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std_detect: Always avoid dlsym on *-linux-{musl,ohos}* targets #1746

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merged 2 commits into from
Mar 26, 2025
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std_detect: Always avoid dlsym on *-linux-{musl,ohos}* targets #1746

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6f1a72f
std_detect: Always avoid dlsym on *-linux-{musl,ohos}* targets
taiki-e Mar 18, 2025
2960b07
std_detect: Move cfgs into getauxval helper function
taiki-e Mar 25, 2025
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6 changes: 6 additions & 0 deletions ci/build-std-detect.sh
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,12 @@ set -ex
cd "$(dirname "$0")"/..

targets=(
# Linux
aarch64-unknown-linux-musl
armv5te-unknown-linux-musleabi
aarch64-unknown-linux-ohos
armv7-unknown-linux-ohos

# Android
aarch64-linux-android
arm-linux-androideabi
Expand Down
9 changes: 6 additions & 3 deletions crates/std_detect/README.md
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Expand Up @@ -30,12 +30,15 @@ run-time feature detection. When this feature is disabled, `std_detect` assumes
that [`getauxval`] is linked to the binary. If that is not the case the behavior
is undefined.

Note: This feature is ignored on `*-linux-gnu*` and `*-android*` targets
Note: This feature is ignored on `*-linux-{gnu,musl,ohos}*` and `*-android*` targets
because we can safely assume `getauxval` is linked to the binary.
* `*-linux-gnu*` targets ([since Rust 1.64](https://blog.rust-lang.org/2022/08/01/Increasing-glibc-kernel-requirements.html))
have glibc requirements higher than [glibc 2.16 that added `getauxval`](https://sourceware.org/legacy-ml/libc-announce/2012/msg00000.html).
have glibc requirements higher than [glibc 2.16 that added `getauxval`](https://sourceware.org/legacy-ml/libc-announce/2012/msg00000.html).
* `*-linux-musl*` targets ([at least since Rust 1.15](https://github.com/rust-lang/rust/blob/1.15.0/src/ci/docker/x86_64-musl/build-musl.sh#L15))
use musl newer than [musl 1.1.0 that added `getauxval`](https://git.musl-libc.org/cgit/musl/tree/WHATSNEW?h=v1.1.0#n1197)
* `*-linux-ohos*` targets use a [fork of musl 1.2](https://gitee.com/openharmony/docs/blob/master/en/application-dev/reference/native-lib/musl.md)
* `*-android*` targets ([since Rust 1.68](https://blog.rust-lang.org/2023/01/09/android-ndk-update-r25.html))
have the minimum supported API level higher than [Android 4.3 (API level 18) that added `getauxval`](https://github.com/aosp-mirror/platform_bionic/blob/d3ebc2f7c49a9893b114124d4a6b315f3a328764/libc/include/sys/auxv.h#L49).
have the minimum supported API level higher than [Android 4.3 (API level 18) that added `getauxval`](https://github.com/aosp-mirror/platform_bionic/blob/d3ebc2f7c49a9893b114124d4a6b315f3a328764/libc/include/sys/auxv.h#L49).

* `std_detect_file_io` (enabled by default, requires `std`): Enable to perform run-time feature
detection using file APIs (e.g. `/proc/cpuinfo`, etc.) if other more performant
Expand Down
123 changes: 39 additions & 84 deletions crates/std_detect/src/detect/os/linux/auxvec.rs
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Expand Up @@ -58,10 +58,13 @@ pub(crate) struct AuxVec {
/// feature detection on some platforms.
///
/// Note: The `std_detect_dlsym_getauxval` cargo feature is ignored on
/// `*-linux-gnu*` and `*-android*` targets because we can safely assume `getauxval`
/// `*-linux-{gnu,musl,ohos}*` and `*-android*` targets because we can safely assume `getauxval`
/// is linked to the binary.
/// - `*-linux-gnu*` targets ([since Rust 1.64](https://blog.rust-lang.org/2022/08/01/Increasing-glibc-kernel-requirements.html))
/// have glibc requirements higher than [glibc 2.16 that added `getauxval`](https://sourceware.org/legacy-ml/libc-announce/2012/msg00000.html).
/// - `*-linux-musl*` targets ([at least since Rust 1.15](https://github.com/rust-lang/rust/blob/1.15.0/src/ci/docker/x86_64-musl/build-musl.sh#L15))
/// use musl newer than [musl 1.1.0 that added `getauxval`](https://git.musl-libc.org/cgit/musl/tree/WHATSNEW?h=v1.1.0#n1197)
/// - `*-linux-ohos*` targets use a [fork of musl 1.2](https://gitee.com/openharmony/docs/blob/master/en/application-dev/reference/native-lib/musl.md)
/// - `*-android*` targets ([since Rust 1.68](https://blog.rust-lang.org/2023/01/09/android-ndk-update-r25.html))
/// have the minimum supported API level higher than [Android 4.3 (API level 18) that added `getauxval`](https://github.com/aosp-mirror/platform_bionic/blob/d3ebc2f7c49a9893b114124d4a6b315f3a328764/libc/include/sys/auxv.h#L49).
///
Expand All @@ -71,62 +74,8 @@ pub(crate) struct AuxVec {
/// [auxvec_h]: https://github.com/torvalds/linux/blob/master/include/uapi/linux/auxvec.h
/// [auxv_docs]: https://docs.rs/auxv/0.3.3/auxv/
pub(crate) fn auxv() -> Result<AuxVec, ()> {
#[cfg(all(
feature = "std_detect_dlsym_getauxval",
not(all(target_os = "linux", target_env = "gnu")),
// TODO: libc crate currently doesn't provide getauxval on 32-bit Android.
not(all(target_os = "android", target_pointer_width = "64")),
))]
{
// Try to call a dynamically-linked getauxval function.
if let Ok(hwcap) = getauxval(AT_HWCAP) {
// Targets with only AT_HWCAP:
#[cfg(any(
target_arch = "riscv32",
target_arch = "riscv64",
target_arch = "mips",
target_arch = "mips64"
))]
{
// Zero could indicate that no features were detected, but it's also used to
// indicate an error. In either case, try the fallback.
if hwcap != 0 {
return Ok(AuxVec { hwcap });
}
}

