Auto merge of #50331 - MartinHusemann:master, r=Kimundi

Map the stack guard page with max protection on NetBSD

On NetBSD the initial mmap() protection of a mapping can not be made
less restrictive with mprotect().

So when mapping a stack guard page, use the maximum protection
we ever want to use, then mprotect() it to the permission we
want it to have initially.

Fixes #50313

Author: bors

src/libstd/sys/unix/thread.rs

@@ -326,13 +326,22 @@ pub mod guard {
             // Reallocate the last page of the stack.
             // This ensures SIGBUS will be raised on
             // stack overflow.
-            let result = mmap(stackaddr, PAGE_SIZE, PROT_NONE,
+            // Systems which enforce strict PAX MPROTECT do not allow
+            // to mprotect() a mapping with less restrictive permissions
+            // than the initial mmap() used, so we mmap() here with
+            // read/write permissions and only then mprotect() it to
+            // no permissions at all. See issue #50313.
+            let result = mmap(stackaddr, PAGE_SIZE, PROT_READ | PROT_WRITE,
                               MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0);
-
             if result != stackaddr || result == MAP_FAILED {
                 panic!("failed to allocate a guard page");
             }
 
+            let result = mprotect(stackaddr, PAGE_SIZE, PROT_NONE);
+            if result != 0 {
+                panic!("failed to protect the guard page");
+            }
+
             let guardaddr = stackaddr as usize;
             let offset = if cfg!(target_os = "freebsd") {
                 2