[RFC][PATCH] linux/uuid.h: hoist uuid_is_null() helper from libnvdimm

[Amir Goldstein <amir73il@gmail.com>]

We need a helper for VFS to check if struct super_block field s_uuid
was filled by the filesystem, before consumers can use it.

The libnvdimm code already has a private helper to check if uuid is null
and the helper name is not using a private namespace prefix, which
prevents us from using the same helper name as a common function.

Hoist the libnvdimm helper as an inline helper to linux/uuid.h.

Suggested-by: Christoph Hellwig <hch@lst.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 drivers/nvdimm/btt_devs.c | 8 +-------
 include/linux/uuid.h      | 7 +++++++
 2 files changed, 8 insertions(+), 7 deletions(-)

Christoph,

Following your suggestion to provide a helper for checking if filesystem
had filled sb->s_uuid, here is a patch to add that helper.

I considered hoisting xfs's uuid_is_nil() helper and uuid_t to uuid.h, but
that would have been more painful, so just moved this simple helper instead.
We can continue debating which implementation is better, but that would be
futile...

If in the future xfs uuid table code is going to be moved to VFS, we should
probably move variants of the xfs/uuid.c functions to lib/uuid.c if those
variants don't already exist.

CC'ing the maintainers of clearcache and EVM/IMA to see if this helper
should be added (as sanity?) to their code before accessing sb->s_uuid.

I truely hope that EVM/IMA signatures do not expect xfs/ubifs to keep
exporting null s_uuid, because if they do, then starting to export s_uuid
for xfs/ubifs may require a new config/mkfs/mount option and that would
be a shame.

Cheers,
Amir.

diff --git a/drivers/nvdimm/btt_devs.c b/drivers/nvdimm/btt_devs.c
index 97dd292..d0fcda2 100644
--- a/drivers/nvdimm/btt_devs.c
+++ b/drivers/nvdimm/btt_devs.c
@@ -17,6 +17,7 @@
 #include <linux/slab.h>
 #include <linux/fs.h>
 #include <linux/mm.h>
+#include <linux/uuid.h>
 #include "nd-core.h"
 #include "btt.h"
 #include "nd.h"
@@ -222,13 +223,6 @@ struct device *nd_btt_create(struct nd_region *nd_region)
 	return dev;
 }
 
-static bool uuid_is_null(u8 *uuid)
-{
-	static const u8 null_uuid[16];
-
-	return (memcmp(uuid, null_uuid, 16) == 0);
-}
-
 /**
  * nd_btt_arena_is_valid - check if the metadata layout is valid
  * @nd_btt:	device with BTT geometry and backing device info
diff --git a/include/linux/uuid.h b/include/linux/uuid.h
index 4dff73a..d3f8656 100644
--- a/include/linux/uuid.h
+++ b/include/linux/uuid.h
@@ -58,6 +58,13 @@ static inline int uuid_be_cmp(const uuid_be u1, const uuid_be u2)
 	return memcmp(&u1, &u2, sizeof(uuid_be));
 }
 
+static inline bool uuid_is_null(u8 *uuid)
+{
+	static const u8 null_uuid[16];
+
+	return (memcmp(uuid, null_uuid, 16) == 0);
+}
+
 void generate_random_uuid(unsigned char uuid[16]);
 
 extern void uuid_le_gen(uuid_le *u);
-- 
2.7.4