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cleancache: forbid overriding cleancache_ops

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Currently, cleancache_register_ops returns the previous value of
cleancache_ops to allow chaining.  However, chaining, as it is
implemented now, is extremely dangerous due to possible pool id
collisions.  Suppose, a new cleancache driver is registered after the
previous one assigned an id to a super block.  If the new driver assigns
the same id to another super block, which is perfectly possible, we will
have two different filesystems using the same id.  No matter if the new
driver implements chaining or not, we are likely to get data corruption
with such a configuration eventually.

This patch therefore disables the ability to override cleancache_ops
altogether as potentially dangerous.  If there is already cleancache
driver registered, all further calls to cleancache_register_ops will
return EBUSY.  Since no user of cleancache implements chaining, we only
need to make minor changes to the code outside the cleancache core.

Signed-off-by: Vladimir Davydov <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: David Vrabel <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Stefan Hengelein <[email protected]>
Cc: Florian Schmaus <[email protected]>
Cc: Andor Daam <[email protected]>
Cc: Dan Magenheimer <[email protected]>
Cc: Bob Liu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Vladimir Davydov 2015-04-14 15:46:45 -07:00 • committed by Linus Torvalds
commit 53d85c9856
4 changed files with 18 additions and 17 deletions
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16
drivers/xen/tmem.c
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@ -397,13 +397,15 @@ static int __init xen_tmem_init(void)
#ifdef CONFIG_CLEANCACHE
BUG_ON(sizeof(struct cleancache_filekey) != sizeof(struct tmem_oid));
if (tmem_enabled && cleancache) {
char *s = "";
struct cleancache_ops *old_ops =
cleancache_register_ops(&tmem_cleancache_ops);
if (old_ops)
s = " (WARNING: cleancache_ops overridden)";
pr_info("cleancache enabled, RAM provided by Xen Transcendent Memory%s\n",
s);
int err;
err = cleancache_register_ops(&tmem_cleancache_ops);
if (err)
pr_warn("xen-tmem: failed to enable cleancache: %d\n",
err);
else
pr_info("cleancache enabled, RAM provided by "
"Xen Transcendent Memory\n");
}
#endif
#ifdef CONFIG_XEN_SELFBALLOONING