[OpenWrt-Devel] [PATCH] *** Fix RENAME_EXCHANGE of jffs2 to handle i_nlink properly ***

[Jing Qiu]

From: Jing Qiu <aqiu0720 at gmail.com>

Current OpenWRT's jffs2 patch includes RENAME_EXCHANGE support for overlayFS. However, it doesn't handle the i_nlink of parent inode for the exchanged dentry properly. To reproduce the issue, I run openwrt:master on a WR703N device, and follow below steps:
```
#mount -t overlay overlay -o lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work / 
#rm -r /bin
#rm -r /lib
#dmesg
[  421.340602] ------------[ cut here ]------------
[  421.343925] WARNING: CPU: 0 PID: 1421 at fs/inode.c:273 drop_nlink+0x28/0x64()
[  421.351275] Modules linked in: ath9k ath9k_common pppoe ppp_async iptable_nat ath9k_hw ath pppox ppp_generic nf_nat_ipv4 nf_conntrack_ipv6 nf_conntrack_ipv4 mac80211 ipt_REJECT ipt_MASQUE
RADE ebtable_nat ebtable_filter ebtable_broute cfg80211 xt_time xt_tcpudp xt_state xt_nat xt_multiport xt_mark xt_mac xt_limit xt_conntrack xt_comment xt_TRACE xt_TCPMSS xt_REDIRECT xt_LOG x
t_CT ums_usbat ums_sddr55 ums_sddr09 ums_karma ums_jumpshot ums_isd200 ums_freecom ums_datafab ums_cypress ums_alauda slhc nf_reject_ipv4 nf_nat_redirect nf_nat_masquerade_ipv4 nf_nat nf_log
_ipv4 nf_defrag_ipv6 nf_defrag_ipv4 nf_conntrack_rtcache nf_conntrack iptable_raw iptable_mangle iptable_filter ip_tables ebtables ebt_vlan ebt_stp ebt_snat ebt_redirect ebt_pkttype ebt_mark
_m ebt_mark ebt_limit ebt_ip ebt_dnat ebt_arpreply ebt_arp ebt_among ebt_802_3 crc_ccitt compat ledtrig_usbdev xt_set ip_set_list_set ip_set_hash_netiface ip_set_hash_netport ip_set_hash_net
net ip_set_hash_net ip_set_hash_netportnet ip_set_hash_mac ip_set_hash_ipportnet ip_set_hash_ipportip ip_set_hash_ipport ip_set_hash_ipmark ip_set_hash_ip ip_set_bitmap_port ip_set_bitmap_ip
mac ip_set_bitmap_ip ip_set nfnetlink ip6t_REJECT nf_reject_ipv6 nf_log_ipv6 nf_log_common ip6table_raw ip6table_mangle ip6table_filter ip6_tables x_tables usb_storage uhci_hcd ohci_platform
 ohci_hcd ehci_platform ehci_hcd sd_mod scsi_mod gpio_button_hotplug usbcore nls_base usb_common
[  421.475042] CPU: 0 PID: 1421 Comm: rm Tainted: G        W       4.4.14 #8
[  421.481810] Stack : 803c81a4 00000000 00000001 80420000 83959180 80410e63 803a9890 0000058d
[  421.481810]    80483790 83b2a800 83573ee0 83573990 00000000 800a678c 803aeed4 80410000
[  421.481810]    00000003 83b2a800 803ad2e0 82ca7d1c 00000000 800a4758 82d274c8 00000000
[  421.481810]    8040ff50 801f2b00 00000000 00000000 00000000 00000000 00000000 00000000
[  421.481810]    00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[  421.481810]    ...
[  421.517209] Call Trace:
[  421.519663] [<8007197c>] show_stack+0x50/0x84
[  421.524042] [<80081580>] warn_slowpath_common+0xa0/0xd0
[  421.529247] [<80081634>] warn_slowpath_null+0x18/0x24
[  421.534244] [<8011b8a4>] drop_nlink+0x28/0x64
[  421.538613] [<8015d144>] jffs2_rmdir+0xa0/0xd8
[  421.543008] [<80111e10>] vfs_rmdir+0x84/0x11c
[  421.547399] [<80173018>] ovl_cleanup+0x48/0xa0
[  421.551859] [<80173ef8>] ovl_do_remove+0x3a4/0x428
[  421.556570] [<80111e10>] vfs_rmdir+0x84/0x11c
[  421.560900] [<80112930>] do_rmdir+0xfc/0x180
[  421.565148] [<8006295c>] syscall_common+0x30/0x54
[  421.569837] 
[  421.571293] ---[ end trace 782b40067d1ca4ff ]---
[  421.575915] jffs2_rmdir: dir_i->i_nlink is -1

```

The problem is caused by the incorrect handling of the parent inode's i_nlink count for the dentry to be RENAME_EXCHANGED. There are 3 cases to consider. Assume we want to RENAME_EXCHANGE struct dentry *a and struct dentry *b, and inode_a is pointed to by dentry_a, inode_b is pointed to by dentry_b:

1. If inode_a is a directory, but inode_b isn't, then we must decrease the i_nlink count of old_dir_i, and increase the i_nlink of new_dir_i.
2. If inode_a isn't a directory, but inode_b is a directory, then we must increase the i_nlink of old_dir_i, and decrease the i_nlink count of new_dir_i.
3. If the types of inode_a and inode_b are the same, we don't change the i_nlink for either old_dir_i or new_dir_i.

I have tested the code on openwrt:master, commit 851a890 and it fixes the issue.

S
jingqiuELE (1):
  Fix the incorrect i_nlink count after jffs2's RENAME_EXCHANGE
    operations.

 .../111-jffs2-add-RENAME_EXCHANGE-support.patch    | 37 ++++++++++++++++++----
 1 file changed, 30 insertions(+), 7 deletions(-)

-- 
2.7.4
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