This is unnecessary because Darwin builds already cannot create
`set{u,g}id` files due to the minimal sandbox policy, because we can
forbid messing with the top‐level build directory directly in the
sandbox rules, and because Darwin builds can trivially avoid isolation
through temporary directories right now anyway.
This does regress the build directory isolation for builds with
`sandbox = false`, but I can’t imagine that mattering given the
above. The sandbox change prepares us for a world where we close
off shared temporary directories for `sandbox = true` builds and try
harder to achieve proper isolation on Darwin, but probably doesn’t
have a meaningful security impact one way or another for now.
With this change, we get down to 41 byte build directory paths on
Darwin, a ~2–3½ byte improvement over the old status quo. We can
also restore the 0710 permissions on Linux.
Change-Id: I6a6a6964a681c0365241fe7234831db656b76799
The minimum build directory length on Darwin with default settings
when we were still using `/tmp` was 22 bytes. Deriving build directory
names from my local store, the median and mode were 43, the mean was
around 44½, and the maximum was 127.
The switch to `/nix/var/nix/builds` over `/tmp` added a 15 byte
penalty, and the additional `/b` directory added another 2.
Now that we use opaque build directory names, the length is 48, so
we’re still at a ~3½–5 byte penalty over the previous status
quo. This change brings us down to 43, matching the previous median.
Note that these calculations do not take into account the fact that
`/tmp` is a symbolic link to `/private/tmp` on Darwin. Anything
that was canonicalizing paths would have had an additional 8 byte
penalty in the previous status quo that is not applicable here,
so we may already be ahead even without this change. If the more
opaque directory name here is undesirable, then that factor could
potentially help us squeeze by without. Alternatively, in combination
with dropping the `/b` on Darwin we could use `/nix/var/nix/bld` or
similar, but I feel that the paths in general are sufficiently opaque
that it should be okay to go with the shorter option here. Given that
some projects already had to reduce filename lengths to avoid this
limit even before the recent changes, I think it is best to try and
improve on the previous status quo.
Note that `/nix/var/nix/builds` will unfortunately not be cleaned up
on Darwin. However, we don’t clean up the directories inside it on
Darwin anyway, so hopefully that’s okay for now?
Closes: #913
Change-Id: I6a6a6964bffce7194bcddcaefb4c4a37569c7df5
We always use the default temporary directory, because
`createUniqueDir` has an interface nice enough to use directly for
the few bespoke uses.
Change-Id: I6a6a696450b7c0a0bd76655632fb14d7c5e38199
This makes the paths more nondeterministic, but more reliably unique,
and lets us remove the retry loop.
Note that this adds random entropy to the build directory visible
inside derivations on Darwin and unsandboxed Linux. It was already
non‐deterministic in the presence of concurrent builds and similar,
but now we can reliably expect it to be different every time. On the
whole I think that’s a good thing, as it is impossible to ensure
a single consistent build directory and derivation outputs should
not depend on it.
Package reproducibility isn’t great on Darwin to begin with,
though, and the reproducibility bugs this will turn up in packages
will be more urgent to fix than when the build directory was mostly
consistent. A quick survey of my local store shows that many C, C++,
and Rust binaries contain build directory references, likely due to
use of `__FILE__` and its equivalents; non‐binary offenders include:
* Install logs included in the Rust and Cargo bootstrap compilers
* Example errors in the Rust documentation referencing build paths
* Configuration information installed with CPython itself
* Python 2 metadata from resholve’s closure
* Cython metadata
* Generated headers in Facebook libraries referencing source paths
* Generated CMake files in Facebook libraries referencing source paths
I haven’t built that much in this store since the last GC, so this is
probably only a small sample of the problems across the tree. These are
all instances of <https://reproducible-builds.org/docs/build-path/>,
though, and should probably just be treated as general reproducibility
bugs outside of contexts like the Linux sandbox where we can normalize
them away entirely.
I have implemented away build directory paths for C/C++, applied some
additional fixes for non‐`__FILE__`‐related issues in binaries
from ATF and LLVM, and fixed the derivation bug causing the CPython
3 issue, and will work on upstreaming these changes. Rust is working
on the problem upstream, with some temporary workarounds we can
potentially apply in Nixpkgs for now. The rest will require some
distributed effort.
Change-Id: I6a6a69645b4915c56c0fdef904aa81684e4136c6
This does change the behaviour when the global temporary directory
does not exist, but other uses of the global temporary directory are
already broken in that circumstance, and it should be fixed centrally
if the use case is considered desirable. The logic was not present
before the recent churn around build directories – it was added now
that Lix is taking ownership of the build directory in the store –
so this should not be a meaningful regression.
Change-Id: I6a6a6964e345ea6803226c5ad759e836de7cb0ed
There is now no risk of race conditions on a system with a functioning
entropy source, and the bespoke prefixes are either redundant to the
default or unnecessary.
Change-Id: I6a6a69641211c6bb979ea48ad30aecb1a53d03f0
Relax the constraints on keeping the exact same filename format to
provide a more robust source of entropy with a simpler interface
(as previously suggested by eldritch horrors). Using 128 bits of
OS‐provided entropy ensures global uniqueness and allows us to
skip any thought of gracefully handling the case where these files
already exist.
My microbenchmark that repeatedly constructed paths like this and
printed them out showed that this takes about 1.23× the time of
the previous implementation, both taking on the order of a couple
microseconds for one iteration. Since everything that uses it is doing
things more expensive than printing to standard output, the actual
performance delta is likely to be lost in the noise. If it somehow
becomes a bottleneck, it can be optimized without sacrificing the
guarantees by reading from the system RNG only to seed a thread‐local
CSPRNG like [ChaCha8Rand], but I think that’s very unlikely.
We also tweak the recommended way of creating a temporary file inside
a directory in anticipation of later changes, and rename the `suffix`
parameter to `prefix` (it’s a prefix to the random characters and
a suffix to the root, but this way is more consistent).
[ChaCha8Rand]: https://c2sp.org/chacha8rand
Change-Id: I6a6a69648502c746d13d8c3bd2768cbbf1b90466
base32Encode now takes std::span<std::byte>, with a base32EncodeStr
convenience wrapper which takes std::string_view.
Co-authored-by: Qyriad <qyriad@qyriad.me>
Change-Id: I6a6a6964f799dc84ecbfb55c7ca03a064cff71d9
The prospective callers of this should probably be doing something
smarter or more abstracted to begin with, but this is useful as an
incremental improvement for call sites with existing `makeTempPath`
logic in the face of filename length limits.
Change-Id: I6a6a69644292f5bbf984a1df90192e06c6022b53
We’re already allowing `/tmp` anyway, so this should be harmless,
and it fixes a regression in the default configuration caused by
moving the build directories out of `temp-dir`. (For instance, that
broke the Lix `guessOrInventPath.sockets` test.)
Note that removing `/tmp` breaks quite a few builds, so although it may
be a good idea in general it would require work on the Nixpkgs side.
Fixes: 749afbbe99
Change-Id: I6a6a69645f429bc50d4cb24283feda3d3091f534
(cherry picked from commit d1db3e5fa3)
