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Comment by clnhlzmn | Hacker News Reader

Would it be reasonable to expect that this MR comes along with a test that shows that it does the thing it’s claiming to do? I’m not sure how that would work in this case.. have a test that is run on a system that is known to have landlock that does something to ensure that it’s enabled? Even that could be subverted, but it seems like demanding that kind of thing before merging “features” is a good step.

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I like the idea of testing build-system behaviors like this, and I don’t think it’s ever really done in practice. Scriptable build systems, for lack of a better name for them, exist at a bad intersection of Turing complete, hard to test different cases, hard to reason about, hard to read the build script code, and most of us treating them as “ugh I hope all this stuff works” and if it does “thank god I get to ignore all this stuff for another 6 months”.

If you mean testing the "disable Landlock if the headers and syscalls are out of sync" functionality then I agree, workarounds for such corner cases are often not fully tested.

But it would have been enough here to have a test just to see that Landlock works in general. That test would have broken with this commit, because that's what the commit actually does - break all Landlock support.

Based on that it sounds like there wasn't a test for Landlock integration, if I've understood things correctly.

Create a tiny fake version of landlock with just the features you're testing for. Since it's only checking for 4 #defines in 3 header files, that's easy.

Then compile your test program against your fake header files (with -Imy-fake-includes). It should compile without errors even if landlock is missing from your actual system.

Then build your test program a second time, this time against the real system headers, to test whether landlock is supported on your system.

I'd say this MR is a bad approach in general. The headers say what interfaces are known, not what features are available. You should be able to compile with landlock support on a system which doesn't enable it. Same situation as seccomp and others. Your build machine doesn't have to match the capabilities of the target runner.

But yeah, to test it, you can have a mock version of landlock which responds with the error/success as you want, regardless of what the system would normally do. It relies on the test not being sabotaged too though...

Read the code of the check again. It mostly checks that the required SYS_* constants are defined to be able to use the syscalls. You can compile this on a system that does not have landlock enabled in the running kernel, but the libc (which imports the kernel system call interface) has to provide the syscall numbers.

You're right. I didn't see SYS... symbols being actually used, but they are: https://git.tukaani.org/?p=xz.git;a=blob;f=src/xz/sandbox.c;...

This doesn't change my opinion in general - that version should be exposed through a library call and knowing about the specific syscalls shouldn't be needed in xv.

I see your point, but suggesting adding an additional library dependency while we're discussing a supply chain attack is quite ironic.

Should've said function call not library call. My bad. Basically if you already have the linux/landlock.h, that should provide everything you need to do without explicit references to SYS...

Now we are running in circles. As you see in the git commit, the compile check was added because the existance of linux/landlock.h alone was not enough to check that the feature can be used.

This header defines the data types for the Linux kernel interface, but not how the syscall landlock_create_ruleset(2) will be issued. That is provided by libc either as a separate wrapper function (does not exist in glibc) or the implementation needs to use syscall(SYS_landlock_create_ruleset, ...), with the constant also being provided by libc. That is how it works for all syscalls and you won't be able to change this.

The only source of the claim that the existence of.linux/landlock.h is insufficient is (AFAICT) the malicious git commit. Why trust the comment, written by the attacker, to explain away a malicious change?

I already explained above why the existence of linux/landlock.h is not sufficient. Why do you still question it? If you know a bit about system programming and how configure checks work, the change in itself is totally reasonable.

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