diff mbox series

[V2,1/3] base-files: sysupgrade: add tar.sh with helpers for building archives

Message ID 20240228080304.22671-1-zajec5@gmail.com
State Changes Requested
Delegated to: Rafał Miłecki
Headers show
Series [V2,1/3] base-files: sysupgrade: add tar.sh with helpers for building archives | expand

Commit Message

Rafał Miłecki Feb. 28, 2024, 8:03 a.m. UTC 
From: Jo-Philipp Wich <jo@mein.io>

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

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
---
V2: Simplify dd in __tar_print_padding (I still think helper is useful)
    Hardcode 0/0/ root/root for now as most likely it'll be enough
    Simplify name validation (leasing slash)
    Reorder some variables

 package/base-files/files/lib/upgrade/tar.sh | 78 +++++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 package/base-files/files/lib/upgrade/tar.sh

Comments

Jo-Philipp Wich Feb. 28, 2024, 8:51 a.m. UTC | #1 
Hi Rafał,

comments inline. Sorry for the bikeshedding ahead.

~ Jo

> [...]
> +
> +__tar_print_padding() {
> +	dd if=/dev/zero bs=$1 count=1 2>/dev/null

$1 may be 0 which is an invalid value for `bs=`:

   root@OpenWrt:~# dd bs=0
   dd: number 0 is not in 1..2147483647 range

A value of "0" is valid for `count=` though:

   root@OpenWrt:~# dd bs=1 count=0
   0+0 records in
   0+0 records out

So either revert to the previous version or hardcode `bs` to 1, pass the
desired size via `count=` and live with slightly less efficiency (which likely
does not matter at all).

> +}
> +
> +__tar_make_member() {
> +	local name="$1"
> +	local content="$2"

If you make this `local mtime=${3:-$(date +%s)}` you can rename this function 
to `tar_make_member_inline()` and drop the extra wrapper.

> +	local mtime="$3"
> +	local mode=644
> [...]
> +}
> +
> +tar_make_member_from_file() {

This `tar_make_member_from_file()` wrapper is unused and very trivial, the
only thing it adds is defaulting the mtime value to the mtime of the given
file path. I suggest to drop `tar_make_member_from_file()` entirely and to
rename  `tar_make_member_inline()` below to simply `tar_make_member()`.

Maybe also consider renaming it to `tar_print_member()` or
`tar_output_member()` since that is what it does.

> +	local name="$1"
> +
> +	__tar_make_member "$name" "$(cat $name)" "$(date +%s -r "$1")"
> +}
> +
> +tar_make_member_inline() {
> +	local name="$1"
> +	local content="$2"
> +	local mtime="${3:-$(date +%s)}"
> +
> +	__tar_make_member "$name" "$content" "$mtime"
> +}
> +

Analogous to the suggested name change above, the `tar_close()` function 
should probably be renamed to `tar_print_trailer()` or `tar_output_trailer()`
as well.

> +tar_close() {
> +	__tar_print_padding 1024
> +}
diff mbox series

Patch

diff --git a/package/base-files/files/lib/upgrade/tar.sh b/package/base-files/files/lib/upgrade/tar.sh
new file mode 100644
index 0000000000..406d5fd71b
--- /dev/null
+++ b/package/base-files/files/lib/upgrade/tar.sh
@@ -0,0 +1,78 @@ 
+# SPDX-License-Identifier: GPL-2.0-or-later OR MIT
+
+__tar_print_padding() {
+	dd if=/dev/zero bs=$1 count=1 2>/dev/null
+}
+
+__tar_make_member() {
+	local name="$1"
+	local content="$2"
+	local mtime="$3"
+	local mode=644
+	local uid=0
+	local gid=0
+	local size=${#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_make_member_from_file() {
+	local name="$1"
+
+	__tar_make_member "$name" "$(cat $name)" "$(date +%s -r "$1")"
+}
+
+tar_make_member_inline() {
+	local name="$1"
+	local content="$2"
+	local mtime="${3:-$(date +%s)}"
+
+	__tar_make_member "$name" "$content" "$mtime"
+}
+
+tar_close() {
+	__tar_print_padding 1024
+}