Support subordinate user and group ID ranges on enterprise set-ups

[debarshiray]

On enterprise FreeIPA set-ups, the subordinate user and group IDs are provided by SSSD's sss plugin for the GNU Name Service Switch (or NSS) functionality of the GNU C Library. They are not listed in /etc/subuid and /etc/subgid. Therefore, its necessary to use libsubid.so to check the subordinate ID ranges.

The CGO interaction with libsubid.so is loosely based on 'readSubid' in github.com/containers/storage/pkg/idtools [1].

However, unlike 'readSubid', this code considers the absence of any range (ie., nRanges == 0) to be an error as well.

More importantly, this code uses dlopen(3) and friends to dynamically load the symbols from libsubid.so, instead of linking to libsubid.so at build-time and having the dependency noted in the /usr/bin/toolbox binary. This is done because libsubid.so itself depends on several other shared libraries, and indirect dependencies can't be influenced by the RUNPATH [2] embedded in the /usr/bin/toolbox binary [3]. Hence, when the binary is used inside Toolbx containers (eg., as the entry point), those indirect dependencies won't be picked from the host's runtime against which the binary was built. This can render the binary useless due to ABI compatibility issues. Using dlopen(3) avoids this problem, especially because libsubid.so is only used when running on the host.

Care was taken to not load and link libsubid.so twice to separately validate the subordinate ID ranges for the user and the group. Note that libsubid_init() must be passed a FILE pointer for logging. Otherwise, it will create it's own for logging, and there's no way to close it during dlclose(3).

Version 4 of the libsubid.so API/ABI [4] was released in Shadow 4.10, which is newer than the versions shipped on RHEL 8 and Debian 10 [5], and even that newer version had some problems [6]. Therefore, support for older versions, with the relevant workarounds, is necessary. Fortunately, the oldest that needs to be support is Shadow 4.9 because that's when libsubid.so was introduced [7].

Note that SUBID_ABI_VERSION was only introduced with version 4 of the libsubid.so API/ABI released in Shadow 4.10 [8]. The first release of libsubid.so in Shadow 4.9 already had an ABI version of 3.0.0 [9], since it was bumped a few times during development, so that's what's assumed when SUBID_ABI_VERSION is absent.

This code doesn't set the public variables Prog and shadow_logfd that older Shadow versions used to expect for logging, because from Shadow 4.9 onwards there's a separate function [4,10] to specify these. This can be changed if there are libsubid.so versions in the wild that really do need those public variables to be set.

Finally, ISO C99 is required because of the use of <stdbool.h> in the libsubid.so API.

Some changes by Debarshi Ray.

[1] https://github.com/containers/storage/blob/main/pkg/idtools/idtools_supported.go

[2] https://man7.org/linux/man-pages/man8/ld.so.8.html

[3] Commit 6063eb2
#821

[4] Shadow commit 32f641b207f6ddff
shadow-maint/shadow@32f641b207f6ddff
shadow-maint/shadow#443

[5] https://packages.debian.org/source/buster/shadow

[6] Shadow commit 79157cbad87f42cd
shadow-maint/shadow@79157cbad87f42cd
shadow-maint/shadow#465

[7] Shadow commit 0a7888b1fad613a0
shadow-maint/shadow@0a7888b1fad613a0
shadow-maint/shadow#154

[8] Shadow commit 0c9f64140852e8d5
shadow-maint/shadow@0c9f64140852e8d5
shadow-maint/shadow#449

[9] Shadow commit 3d670ba7ed58f910
shadow-maint/shadow@3d670ba7ed58f910
shadow-maint/shadow#339

[10] Shadow commit 2b22a6909dba60d
shadow-maint/shadow@2b22a6909dba60d
shadow-maint/shadow#325

#1074

Signed-off-by: Martin Jackson [email protected]

[softwarefactory-project-zuul]

Build succeeded.

✔️ unit-test SUCCESS in 8m 43s
✔️ unit-test-migration-path-for-coreos-toolbox SUCCESS in 3m 03s
✔️ system-test-fedora-rawhide SUCCESS in 12m 58s
✔️ system-test-fedora-36 SUCCESS in 12m 16s
✔️ system-test-fedora-35 SUCCESS in 13m 42s

[debarshiray]

See #1180 for the whole story.