base-files: sysupgrade: add tar.sh with helpers for building archives

This allows building uncompressed tar archives from shell scripts (and
compressing them later if needed)

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
[rmilecki: adapt to sysupgrade needs]
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

diff --git a/package/base-files/files/lib/upgrade/tar.sh b/package/base-files/files/lib/upgrade/tar.sh
new file mode 100644 (file)
index 0000000..a9d1d55
--- /dev/null
+++ b/package/base-files/files/lib/upgrade/tar.sh
@@ -0,0 +1,71 @@
+# SPDX-License-Identifier: GPL-2.0-or-later OR MIT
+
+# Example usage:
+#
+# {
+#         tar_print_member "date.txt" "It's $(date +"%Y")"
+#         tar_print_trailer
+# } > test.tar
+
+__tar_print_padding() {
+       dd if=/dev/zero bs=1 count=$1 2>/dev/null
+}
+
+tar_print_member() {
+       local name="$1"
+       local content="$2"
+       local mtime="${3:-$(date +%s)}"
+       local mode=644
+       local uid=0
+       local gid=0
+       local size=${#content}
+       local type=0
+       local link=""
+       local username="root"
+       local groupname="root"
+
+       # 100 byte of padding bytes, using 0x01 since the shell does not tolerate null bytes in strings
+       local pad=$'\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1'
+
+       # validate name (strip leading slash if present)
+       name=${name#/}
+
+       # truncate string header values to their maximum length
+       name=${name:0:100}
+       link=${link:0:100}
+       username=${username:0:32}
+       groupname=${groupname:0:32}
+
+       # construct header part before checksum field
+       local header1="${name}${pad:0:$((100 - ${#name}))}"
+       header1="${header1}$(printf '%07d\1' $mode)"
+       header1="${header1}$(printf '%07o\1' $uid)"
+       header1="${header1}$(printf '%07o\1' $gid)"
+       header1="${header1}$(printf '%011o\1' $size)"
+       header1="${header1}$(printf '%011o\1' $mtime)"
+
+       # construct header part after checksum field
+       local header2="$(printf '%d' $type)"
+       header2="${header2}${link}${pad:0:$((100 - ${#link}))}"
+       header2="${header2}ustar  ${pad:0:1}"
+       header2="${header2}${username}${pad:0:$((32 - ${#username}))}"
+       header2="${header2}${groupname}${pad:0:$((32 - ${#groupname}))}"
+
+       # calculate checksum over header fields
+       local checksum=0
+       for byte in $(printf '%s%8s%s' "$header1" "" "$header2" | tr '\1' '\0' | hexdump -ve '1/1 "%u "'); do
+               checksum=$((checksum + byte))
+       done
+
+       # print member header, padded to 512 byte
+       printf '%s%06o\0 %s' "$header1" $checksum "$header2" | tr '\1' '\0'
+       __tar_print_padding 183
+
+       # print content data, padded to multiple of 512 byte
+       printf "%s" "$content"
+       __tar_print_padding $((512 - (size % 512)))
+}
+
+tar_print_trailer() {
+       __tar_print_padding 1024
+}