#576 Incorrect absolute symbolic links upon extraction

[Mike]

Description

When extracting an archive/image using -snld and -o switches, the absolute symlinks are prefixed with the output directory.

In practice, if we extract a root filesystem from an image file, links are damaged in a chroot-ed environment. The logic is also ambiguous as we are explicitly allowing insecure symlinks with the -snld switch, and it doesn't rewrite insecure relative links.

Version

7-Zip 23.01 (arm64) : Copyright (c) 1999-2023 Igor Pavlov : 2023-06-20
 64-bit arm_v:8 locale=C.UTF-8 Threads:8 OPEN_MAX:1048576

Reproduction Path

mkdir -p rootfs/a rootfs/b
touch rootfs/a/1
ln -s /a/1 rootfs/b/2
ln -s ../a/1 rootfs/b/3
(cd rootfs && 7z -snl a ../rootfs.tar .)
7z -otest -snld x rootfs.tar 

The links are stored correctly, as we can see in the debug output:

# 7z -slt l rootfs.tar

7-Zip 23.01 (arm64) : Copyright (c) 1999-2023 Igor Pavlov : 2023-06-20
 64-bit arm_v:8 locale=C.UTF-8 Threads:8 OPEN_MAX:1048576

Scanning the drive for archives:
1 file, 3584 bytes (4 KiB)

Listing archive: rootfs.tar

--
Path = rootfs.tar
Type = tar
Physical Size = 3584
Headers Size = 3584
Code Page = UTF-8
Characteristics = GNU ASCII

----------
Path = a
Folder = +
Size = 0
Packed Size = 0
Modified = 2024-07-31 12:34:13
Created = 
Accessed = 
Mode = drwxr-xr-x
User = 
Group = 
User ID = 0
Group ID = 0
Symbolic Link = 
Hard Link = 
Characteristics = 5 GNU ASCII
Comment = 
Device Major = 
Device Minor = 

Path = a/1
Folder = -
Size = 0
Packed Size = 0
Modified = 2024-07-31 12:34:13
Created = 
Accessed = 
Mode = -rw-r--r--
User = 
Group = 
User ID = 0
Group ID = 0
Symbolic Link = 
Hard Link = 
Characteristics = 0 GNU ASCII
Comment = 
Device Major = 
Device Minor = 

Path = b
Folder = +
Size = 0
Packed Size = 0
Modified = 2024-07-31 12:34:13
Created = 
Accessed = 
Mode = drwxr-xr-x
User = 
Group = 
User ID = 0
Group ID = 0
Symbolic Link = 
Hard Link = 
Characteristics = 5 GNU ASCII
Comment = 
Device Major = 
Device Minor = 

Path = b/2
Folder = -
Size = 4
Packed Size = 0
Modified = 2024-07-31 12:34:13
Created = 
Accessed = 
Mode = lrwxrwxrwx
User = 
Group = 
User ID = 0
Group ID = 0
Symbolic Link = /a/1
Hard Link = 
Characteristics = 2 GNU ASCII
Comment = 
Device Major = 
Device Minor = 

Path = b/3
Folder = -
Size = 6
Packed Size = 0
Modified = 2024-07-31 12:34:13
Created = 
Accessed = 
Mode = lrwxrwxrwx
User = 
Group = 
User ID = 0
Group ID = 0
Symbolic Link = ../a/1
Hard Link = 
Characteristics = 2 GNU ASCII
Comment = 
Device Major = 
Device Minor = 

But in the extracted folder, the absolute link (test/b/2) is being rewritten:

# ls -la test/b
total 8
drwxr-xr-x 2 root root 4096 Jul 31 12:34 .
drwxr-xr-x 4 root root 4096 Jul 31 12:39 ..
lrwxrwxrwx 1 root root   13 Jul 31 12:39 2 -> /tmp/test/a/1
lrwxrwxrwx 1 root root    6 Jul 31 12:39 3 -> ../a/1

[Igor Pavlov]

Probably there were reasons for that scheme in 7-zip.
As I remember windows for wim stores links with some transformations. And I tried to follow that scheme in 7-Zip's WIM and then it TAR.

[Mike]

@ipavlov The logic makes sense for security considerations. The main concern is that it's expected to extract the links as they are without any transformations if the -snld switch is used.

7-Zip is an excellent Linux tool as it supports many different formats. It also has one feature that makes it stand out from its alternatives: support for Linux filesystems and image files. With 7-Zip, it's possible to extract those images in a rootless environment such as Docker. I've spent hours trying to find a way to extract images during the Docker image build without granting insecure privileges, and 7-Zip is the only reliable option available at the moment.

My point is that it would be great to be able to extract filesystems as they are with 7-Zip. So far, I found only two issues blocking seamless extraction in a rootless environment — this one and #577. Is there any chance we can consider those as bugs or feature requests?

[Igor Pavlov]

I don't plan to work for that features now. Maybe later.
You can fix it in source code and recompile binary for youself .