Flags [P.], seq 365:465, ack 628, win 257, options [nop,nop,TS val 1813851032 ecr 296853323], length 100: NFS reply xid 2973536849 reply ok 96 getattr ERROR: Permission denied

(the RHEL nfs server responds to the request without error)



Note: this BZ is regarding the inaccurate error message.  I will address the issue of the inability to unmount in this condition in a separate BZ against the kernel.