libc backend: `fs`: `FsWord` does not match `StatFs::f_type` on s390x-musl
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Assessment
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Newbie friendliness
- 66/100
- Issue type
- Bug
- Clarity
- Mostly clear
- Activity status
- Active
- Tech stack
- docker, linux, rust
- Domain
- operating-systems
Research direction
Start with src/backend/libc/fs/types.rs and src/fs/fd.rs, comparing the referenced libc and linux_raw definitions for FsWord, StatFs::f_type, and filesystem constants. Run the provided Docker reproducer with the s390x-alpine-linux-musl target first. Done means the s390x-musl cargo check and the shown type comparison compile successfully without breaking the other backends.
Written by the indexing model from the issue text.
Description
rustix::fs::FsWord in the libc backend mode does not match rustix::fs::StatFs::f_type on s390x-musl, while it matches on other toolchains such as s390x-gnu.
error[E0308]: mismatched types
--> src/main.rs:3:27
|
| assert_eq!(st.f_type, rustix::fs::PROC_SUPER_MAGIC);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `u32`, found `u64`
Affects:
Relevant code
libc backend
https://github.com/bytecodealliance/rustix/blob/287214b889865d8e1406a0ee71cc409b6f6191c8/src/backend/libc/fs/types.rs#L1138-L1170
https://github.com/bytecodealliance/rustix/blob/287214b889865d8e1406a0ee71cc409b6f6191c8/src/backend/libc/fs/types.rs#L1081-L1086
linux_raw backend
https://github.com/bytecodealliance/rustix/blob/287214b889865d8e1406a0ee71cc409b6f6191c8/src/backend/linux_raw/fs/types.rs#L837-L851
https://github.com/bytecodealliance/rustix/blob/287214b889865d8e1406a0ee71cc409b6f6191c8/src/backend/linux_raw/fs/types.rs#L763-L801
Reproducer
Dockerfile
# Usage: docker build --platform amd64,arm64,ppc64le,s390x,riscv64,linux/arm/v6,linux/arm/v7 .
#
# On s390x, rustix uses the libc backend by default, which fails to compile.
# Forcing the linux_raw backend compiles fine:
# docker build --platform s390x \
# --build-arg RUSTFLAGS="--cfg rustix_use_experimental_asm" \
# --build-arg RUSTC_BOOTSTRAP=1 .
ARG ALPINE_VERSION=3.24.2
ARG RUSTIX_VERSION=1.1.5
FROM --platform=$BUILDPLATFORM alpine:${ALPINE_VERSION}
RUN apk add --no-cache cargo rust
WORKDIR /repro
ARG RUSTIX_VERSION
RUN cargo init -q --name repro && cargo add -q rustix@=${RUSTIX_VERSION} --features fs
COPY <<RS src/main.rs
fn main() {
let st = rustix::fs::statfs("/proc").unwrap();
assert_eq!(st.f_type, rustix::fs::PROC_SUPER_MAGIC);
}
RS
ARG TARGETARCH
ARG TARGETVARIANT
ARG RUSTFLAGS=
ARG RUSTC_BOOTSTRAP=
RUN set -ex; \
case "$TARGETARCH/$TARGETVARIANT" in \
amd64/*) apk_arch=x86_64; triple=x86_64-alpine-linux-musl ;; \
arm64/*) apk_arch=aarch64; triple=aarch64-alpine-linux-musl ;; \
arm/v6) apk_arch=armhf; triple=armv6-alpine-linux-musleabihf ;; \
arm/v7|arm/) apk_arch=armv7; triple=armv7-alpine-linux-musleabihf ;; \
ppc64le/*) apk_arch=ppc64le; triple=powerpc64le-alpine-linux-musl ;; \
s390x/*) apk_arch=s390x; triple=s390x-alpine-linux-musl ;; \
riscv64/*) apk_arch=riscv64; triple=riscv64-alpine-linux-musl ;; \
*) echo "unsupported platform $TARGETARCH/$TARGETVARIANT" >&2; exit 1 ;; \
esac; \
rustc --print target-list | grep -qx "$triple" \
|| { echo "rustc has no target spec for $triple" >&2; exit 1; }; \
mkdir -p /sysroot/etc/apk/keys; \
cp /etc/apk/repositories /sysroot/etc/apk/; \
echo "$apk_arch" >/sysroot/etc/apk/arch; \
apk add --no-cache --initdb --root /sysroot --allow-untrusted alpine-keys; \
apk add --no-cache --root /sysroot rust; \
ln -s "/sysroot/usr/lib/rustlib/$triple" \
"$(rustc --print sysroot)/lib/rustlib/$triple"; \
rc=0; cargo check --target "$triple" || rc=$?; \
grep -h 'rustc-cfg=\(libc\|linux_raw\)$' target/$triple/debug/build/rustix-*/output; \
exit $rc
$ docker build --platform s390x .
[...]
8.770 Downloading crates ...
8.846 Downloaded errno v0.3.14
8.858 Downloaded bitflags v2.13.2
8.919 Downloaded rustix v1.1.5
8.943 Downloaded libc v0.2.190
8.988 Downloaded linux-raw-sys v0.12.1
9.030 Compiling libc v0.2.190
9.030 Compiling rustix v1.1.5
9.030 Checking bitflags v2.13.2
9.030 Checking linux-raw-sys v0.12.1
9.583 Checking errno v0.3.14
10.19 Checking repro v0.1.0 (/repro)
10.21 error[E0308]: mismatched types
10.21 --> src/main.rs:3:27
10.21 |
10.21 3 | assert_eq!(st.f_type, rustix::fs::PROC_SUPER_MAGIC);
10.21 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `u32`, found `u64`
10.21 |
10.21 help: you can convert `st.f_type` from `u32` to `u64`, matching the type of `rustix::fs::PROC_SUPER_MAGIC`
10.21 |
10.21 3 | assert_eq!(u64::from(st.f_type), rustix::fs::PROC_SUPER_MAGIC);
10.21 | ++++++++++ +
10.21
10.21 For more information about this error, try `rustc --explain E0308`.
10.21 error: could not compile `repro` (bin "repro") due to 1 previous error
[...]
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