// Targets with AT_HWCAP and AT_HWCAP2:
#[cfg(any(
target_arch = "aarch64",
target_arch = "arm",
target_arch = "powerpc",
target_arch = "powerpc64",
target_arch = "s390x",
))]
{
if let Ok(hwcap2) = getauxval(AT_HWCAP2) {
// Zero could indicate that no features were detected, but it's also used to
// indicate an error. In particular, on many platforms AT_HWCAP2 will be
// legitimately zero, since it contains the most recent feature flags. Use the
// fallback only if no features were detected at all.
if hwcap != 0 || hwcap2 != 0 {
return Ok(AuxVec { hwcap, hwcap2 });
}
}
}

// Intentionnaly not used
let _ = hwcap;
}
}

#[cfg(any(
not(feature = "std_detect_dlsym_getauxval"),
all(target_os = "linux", target_env = "gnu"),
// TODO: libc crate currently doesn't provide getauxval on 32-bit Android.
all(target_os = "android", target_pointer_width = "64"),
))]
{
// Try to call a getauxval function.
if let Ok(hwcap) = getauxval(AT_HWCAP) {
// Targets with only AT_HWCAP:
#[cfg(any(
target_arch = "riscv32",
Expand All @@ -136,7 +85,6 @@ pub(crate) fn auxv() -> Result<AuxVec, ()> {
target_arch = "loongarch64",
))]
{
let hwcap = unsafe { libc::getauxval(AT_HWCAP as libc::c_ulong) as usize };
// Zero could indicate that no features were detected, but it's also used to indicate
// an error. In either case, try the fallback.
if hwcap != 0 {
Expand All @@ -153,16 +101,19 @@ pub(crate) fn auxv() -> Result<AuxVec, ()> {
target_arch = "s390x",
))]
{
let hwcap = unsafe { libc::getauxval(AT_HWCAP as libc::c_ulong) as usize };
let hwcap2 = unsafe { libc::getauxval(AT_HWCAP2 as libc::c_ulong) as usize };
// Zero could indicate that no features were detected, but it's also used to indicate
// an error. In particular, on many platforms AT_HWCAP2 will be legitimately zero,
// since it contains the most recent feature flags. Use the fallback only if no
// features were detected at all.
if hwcap != 0 || hwcap2 != 0 {
return Ok(AuxVec { hwcap, hwcap2 });
if let Ok(hwcap2) = getauxval(AT_HWCAP2) {
// Zero could indicate that no features were detected, but it's also used to indicate
// an error. In particular, on many platforms AT_HWCAP2 will be legitimately zero,
// since it contains the most recent feature flags. Use the fallback only if no
// features were detected at all.
if hwcap != 0 || hwcap2 != 0 {
return Ok(AuxVec { hwcap, hwcap2 });
}
}
}

// Intentionnaly not used
let _ = hwcap;
}

#[cfg(feature = "std_detect_file_io")]
Expand All @@ -178,27 +129,31 @@ pub(crate) fn auxv() -> Result<AuxVec, ()> {
}

/// Tries to read the `key` from the auxiliary vector by calling the
/// dynamically-linked `getauxval` function. If the function is not linked,
/// this function return `Err`.
#[cfg(any(
test,
all(
feature = "std_detect_dlsym_getauxval",
not(all(target_os = "linux", target_env = "gnu"))
)
))]
/// `getauxval` function. If the function is not linked, this function return `Err`.
fn getauxval(key: usize) -> Result<usize, ()> {
use libc;
pub type F = unsafe extern "C" fn(usize) -> usize;
unsafe {
let ptr = libc::dlsym(libc::RTLD_DEFAULT, c"getauxval".as_ptr());
if ptr.is_null() {
return Err(());
type F = unsafe extern "C" fn(libc::c_ulong) -> libc::c_ulong;
cfg_if::cfg_if! {
if #[cfg(all(
feature = "std_detect_dlsym_getauxval",
not(all(
target_os = "linux",
any(target_env = "gnu", target_env = "musl", target_env = "ohos"),
)),
// TODO: libc crate currently doesn't provide getauxval on 32-bit Android.
not(all(target_os = "android", target_pointer_width = "64")),
))] {
let ffi_getauxval: F = unsafe {
let ptr = libc::dlsym(libc::RTLD_DEFAULT, c"getauxval".as_ptr());
if ptr.is_null() {
return Err(());
}
core::mem::transmute(ptr)
};
} else {
let ffi_getauxval: F = libc::getauxval;
}

let ffi_getauxval: F = core::mem::transmute(ptr);
Ok(ffi_getauxval(key))
}
Ok(unsafe { ffi_getauxval(key as libc::c_ulong) as usize })
}

/// Tries to read the auxiliary vector from the `file`. If this fails, this
Expand Down