lib/systems: strip kernel to avoid reference cycles

[ghost]

Description of changes

lib/systems: strip kernel to avoid reference cycles

Since #221707 we must ensure that the image resulting from a kernel build is stripped. If we do not do this, there will be circular dependencies among its outpaths.

This commit fixes the breakage on mips.

Closes #244438

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[vkleen]

It turns out that I can't actually get the PowerPC zImage to boot with kexec. Sorry for the half-baked suggestion... Unfortunately, for reasons that I don't understand, doing the same stripping on vmlinux as you're doing for MIPS doesn't seem to be enough to get rid of cyclic references.

Stupid workaround: the linux kernel build produces a vmlinux.strip.gz; if we set the platform kernel target to that instead of vmlinux, this might sidestep the issue.

[vkleen]

Further testing: setting KBUILD_IMAGE=vmlinux.strip.gz and getting rid of the extra makeFlags produces a bootable image. At least after manually gunziping, either in the bootloader (which for my machine is a full Linux system in itself) or before.

[ghost]

It turns out that I can't actually get the PowerPC zImage to boot with kexec. Sorry for the half-baked suggestion... Unfortunately, for reasons that I don't understand, doing the same stripping on vmlinux as you're doing for MIPS doesn't seem to be enough to get rid of cyclic references.

MIPS kernels are actually fully ELF-compliant images, so a lot of stuff is easier there.

[ghost]

I'm running test builds of this now...

[ghost]

@alyssais wrote

This is what I was trying to avoid — I'd rather have the stripping done by the kernel build system where it supports it, because they'll do a better job of setting the right flags, I think.

I know that, stylistically, you were trying to avoid doing things this way, but no other solutions for the breakage caused by #221707 have arisen. I can't get anything else to work and I have run out of time.

[alyssais]

(Originally posted here by mistake.)

I've done some investigation of my own. Here are the results:

PowerPC

PowerPC ends up with two sets of references. One is from kexec purgatory, and is probably an oversight. I expect I could upstream a patch to fix this, like I recently did for x86 and RISC-V¹. The other is from the VDSO. Unlike every other architecture, PowerPC deliberately uses a non-stripped vdso, apparently because it made perf traces nicer. So unless things I've changed there (I'll try to have a look — it seems a bit strange that PowerPC would be special here), we won't be able to get the kernel build system to give us a reference-free PowerPC image. So even though I don't like it, ack to stripping manually on PowerPC.

MIPS

MIPS offers vmlinux, vmlinuz, and uImage. vmlinuz and uImage should both be reference-free, but I don't know enough about how MIPS boots to know whether either of those would be sensible default targets. I'd appreciate your insight here. If it's not feasible to change to either of those images, then we probably have to strip manually on MIPS too.

Footnotes

1. the only thing getting in the way here is that it's a bit annoying to test, because our kexec test is currently broken. cc @RaitoBezarius. ↩

[ghost]

MIPS offers vmlinux, vmlinuz, and uImage. vmlinuz and uImage should both be reference-free, but I don't know enough about how MIPS boots to know whether either of those would be sensible default targets.

make vmlinuz requires CONFIG_SYS_SUPPORTS_ZBOOT, which isn't supported by all mips machines. Specifically Cavium Octeon routers, the most popular mips64 machines, don't have CONFIG_SYS_SUPPORTS_ZBOOT (the vendor-supplied bootloader is simply awful).

So make vmlinuz gives you:

CONFIG_SYS_SUPPORTS_ZBOOT is not enabled

I tried make uImage but it appears that is the same as make vmlinux, plus perhaps additional steps afterwards. It's also uboot-specific?

So even though I don't like it, ack to stripping manually on PowerPC.

@alyssais would you please update your review to reflect this? It still shows the big-red-changes-requested-of-doom.

[ghost]

@ofborg eval

[ghost]

Rebased.

@alyssais said "If it's not feasible to change to either of those images, then we probably have to strip manually on MIPS too." and that is the case #228018 (comment)

[ghost]

From #194153, which includes this PR:

pkgsCross.octeon.linux on x86_64-linux — Success Details

[ghost]

I have moved the powerpc fix into

#244436

[ghost]

Rebased.

It seems that ofborg-eval is on vacation today.