[bug report] CIFS: Respect epoch value from create lease context v2

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

[ This code is 10 years old now.  This is from an unreleased Smatch
  check.  We moved the files around recently so these showed up as new
  warnings and I was just curious what was going on. - dan ]

The patch 42873b0a282a: "CIFS: Respect epoch value from create lease
context v2" from Sep 5, 2013, leads to the following Smatch static
checker warning:

fs/smb/client/smb2ops.c:4106 smb3_set_oplock_level() warn: unsigned subtraction: 'epoch - cinode->epoch' use '!='
fs/smb/client/smb2ops.c:4115 smb3_set_oplock_level() warn: unsigned subtraction: 'epoch - cinode->epoch' use '!='
fs/smb/client/smb2ops.c:4119 smb3_set_oplock_level() warn: unsigned subtraction: 'epoch - cinode->epoch' use '!='

fs/smb/client/smb2ops.c
    4095 static void
    4096 smb3_set_oplock_level(struct cifsInodeInfo *cinode, __u32 oplock,
    4097                       unsigned int epoch, bool *purge_cache)
    4098 {
    4099         unsigned int old_oplock = cinode->oplock;
    4100 
    4101         smb21_set_oplock_level(cinode, oplock, epoch, purge_cache);
    4102 
    4103         if (purge_cache) {
    4104                 *purge_cache = false;
    4105                 if (old_oplock == CIFS_CACHE_READ_FLG) {
--> 4106                         if (cinode->oplock == CIFS_CACHE_READ_FLG &&
    4107                             (epoch - cinode->epoch > 0))

"epoch" is zero for a new file.  I guess for an existing file they
would be the modification time or something?  These values are unsigned
so this is the equivalent of:

	if (epoch != cinode->epoch)

Do we care about greater than less than or just not equal?

    4108                                 *purge_cache = true;
    4109                         else if (cinode->oplock == CIFS_CACHE_RH_FLG &&
    4110                                  (epoch - cinode->epoch > 1))
    4111                                 *purge_cache = true;
    4112                         else if (cinode->oplock == CIFS_CACHE_RHW_FLG &&
    4113                                  (epoch - cinode->epoch > 1))
    4114                                 *purge_cache = true;
    4115                         else if (cinode->oplock == 0 &&
    4116                                  (epoch - cinode->epoch > 0))

same

    4117                                 *purge_cache = true;
    4118                 } else if (old_oplock == CIFS_CACHE_RH_FLG) {
    4119                         if (cinode->oplock == CIFS_CACHE_RH_FLG &&
    4120                             (epoch - cinode->epoch > 0))

same

    4121                                 *purge_cache = true;
    4122                         else if (cinode->oplock == CIFS_CACHE_RHW_FLG &&
    4123                                  (epoch - cinode->epoch > 1))
    4124                                 *purge_cache = true;
    4125                 }
    4126                 cinode->epoch = epoch;
    4127         }
    4128 }

regards,
dan carpenter