From e551954defe7f782597a2a96b5be9320c5658999 Mon Sep 17 00:00:00 2001
From: Yu Watanabe <watanabe.yu+github@gmail.com>
Date: Mon, 13 Mar 2023 23:28:58 +0900
Subject: [PATCH 1/2] udev: mention that the kernel silently truncates
 lo_file_name if too long

Resolves the confusion in https://github.com/systemd/systemd/pull/26693#discussion_r1131151335.
---
 src/udev/udev-builtin-blkid.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/udev/udev-builtin-blkid.c b/src/udev/udev-builtin-blkid.c
index 154cf7000f9..2b2205513b1 100644
--- a/src/udev/udev-builtin-blkid.c
+++ b/src/udev/udev-builtin-blkid.c
@@ -287,12 +287,14 @@ static int read_loopback_backing_inode(
 
         if (isempty((char*) info.lo_file_name) ||
             strnlen((char*) info.lo_file_name, sizeof(info.lo_file_name)-1) == sizeof(info.lo_file_name)-1)
-                /* Don't pick up file name if it is unset or possibly truncated. (Note: we can't really know
-                 * the file file name is truncated if it uses sizeof(info.lo_file_name)-1 as length; it could
-                 * also just mean the string is just that long and wasn't truncated — but the fact is simply
-                 * that we cannot know in that case if it was truncated or not, hence we assume the worst and
-                 * suppress — at least for now. For shorter strings we know for sure it wasn't truncated,
-                 * hence that's always safe.) */
+                /* Don't pick up file name if it is unset or possibly truncated. (Note: the kernel silently
+                 * truncates the string passed from userspace by LOOP_SET_STATUS64 ioctl. See
+                 * loop_set_status_from_info() in drivers/block/loop.c. Hence, we can't really know the file
+                 * name is truncated if it uses sizeof(info.lo_file_name)-1 as length; it could also mean the
+                 * string is just that long and wasn't truncated — but the fact is simply that we cannot know
+                 * in that case if it was truncated or not. Thus, we assume the worst and suppress — at least
+                 * for now. For shorter strings we know for sure it wasn't truncated, hence that's always
+                 * safe.) */
                 fn = NULL;
         else {
                 fn = memdup_suffix0(info.lo_file_name, sizeof(info.lo_file_name));

From 91ea97356c1799d7ac6b54713eb059c220f66d29 Mon Sep 17 00:00:00 2001
From: Yu Watanabe <watanabe.yu+github@gmail.com>
Date: Mon, 13 Mar 2023 23:36:32 +0900
Subject: [PATCH 2/2] test-50-dissect: add test for long reference name

---
 test/units/testsuite-50.sh | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/test/units/testsuite-50.sh b/test/units/testsuite-50.sh
index 741167a2154..af7d3511546 100755
--- a/test/units/testsuite-50.sh
+++ b/test/units/testsuite-50.sh
@@ -447,8 +447,28 @@ fi
 # Detach by loopback device
 systemd-dissect --detach "$LOOP"
 
+# Test long reference name.
+# Note, sizeof_field(struct loop_info64, lo_file_name) == 64,
+# and --loop-ref accepts upto 63 characters, and udev creates symlink
+# based on the name when it has upto _62_ characters.
+name="$(for (( i = 0; i < 62; i++ )); do echo -n 'x'; done)"
+LOOP="$(systemd-dissect --attach --loop-ref="$name" "${image}.raw")"
+udevadm trigger -w "$LOOP"
+
+# Check if the /dev/loop/by-ref/$name symlink really references the right device
+test "/dev/loop/by-ref/$name" -ef "$LOOP"
+
+# Detach by the /dev/loop/by-ref symlink
+systemd-dissect --detach "/dev/loop/by-ref/$name"
+
+name="$(for (( i = 0; i < 63; i++ )); do echo -n 'x'; done)"
+LOOP="$(systemd-dissect --attach --loop-ref="$name" "${image}.raw")"
+udevadm trigger -w "$LOOP"
+
+# Check if the /dev/loop/by-ref/$name symlink does not exist
+test ! -e "/dev/loop/by-ref/$name"
+
 # Detach by backing inode
-systemd-dissect --attach --loop-ref=waldo "${image}.raw"
 systemd-dissect --detach "${image}.raw"
 (! systemd-dissect --detach "${image}.raw")