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

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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>
This commit is contained in:
Jo-Philipp Wich 2024-02-28 11:51:00 +01:00 committed by Rafał Miłecki
parent 043da3fe5a
commit 08495b7f24
1 changed files with 71 additions and 0 deletions
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71
package/base-files/files/lib/upgrade/tar.sh Normal file
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# 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
}