Dynamically size sigaltstk in std

On modern Linux with Intel AMX and 1KiB matrices,
Arm SVE with potentially 2KiB vectors,
and RISCV Vectors with up to 16KiB vectors,
we must handle dynamic signal stack sizes.

We can do so unconditionally by using getauxval,
but assuming it may return 0 as an answer,
thus falling back to the old constant if needed.

library/std/src/sys/unix/stack_overflow.rs

@@ -50,7 +50,7 @@ mod imp {
     #[cfg(all(target_os = "linux", target_env = "gnu"))]
     use libc::{mmap64, munmap};
     use libc::{sigaction, sighandler_t, SA_ONSTACK, SA_SIGINFO, SIGBUS, SIG_DFL};
-    use libc::{sigaltstack, SIGSTKSZ, SS_DISABLE};
+    use libc::{sigaltstack, SS_DISABLE};
     use libc::{MAP_ANON, MAP_PRIVATE, PROT_NONE, PROT_READ, PROT_WRITE, SIGSEGV};
 
     use crate::sync::atomic::{AtomicBool, AtomicPtr, Ordering};
@@ -129,28 +129,36 @@ mod imp {
         drop_handler(MAIN_ALTSTACK.load(Ordering::Relaxed));
     }
 
-    unsafe fn get_stackp() -> *mut libc::c_void {
+    unsafe fn get_stack() -> libc::stack_t {
         // OpenBSD requires this flag for stack mapping
         // otherwise the said mapping will fail as a no-op on most systems
         // and has a different meaning on FreeBSD
         #[cfg(any(target_os = "openbsd", target_os = "netbsd", target_os = "linux",))]
         let flags = MAP_PRIVATE | MAP_ANON | libc::MAP_STACK;
         #[cfg(not(any(target_os = "openbsd", target_os = "netbsd", target_os = "linux",)))]
         let flags = MAP_PRIVATE | MAP_ANON;
-        let stackp =
-            mmap64(ptr::null_mut(), SIGSTKSZ + page_size(), PROT_READ | PROT_WRITE, flags, -1, 0);
+
+        let sigstack_size = sigstack_size();
+        let page_size = page_size();
+
+        let stackp = mmap64(
+            ptr::null_mut(),
+            sigstack_size + page_size,
+            PROT_READ | PROT_WRITE,
+            flags,
+            -1,
+            0,
+        );
         if stackp == MAP_FAILED {
             panic!("failed to allocate an alternative stack: {}", io::Error::last_os_error());
         }
-        let guard_result = libc::mprotect(stackp, page_size(), PROT_NONE);
+        let guard_result = libc::mprotect(stackp, page_size, PROT_NONE);
         if guard_result != 0 {
             panic!("failed to set up alternative stack guard page: {}", io::Error::last_os_error());
         }
-        stackp.add(page_size())
-    }
+        let stackp = stackp.add(page_size);
 
-    unsafe fn get_stack() -> libc::stack_t {
-        libc::stack_t { ss_sp: get_stackp(), ss_flags: 0, ss_size: SIGSTKSZ }
+        libc::stack_t { ss_sp: stackp, ss_flags: 0, ss_size: sigstack_size }
     }
 
     pub unsafe fn make_handler() -> Handler {
@@ -171,21 +179,41 @@ mod imp {
 
     pub unsafe fn drop_handler(data: *mut libc::c_void) {
         if !data.is_null() {
+            let sigstack_size = sigstack_size();
+            let page_size = page_size();
             let stack = libc::stack_t {
                 ss_sp: ptr::null_mut(),
                 ss_flags: SS_DISABLE,
                 // Workaround for bug in macOS implementation of sigaltstack
                 // UNIX2003 which returns ENOMEM when disabling a stack while
                 // passing ss_size smaller than MINSIGSTKSZ. According to POSIX
                 // both ss_sp and ss_size should be ignored in this case.
-                ss_size: SIGSTKSZ,
+                ss_size: sigstack_size,
             };
             sigaltstack(&stack, ptr::null_mut());
             // We know from `get_stackp` that the alternate stack we installed is part of a mapping
             // that started one page earlier, so walk back a page and unmap from there.
-            munmap(data.sub(page_size()), SIGSTKSZ + page_size());
+            munmap(data.sub(page_size), sigstack_size + page_size);
         }
     }
+
+    /// Modern kernels on modern hardware can have dynamic signal stack sizes.
+    #[cfg(any(target_os = "linux", target_os = "android"))]
+    fn sigstack_size() -> usize {
+        // FIXME: reuse const from libc when available?
+        const AT_MINSIGSTKSZ: crate::ffi::c_ulong = 51;
+        let dynamic_sigstksz = unsafe { libc::getauxval(AT_MINSIGSTKSZ) };
+        // If getauxval couldn't find the entry, it returns 0,
+        // so take the higher of the "constant" and auxval.
+        // This transparently supports older kernels which don't provide AT_MINSIGSTKSZ
+        libc::SIGSTKSZ.max(dynamic_sigstksz as _)
+    }
+
+    /// Not all OS support hardware where this is needed.
+    #[cfg(not(any(target_os = "linux", target_os = "android")))]
+    fn sigstack_size() -> usize {
+        libc::SIGSTKSZ
+    }
 }
 
 #[cfg(not(any(