[Bug 199152] msdosfs writes on umount to readonly mounted fs

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=199152

--- Comment #8 from longwitz at incore.de ---
Seen in FreeBSD 10.3-STABLE r317782: 

1. The problem with the first failing umount on a readonly mounted partition is
solved by commit 281457.

2. There is probably in issue with mount -r /mydosfs, it is a little bit
suprising that the bit FSIMOD is set on a readonly mount. That is the reason
for the failing first umount in absence of the patch mentioned above. The
backtrace for this situation is included in the output of a DTrace script:

  1   4751   msdosfs_mount:entry       called with mnt_flag=0x1
  1  18090   mountmsdosfs:entry        called with mnt_flag=0x1
  1  15625   fillinusemap:entry        called with pm_flags=0x20000000,
pm_maxcluster=98305, pm_inusemap=fffffe0003e5b000
  1  18456   usemap_free:entry         called with pm_flags=0x20000000,
pm_maxcluster=98305 and cn=67

              kernel`fillinusemap+0x16d
              kernel`mountmsdosfs+0x9be
              kernel`msdosfs_mount+0x66a
              kernel`vfs_donmount+0xf50
              kernel`sys_nmount+0x71
              kernel`amd64_syscall+0x452
              kernel`0xffffffff807f364b

  1  18457   usemap_free:return       returned with pm_flags=0x21000000
  1  15626   fillinusemap:return      returncode=0, pm_flags=0x21000000,
pm_maxcluster=98305, pm_inusemap=fffffe0003e5b000
  1  18091   mountmsdosfs:return      returncode=0, mnt_flag=0x1001,
pm_flags=0xa1000000, pm_inusemap=fffffe0003e5b000
  1   4752   msdosfs_mount:return     returncode=0

FSIMOD (=0x1000000) is set on call number 67 in usemap_free().

3. To compile the kernel with MSDOSFS_DEBUG some patches are nesessary.

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