Fix test_linux_hwcap to work on old libc versions.

Use `weak!` to detect support for `getauxval` so that we can test on
platforms which don't have it.

tests/process/auxv.rs

@@ -14,11 +14,15 @@ fn test_page_size() {
 #[test]
 #[cfg(any(target_os = "android", target_os = "linux"))]
 fn test_linux_hwcap() {
-    let (_hwcap, hwcap2) = linux_hwcap();
+    weak!(fn getauxval(libc::c_ulong) -> libc::c_ulong);
 
-    // GLIBC seems to return a different value than `LD_SHOW_AUXV=1` reports.
-    #[cfg(not(target_env = "gnu"))]
-    assert_eq!(_hwcap, unsafe { libc::getauxval(libc::AT_HWCAP) } as usize);
+    if let Some(libc_getauxval) = getauxval.get() {
+        let (_hwcap, hwcap2) = linux_hwcap();
 
-    assert_eq!(hwcap2, unsafe { libc::getauxval(libc::AT_HWCAP2) } as usize);
+        // GLIBC seems to return a different value than `LD_SHOW_AUXV=1` reports.
+        #[cfg(not(target_env = "gnu"))]
+        assert_eq!(_hwcap, unsafe { libc_getauxval(libc::AT_HWCAP) } as usize);
+
+        assert_eq!(hwcap2, unsafe { libc_getauxval(libc::AT_HWCAP2) } as usize);
+    }
 }

tests/process/main.rs

@@ -4,6 +4,10 @@
 #![cfg_attr(target_os = "wasi", feature(wasi_ext))]
 #![cfg_attr(io_lifetimes_use_std, feature(io_safety))]
 
+#[cfg(not(target_os = "wasi"))]
+#[macro_use]
+mod weak;
+
 mod auxv;
 mod cpu_set;
 #[cfg(not(target_os = "wasi"))] // WASI doesn't have get[gpu]id.

tests/process/weak.rs

@@ -0,0 +1,159 @@
+// Implementation derived from `weak` in Rust's
+// library/std/src/sys/unix/weak.rs at revision
+// fd0cb0cdc21dd9c06025277d772108f8d42cb25f.
+
+//! Support for "weak linkage" to symbols on Unix
+//!
+//! Some I/O operations we do in libstd require newer versions of OSes but we
+//! need to maintain binary compatibility with older releases for now. In order
+//! to use the new functionality when available we use this module for
+//! detection.
+//!
+//! One option to use here is weak linkage, but that is unfortunately only
+//! really workable on Linux. Hence, use dlsym to get the symbol value at
+//! runtime. This is also done for compatibility with older versions of glibc,
+//! and to avoid creating dependencies on `GLIBC_PRIVATE` symbols. It assumes
+//! that we've been dynamically linked to the library the symbol comes from,
+//! but that is currently always the case for things like libpthread/libc.
+//!
+//! A long time ago this used weak linkage for the `__pthread_get_minstack`
+//! symbol, but that caused Debian to detect an unnecessarily strict versioned
+//! dependency on libc6 (#23628).
+
+// There are a variety of `#[cfg]`s controlling which targets are involved in
+// each instance of `weak!` and `syscall!`. Rather than trying to unify all of
+// that, we'll just allow that some unix targets don't use this module at all.
+#![allow(dead_code, unused_macros)]
+#![allow(clippy::doc_markdown)]
+
+use core::sync::atomic::{self, AtomicUsize, Ordering};
+use core::{marker, mem};
+use rustix::ffi::ZStr;
+
+macro_rules! weak {
+    (fn $name:ident($($t:ty),*) -> $ret:ty) => (
+        #[allow(non_upper_case_globals)]
+        static $name: $crate::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
+            $crate::weak::Weak::new(concat!(stringify!($name), '\0'));
+    )
+}
+
+pub(crate) struct Weak<F> {
+    name: &'static str,
+    addr: AtomicUsize,
+    _marker: marker::PhantomData<F>,
+}
+
+impl<F> Weak<F> {
+    pub(crate) const fn new(name: &'static str) -> Self {
+        Self {
+            name,
+            addr: AtomicUsize::new(1),
+            _marker: marker::PhantomData,
+        }
+    }
+
+    pub(crate) fn get(&self) -> Option<F> {
+        assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
+        unsafe {
+            // Relaxed is fine here because we fence before reading through the
+            // pointer (see the comment below).
+            match self.addr.load(Ordering::Relaxed) {
+                1 => self.initialize(),
+                0 => None,
+                addr => {
+                    let func = mem::transmute_copy::<usize, F>(&addr);
+                    // The caller is presumably going to read through this value
+                    // (by calling the function we've dlsymed). This means we'd
+                    // need to have loaded it with at least C11's consume
+                    // ordering in order to be guaranteed that the data we read
+                    // from the pointer isn't from before the pointer was
+                    // stored. Rust has no equivalent to memory_order_consume,
+                    // so we use an acquire fence (sorry, ARM).
+                    //
+                    // Now, in practice this likely isn't needed even on CPUs
+                    // where relaxed and consume mean different things. The
+                    // symbols we're loading are probably present (or not) at
+                    // init, and even if they aren't the runtime dynamic loader
+                    // is extremely likely have sufficient barriers internally
+                    // (possibly implicitly, for example the ones provided by
+                    // invoking `mprotect`).
+                    //
+                    // That said, none of that's *guaranteed*, and so we fence.
+                    atomic::fence(Ordering::Acquire);
+                    Some(func)
+                }
+            }
+        }
+    }
+
+    // Cold because it should only happen during first-time initialization.
+    #[cold]
+    unsafe fn initialize(&self) -> Option<F> {
+        let val = fetch(self.name);
+        // This synchronizes with the acquire fence in `get`.
+        self.addr.store(val, Ordering::Release);
+
+        match val {
+            0 => None,
+            addr => Some(mem::transmute_copy::<usize, F>(&addr)),
+        }
+    }
+}
+
+unsafe fn fetch(name: &str) -> usize {
+    let name = match ZStr::from_bytes_with_nul(name.as_bytes()) {
+        Ok(c_str) => c_str,
+        Err(..) => return 0,
+    };
+    libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr().cast()) as usize
+}
+
+#[cfg(not(any(target_os = "android", target_os = "linux")))]
+macro_rules! syscall {
+    (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
+        unsafe fn $name($($arg_name: $t),*) -> $ret {
+            weak! { fn $name($($t),*) -> $ret }
+
+            if let Some(fun) = $name.get() {
+                fun($($arg_name),*)
+            } else {
+                errno::set_errno(errno::Errno(libc::ENOSYS));
+                -1
+            }
+        }
+    )
+}
+
+#[cfg(any(target_os = "android", target_os = "linux"))]
+macro_rules! syscall {
+    (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
+        unsafe fn $name($($arg_name:$t),*) -> $ret {
+            // This looks like a hack, but concat_idents only accepts idents
+            // (not paths).
+            use libc::*;
+
+            syscall(
+                concat_idents!(SYS_, $name),
+                $($arg_name as c_long),*
+            ) as $ret
+        }
+    )
+}
+
+macro_rules! weakcall {
+    ($vis:vis fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
+        $vis unsafe fn $name($($arg_name: $t),*) -> $ret {
+            weak! { fn $name($($t),*) -> $ret }
+
+            // Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
+            // interposition, but if it's not found just fail.
+            if let Some(fun) = $name.get() {
+                fun($($arg_name),*)
+            } else {
+                errno::set_errno(errno::Errno(libc::ENOSYS));
+                -1
+            }
+        }
+    )
+}