[RFC 1/6] vfs: Add file timestamp range support

[Deepa Dinamani <deepa.kernel@gmail.com>]

Add fields to the superblock to track the min and max
timestamps supported by filesystems.

Initially, when a superblock is allocated, initialize
it to the max and min values the fields can hold.
Individual filesystems override these to match their
actual limits.

Pseudo filesystems are assumed to always support the
min and max allowable values for the fields.

Note that the time ranges are save in type time64_t
rather than time_t.
This is required because if we save ranges in time_t
then we would not be able to save timestamp ranges
for files that support timestamps beyond y2038.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
---
 fs/libfs.c             | 4 ++++
 fs/super.c             | 2 ++
 include/linux/fs.h     | 3 +++
 include/linux/time64.h | 2 ++
 4 files changed, 11 insertions(+)

diff --git a/fs/libfs.c b/fs/libfs.c
index 48826d4..f03c904 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -256,6 +256,8 @@ struct dentry *mount_pseudo_xattr(struct file_system_type *fs_type, char *name,
 	s->s_op = ops ? ops : &simple_super_operations;
 	s->s_xattr = xattr;
 	s->s_time_gran = 1;
+	s->s_time_min = TIME64_MIN;
+	s->s_time_max = TIME64_MAX;
 	root = new_inode(s);
 	if (!root)
 		goto Enomem;
@@ -515,6 +517,8 @@ int simple_fill_super(struct super_block *s, unsigned long magic,
 	s->s_magic = magic;
 	s->s_op = &simple_super_operations;
 	s->s_time_gran = 1;
+	s->s_time_min = TIME64_MIN;
+	s->s_time_max = TIME64_MAX;
 
 	inode = new_inode(s);
 	if (!inode)
diff --git a/fs/super.c b/fs/super.c
index c183835..27c973e 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -248,6 +248,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 	s->s_maxbytes = MAX_NON_LFS;
 	s->s_op = &default_op;
 	s->s_time_gran = 1000000000;
+	s->s_time_min = TIME64_MIN;
+	s->s_time_max = TIME64_MAX;
 	s->cleancache_poolid = CLEANCACHE_NO_POOL;
 
 	s->s_shrink.seeks = DEFAULT_SEEKS;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 16d2b6e..6d1346b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1380,6 +1380,9 @@ struct super_block {
 	/* Granularity of c/m/atime in ns.
 	   Cannot be worse than a second */
 	u32		   s_time_gran;
+	/* Time limits for c/m/atime in seconds. */
+	time64_t           s_time_min;
+	time64_t           s_time_max;
 
 	/*
 	 * The next field is for VFS *only*. No filesystems have any business
diff --git a/include/linux/time64.h b/include/linux/time64.h
index 980c71b..25433b18 100644
--- a/include/linux/time64.h
+++ b/include/linux/time64.h
@@ -38,6 +38,8 @@ struct itimerspec64 {
 
 /* Located here for timespec[64]_valid_strict */
 #define TIME64_MAX			((s64)~((u64)1 << 63))
+#define TIME64_MIN			(-TIME64_MAX - 1)
+
 #define KTIME_MAX			((s64)~((u64)1 << 63))
 #define KTIME_SEC_MAX			(KTIME_MAX / NSEC_PER_SEC)
 
-- 
2.7.4