tests: don't use dd anymore for allocating empty test disk images (#40583)

Let's define a way how to mark codepaths that are subject to
deletion once the kernel baseline reaches a certain version, to make it
easier to find these cases.

WHile we are at it, introuce a whole section in CODING_STYLE about
kernel version compat.

I followed the new scheme in #39621, but we can merge the coding style
guidelines on this already.

docs/CODING_STYLE.md

@@ -1033,3 +1033,38 @@ SPDX-License-Identifier: LGPL-2.1-or-later
 - Never use `grep -q` in a pipeline, use `grep >/dev/null` instead. The former
   will generate `SIGPIPE` for the previous command in the pipeline when it finds
   a match which will cause the test to fail unexpectedly.
+
+## Kernel Version Dependencies
+
+- For entirely new functionality it's fine to rely on features of very recent
+  (released!) kernel versions. If a feature is added to the upstream kernel,
+  and a stable release is made, then it's immediately OK to merge *new*
+  functionality into systemd relying on it, as long as that functionality is
+  optional. (In some cases, it might be OK to merge a feature into systemd
+  slightly before the final kernel release that it is based on, as long as the
+  kernel development cycle has already progressed far enough that the feature
+  is unlikely to be still reverted – for example once RC2 of the kernel release
+  has been released.)
+
+- For components that already have been released in a stable version
+  compatibility with older kernels must be retained, down to the "minimum
+  baseline" version as listed in the README, or the version current when the
+  component was added to our tree, whichever is newer.
+
+- When adding a fallback path, please avoid checking for kernel versions, as
+  downstream distributions tend to backport features, and version checks are
+  not great replacements for feature checks hence.
+
+- When adding a compatibility code path for an older kernel version, please add
+  a comment in the following style to the relevant codepath:
+
+```c
+        // FIXME: This compatibility code path shall be removed once kernel X.Y
+        //        becomes the new minimal baseline
+```
+
+  When this syntax is followed we'll have an easier time tracking down these
+  codepaths and removing them when bumping baselines.
+
+- Whenever support for a new kernel API feature is added, please update the
+  kernel feature/version list in README as well (as part of the same PR).

src/login/inhibit.c

@@ -153,8 +153,6 @@ static int print_inhibitors(sd_bus *bus) {
                 if (r < 0)
                         return table_log_sort_error(r);
 
-                table_set_header(table, arg_legend);
-
                 r = table_print_with_pager(table, arg_json_format_flags, arg_pager_flags, arg_legend);
                 if (r < 0)
                         return r;

test/units/TEST-13-NSPAWN.machined.sh

@@ -190,7 +190,7 @@ test ! -d /var/lib/machines/.hidden1
 
 # Prepare a simple raw container
 mkdir -p /tmp/mnt
-dd if=/dev/zero of=/var/tmp/container.raw bs=1M count=256
+truncate -s 384M /var/tmp/container.raw
 mkfs.ext4 /var/tmp/container.raw
 mount -o loop /var/tmp/container.raw /tmp/mnt
 cp -r /var/lib/machines/container1/* /tmp/mnt

test/units/TEST-13-NSPAWN.nspawn.sh

@@ -77,7 +77,7 @@ testcase_sanity() {
     create_dummy_container "$template"
     # Create a simple image from the just created container template
     image="$(mktemp /var/lib/machines/TEST-13-NSPAWN.image-XXX.img)"
-    dd if=/dev/zero of="$image" bs=1M count=256
+    truncate -s 384M "$image"
     mkfs.ext4 "$image"
     mkdir -p /mnt
     mount -o loop "$image" /mnt
@@ -810,7 +810,7 @@ testcase_rootidmap() {
     root="$(mktemp -d /var/lib/machines/TEST-13-NSPAWN.rootidmap-path.XXX)"
     # Create ext4 image, as ext4 supports idmapped-mounts.
     mkdir -p /tmp/rootidmap/bind
-    dd if=/dev/zero of=/tmp/rootidmap/ext4.img bs=4k count=2048
+    truncate -s $((4096*2048)) /tmp/rootidmap/ext4.img
     mkfs.ext4 /tmp/rootidmap/ext4.img
     mount /tmp/rootidmap/ext4.img /tmp/rootidmap/bind
     trap "rootidmap_cleanup /tmp/rootidmap/" RETURN
@@ -854,7 +854,7 @@ testcase_owneridmap() {
     root="$(mktemp -d /var/lib/machines/TEST-13-NSPAWN.owneridmap-path.XXX)"
     # Create ext4 image, as ext4 supports idmapped-mounts.
     mkdir -p /tmp/owneridmap/bind
-    dd if=/dev/zero of=/tmp/owneridmap/ext4.img bs=4k count=2048
+    truncate -s $((4096*2048)) /tmp/owneridmap/ext4.img
     mkfs.ext4 /tmp/owneridmap/ext4.img
     mount /tmp/owneridmap/ext4.img /tmp/owneridmap/bind
     trap "owneridmap_cleanup /tmp/owneridmap/" RETURN

