83 lines
4 KiB
Markdown
83 lines
4 KiB
Markdown
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# What is this for?
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NixOS's traditional initrd is generated by listing the paths that
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should be included in initrd and copying the full runtime closure of
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those paths into the archive. For most things, like almost any
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executable, this involves copying the entirety of huge packages like
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glibc, when only things like the shared library files are needed. To
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solve this, NixOS does a variety of patchwork to edit the files being
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copied in so they only refer to small, patched up paths. For instance,
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executables and their shared library dependencies are copied into an
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`extraUtils` derivation, and every ELF file is patched to refer to
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files in that output.
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The problem with this is that it is often difficult to correctly patch
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some things. For instance, systemd bakes the path to the `mount`
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command into the binary, so patchelf is no help. Instead, it's very
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often easier to simply copy the desired files to their original store
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locations in initrd and not copy their entire runtime closure. This
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does mean that it is the burden of the developer to ensure that all
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necessary dependencies are copied in, as closures won't be
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consulted. However, it is rare that full closures are actually
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desirable, so in the traditional initrd, the developer was likely to
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do manual work on patching the dependencies explicitly anyway.
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# How it works
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This program is similar to its inspiration (`find-libs` from the
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traditional initrd), except that it also handles symlinks and
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directories according to certain rules. As input, it receives a
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sequence of pairs of paths. The first path is an object to copy into
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initrd. The second path (if not empty) is the path to a symlink that
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should be placed in the initrd, pointing to that object. How that
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object is copied depends on its type.
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1. A regular file is copied directly to the same absolute path in the
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initrd.
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- If it is *also* an ELF file, then all of its direct shared
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library dependencies are also listed as objects to be copied.
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- If an unwrapped file exists as `.[filename]-wrapped`, then it is
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also listed as an object to be copied.
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2. A directory's direct children are listed as objects to be copied,
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and a directory at the same absolute path in the initrd is created.
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3. A symlink's target is listed as an object to be copied.
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There are a couple of quirks to mention here. First, the term "object"
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refers to the final file path that the developer intends to have
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copied into initrd. This means any parent directory is not considered
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an object just because its child was listed as an object in the
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program input; instead those intermediate directories are simply
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created in support of the target object. Second, shared libraries,
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directory children, and symlink targets aren't immediately recursed,
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because they simply get listed as objects themselves, and are
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therefore traversed when they themselves are processed. Finally,
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symlinks in the intermediate directories leading to an object are
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preserved, meaning an input object `/a/symlink/b` will just result in
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initrd containing `/a/symlink -> /target/b` and `/target/b`, even if
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`/target` has other children. Preserving symlinks in this manner is
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important for things like systemd.
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These rules automate the most important and obviously necessary
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copying that needs to be done in most cases, allowing programs and
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configuration files to go unpatched, while keeping the content of the
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initrd to a minimum.
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# Why Rust?
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- A prototype of this logic was written in Bash, in an attempt to keep
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with its `find-libs` ancestor, but that program was difficult to
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write, and ended up taking several minutes to run. This program runs
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in less than a second, and the code is substantially easier to work
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with.
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- This will not require end users to install a rust toolchain to use
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NixOS, as long as this tool is cached by Hydra. And if you're
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bootstrapping NixOS from source, rustc is already required anyway.
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- Rust was favored over Python for its type system, and because if you
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want to go fast, why not go *really fast*?
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