Fix truncate(2) mtime and ctime handling

On Linux, ftruncate(2) always changes the file timestamps, even if the
file size is not changed. However, in case of a successfull
truncate(2), the timestamps are updated only if the file size changes.
This translates to the VFS calling the ZFS Posix Layer "setattr"
function (zpl_setattr) with ATTR_MTIME and ATTR_CTIME unconditionally
set on the iattr mask only when doing a ftruncate(2), while the
truncate(2) is left to the filesystem implementation to be dealt with.

This behaviour is consistent with POSIX:2004/SUSv3 specifications
where there's no explicit requirement for file size changes to update
the timestamps only for ftruncate(2):

http://pubs.opengroup.org/onlinepubs/009695399/functions/truncate.html
http://pubs.opengroup.org/onlinepubs/009695399/functions/ftruncate.html

This has been later updated in POSIX:2008/SUSv4 where, for both
truncate(2)/ftruncate(2), there's no mention of this size change
requirement:

http://austingroupbugs.net/view.php?id=489
http://pubs.opengroup.org/onlinepubs/9699919799/functions/truncate.html
http://pubs.opengroup.org/onlinepubs/9699919799/functions/ftruncate.html

Unfortunately the Linux VFS is still calling into the ZPL without
ATTR_MTIME/ATTR_CTIME set in the truncate(2) case: we fix this by
explicitly updating the timestamps when detecting the ATTR_SIZE bit,
which is always set in do_truncate(), on the iattr mask.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: loli10K <[email protected]>
Closes #6811 
Closes #6819 

1 files changed, 30 insertions, 0 deletions

diff --git a/tests/zfs-tests/include/math.shlib b/tests/zfs-tests/include/math.shlib
index 4ed11fb4b..66658cdda 100644
--- a/tests/zfs-tests/include/math.shlib
+++ b/tests/zfs-tests/include/math.shlib
@@ -66,3 +66,33 @@ function to_bytes
 
 	return 0
 }
+
+#
+# Verify $a is equal to $b, otherwise raise an error specifying
+# the $type of values being compared
+#
+function verify_eq # <a> <b> <type>
+{
+	typeset a=$1
+	typeset b=$2
+	typeset type=$3
+
+	if [[ $a -ne $b ]]; then
+		log_fail "Compared $type should be equal: $a != $b"
+	fi
+}
+
+#
+# Verify $a is not equal to $b, otherwise raise an error specifying
+# the $type of values being compared
+#
+function verify_ne # <a> <b> <type>
+{
+	typeset a=$1
+	typeset b=$2
+	typeset type=$3
+
+	if [[ $a -eq $b ]]; then
+		log_fail "Compared $type should be not equal: $a == $b"
+	fi
+}