test/units/TEST-17-UDEV.sanity-check.sh

@@ -25,7 +25,7 @@ netdev=hoge
 ip link add $netdev type dummy
 
 blk="$(mktemp)"
-dd if=/dev/zero of="$blk" bs=1M count=1
+truncate -s 1M "$blk"
 loopdev="$(losetup --show -f "$blk")"
 
 # Wait for devices created in the above being processed.

test/units/TEST-50-DISSECT.sh

@@ -153,7 +153,7 @@ root_size="$(du --apparent-size -k "$MINIMAL_IMAGE.raw" | cut -f1)"
 verity_size="$(du --apparent-size -k "$MINIMAL_IMAGE.verity" | cut -f1)"
 signature_size=4
 # 4MB seems to be the minimum size blkid will accept, below that probing fails
-dd if=/dev/zero of="$MINIMAL_IMAGE.gpt" bs=512 count=$((8192+root_size*2+verity_size*2+signature_size*2))
+truncate -s $(((8192+root_size*2+verity_size*2+signature_size*2)*512)) "$MINIMAL_IMAGE.gpt"
 # sfdisk seems unhappy if the size overflows into the next unit, eg: 1580KiB will be interpreted as 1MiB
 # so do some basic rounding up if the minimal image is more than 1 MB
 if [[ "$root_size" -ge 1024 ]]; then

test/units/TEST-55-OOMD.sh

@@ -26,7 +26,7 @@ if systemd-detect-virt --vm --quiet; then
     if [[ "$(findmnt -n -o FSTYPE /)" == btrfs ]]; then
         btrfs filesystem mkswapfile -s 64M /swapfile
     else
-        dd if=/dev/zero of=/swapfile bs=1M count=64
+        fallocate -l 64M /swapfile
         chmod 0600 /swapfile
         mkswap /swapfile
     fi

test/units/TEST-58-REPART.sh

@@ -1708,7 +1708,7 @@ testcase_btrfs_compression() {
     # Must not be in tmpfs due to exclusions. It also must be large and
     # compressible so that the compression check succeeds later.
     src=/etc/test-source-file
-    dd if=/dev/zero of="$src" bs=1M count=1 2>/dev/null
+    fallocate -l 1M "$src"
 
     tee "$defs/btrfs-compressed.conf" <<EOF
 [Partition]

test/units/TEST-64-UDEV-STORAGE.sh

@@ -1013,7 +1013,7 @@ testcase_iscsi_lvm() {
     udevadm wait --settle --timeout=30 "${devices[0]}"
     mount "${devices[0]}" "$mpoint"
     for i in {1..4}; do
-        dd if=/dev/zero of="$mpoint/lun$i.img" bs=1M count=32
+        truncate -s 32M "$mpoint/lun$i.img"
     done
     # Initialize a new iSCSI target <$target_name> consisting of 4 LUNs, each
     # backed by a file

test/units/TEST-67-INTEGRITY.sh

@@ -48,8 +48,8 @@ EOF
 udevadm control --reload
 
 TMP_DIR="$(mktemp -d -t -p / integrity.tmp.XXXXXX)"
-dd if=/dev/zero of="${TMP_DIR}/image" bs=1048576 count=64
-dd if=/dev/zero of="${TMP_DIR}/data" bs=1048576 count=64
+truncate -s 64M "${TMP_DIR}/image"
+truncate -s 64M "${TMP_DIR}/data"
 LOOP="$(losetup --show -f "${TMP_DIR}/image")"
 udevadm wait --timeout=30 --settle "${LOOP}"
 

test/units/TEST-87-AUX-UTILS-VM.mount.sh

@@ -43,7 +43,7 @@ test -e "$WORK_DIR/upper/foo"
 systemd-umount "$WORK_DIR/overlay"
 
 # Set up a simple block device for further tests
-dd if=/dev/zero of="$WORK_DIR/simple.img" bs=1M count=16
+truncate -s 16M "$WORK_DIR/simple.img"
 mkfs.ext4 -L sd-mount-test "$WORK_DIR/simple.img"
 LOOP="$(losetup --show --find "$WORK_DIR/simple.img")"
 udevadm wait --timeout=60 --settle "$LOOP"