[PATCH] vfs: preserve subsecond timestamps at filesystem time limits

[Hungyu Lin <dennylin0707@gmail.com>]

timestamp_truncate() clears tv_nsec whenever the clamped seconds equal
either filesystem time limit. This also drops the fractional part of
timestamps whose seconds were already within the supported range.

For example, setting a JFS mtime to 0.123456789 produces 0.000000000,
although the filesystem can store the requested value. Utilities such
as touch and cp -p consequently lose valid subsecond timestamps when
preserving these endpoint values. Timestamps strictly between the
filesystem limits are unaffected.

Only clear tv_nsec when clamping actually changes tv_sec. Continue to
truncate in-range nanoseconds to the filesystem's timestamp granularity,
including when the seconds equal either endpoint.

Ben Hutchings suggested comparing clamped and original seconds in the
2019 utimes review. That caller was switched to timestamp_truncate(),
but the helper retained its endpoint equality check.

Link: https://lore.kernel.org/lkml/12c095e595836a7ff7f2c7b2a32cb5544dd29b55.camel@codethink.co.uk/
Fixes: 50e17c000c46 ("vfs: Add timestamp_truncate() api")
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 fs/inode.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index 1cb6293b237a..7f8e8feec4db 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2847,9 +2847,10 @@ struct timespec64 timestamp_truncate(struct timespec64 t, struct inode *inode)
 {
 	struct super_block *sb = inode->i_sb;
 	unsigned int gran = sb->s_time_gran;
+	time64_t seconds = t.tv_sec;
 
 	t.tv_sec = clamp(t.tv_sec, sb->s_time_min, sb->s_time_max);
-	if (unlikely(t.tv_sec == sb->s_time_max || t.tv_sec == sb->s_time_min))
+	if (unlikely(t.tv_sec != seconds))
 		t.tv_nsec = 0;
 
 	/* Avoid division in the common cases 1 ns and 1 s. */
-- 
2.43.0