package/base-files/files/lib/upgrade/tar.sh

   1 # SPDX-License-Identifier: GPL-2.0-or-later OR MIT
   2
   3 # Example usage:
   4 #
   5 # {
   6 #         tar_print_member "date.txt" "It's $(date +"%Y")"
   7 #         tar_print_trailer
   8 # } > test.tar
   9
  10 __tar_print_padding() {
  11         dd if=/dev/zero bs=1 count=$1 2>/dev/null
  12 }
  13
  14 tar_print_member() {
  15         local name="$1"
  16         local content="$2"
  17         local mtime="${3:-$(date +%s)}"
  18         local mode=644
  19         local uid=0
  20         local gid=0
  21         local size=${#content}
  22         local type=0
  23         local link=""
  24         local username="root"
  25         local groupname="root"
  26
  27         # 100 byte of padding bytes, using 0x01 since the shell does not tolerate null bytes in strings
  28         local pad=$'\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1'
  29
  30         # validate name (strip leading slash if present)
  31         name=${name#/}
  32
  33         # truncate string header values to their maximum length
  34         name=${name:0:100}
  35         link=${link:0:100}
  36         username=${username:0:32}
  37         groupname=${groupname:0:32}
  38
  39         # construct header part before checksum field
  40         local header1="${name}${pad:0:$((100 - ${#name}))}"
  41         header1="${header1}$(printf '%07d\1' $mode)"
  42         header1="${header1}$(printf '%07o\1' $uid)"
  43         header1="${header1}$(printf '%07o\1' $gid)"
  44         header1="${header1}$(printf '%011o\1' $size)"
  45         header1="${header1}$(printf '%011o\1' $mtime)"
  46
  47         # construct header part after checksum field
  48         local header2="$(printf '%d' $type)"
  49         header2="${header2}${link}${pad:0:$((100 - ${#link}))}"
  50         header2="${header2}ustar  ${pad:0:1}"
  51         header2="${header2}${username}${pad:0:$((32 - ${#username}))}"
  52         header2="${header2}${groupname}${pad:0:$((32 - ${#groupname}))}"
  53
  54         # calculate checksum over header fields
  55         local checksum=0
  56         for byte in $(printf '%s%8s%s' "$header1" "" "$header2" | tr '\1' '\0' | hexdump -ve '1/1 "%u "'); do
  57                 checksum=$((checksum + byte))
  58         done
  59
  60         # print member header, padded to 512 byte
  61         printf '%s%06o\0 %s' "$header1" $checksum "$header2" | tr '\1' '\0'
  62         __tar_print_padding 183
  63
  64         # print content data, padded to multiple of 512 byte
  65         printf "%s" "$content"
  66         __tar_print_padding $((512 - (size % 512)))
  67 }
  68
  69 tar_print_trailer() {
  70         __tar_print_padding 1024
  71 }