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Support xattrs in the build sandbox #838

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opened 2025-05-22 23:21:28 +00:00 by jade · 0 comments
jade commented 2025-05-22 23:21:28 +00:00
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I found out today that we block xattrs usage with seccomp on Linux: https://git.lix.systems/lix-project/lix/src/5ca168f6f2dfda3aa047386de6d8733fcddff1bd/lix/libstore/platform/linux.cc#L701-L706

This is not ideal and we probably want to allow it especially if we do #298, since it will prevent running our own test suite inside the nix sandbox.

The reason that xattrs are blocked in the first place is that they aren't representable in NARs so you could have local builds with different results than substitutions. There's a couple of issues with the way this is implemented:

  • It blocks xattr usage in the build directory unnecessarily by using too big a hammer to try to block them landing in derivation outputs. It's not easy to block xattrs just on build directories, but:
    • We could block setting xattrs via something like bindfs's option to make them readonly and use that on input-addressed derivations' output paths https://bindfs.org/docs/bindfs.1.html
    • We could scan outputs for xattrs and either fail the build or delete them; this is likely to have some performance cost, but it's unclear if scanning this is that expensive (and we already do have to do a ref-scanning pass anyway!).
  • It turns out that for lol lmao reasons setting xattrs on build output is not actually blocked on macOS at all, today. It might well just be Fine and Ignorable even if Not Ideal!
$ nix-build --expr 'with import <nixpkgs> {}; runCommand "meow" {} "touch $out; /usr/bin/xattr -w meow kitty $out"'
this derivation will be built:
  /nix/store/pwinv2bgjb5zdv0jx58l08chfvp3hw7y-meow.drv
building '/nix/store/pwinv2bgjb5zdv0jx58l08chfvp3hw7y-meow.drv'...
/nix/store/gh1m65y27b93afxsrfqw9d7dkxja060v-meow

$ xattr -r /nix/store/gh1m65y27b93afxsrfqw9d7dkxja060v-meow
/nix/store/gh1m65y27b93afxsrfqw9d7dkxja060v-meow: meow

So like, you know...

I found out today that we block xattrs usage with seccomp on Linux: https://git.lix.systems/lix-project/lix/src/5ca168f6f2dfda3aa047386de6d8733fcddff1bd/lix/libstore/platform/linux.cc#L701-L706 This is not ideal and we probably want to allow it especially if we do https://git.lix.systems/lix-project/lix/issues/298, since it will prevent running our own test suite inside the nix sandbox. The reason that xattrs are blocked in the first place is that they aren't representable in NARs so you could have local builds with different results than substitutions. There's a couple of issues with the way this is implemented: - It blocks xattr usage in the build directory unnecessarily by using too big a hammer to try to block them landing in derivation outputs. It's not *easy* to block xattrs just on build directories, but: - We could block setting xattrs via something like bindfs's option to make them readonly and use that on input-addressed derivations' output paths https://bindfs.org/docs/bindfs.1.html - We could scan outputs for xattrs and either fail the build or delete them; this is likely to have some performance cost, but it's unclear if scanning this is that expensive (and we already do have to do a ref-scanning pass anyway!). - It turns out that for lol lmao reasons *setting xattrs on build output* is not actually blocked on macOS at all, today. It might well just be Fine and Ignorable even if Not Ideal! ``` $ nix-build --expr 'with import <nixpkgs> {}; runCommand "meow" {} "touch $out; /usr/bin/xattr -w meow kitty $out"' this derivation will be built: /nix/store/pwinv2bgjb5zdv0jx58l08chfvp3hw7y-meow.drv building '/nix/store/pwinv2bgjb5zdv0jx58l08chfvp3hw7y-meow.drv'... /nix/store/gh1m65y27b93afxsrfqw9d7dkxja060v-meow $ xattr -r /nix/store/gh1m65y27b93afxsrfqw9d7dkxja060v-meow /nix/store/gh1m65y27b93afxsrfqw9d7dkxja060v-meow: meow ``` So like, you know...
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Reference: lix-project/lix#838
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