Add a "fork" of musl as //fusl.

fusl/src/malloc/expand_heap.c

+#include <limits.h>
+#include <stdint.h>
+#include <errno.h>
+#include <sys/mman.h>
+#include "libc.h"
+#include "syscall.h"
+
+/* This function returns true if the interval [old,new]
+ * intersects the 'len'-sized interval below &libc.auxv
+ * (interpreted as the main-thread stack) or below &b
+ * (the current stack). It is used to defend against
+ * buggy brk implementations that can cross the stack. */
+
+static int traverses_stack_p(uintptr_t old, uintptr_t new)
+{
+        const uintptr_t len = 8<<20;
+        uintptr_t a, b;
+
+        b = (uintptr_t)libc.auxv;
+        a = b > len ? b-len : 0;
+        if (new>a && old<b) return 1;
+
+        b = (uintptr_t)&b;
+        a = b > len ? b-len : 0;
+        if (new>a && old<b) return 1;
+
+        return 0;
+}
+
+void *__mmap(void *, size_t, int, int, int, off_t);
+
+/* Expand the heap in-place if brk can be used, or otherwise via mmap,
+ * using an exponential lower bound on growth by mmap to make
+ * fragmentation asymptotically irrelevant. The size argument is both
+ * an input and an output, since the caller needs to know the size
+ * allocated, which will be larger than requested due to page alignment
+ * and mmap minimum size rules. The caller is responsible for locking
+ * to prevent concurrent calls. */
+
+void *__expand_heap(size_t *pn)
+{
+        static uintptr_t brk;
+        static unsigned mmap_step;
+        size_t n = *pn;
+
+        if (n > SIZE_MAX/2 - PAGE_SIZE) {
+                errno = ENOMEM;
+                return 0;
+        }
+        n += -n & PAGE_SIZE-1;
+
+        if (!brk) {
+                brk = __syscall(SYS_brk, 0);
+                brk += -brk & PAGE_SIZE-1;
+        }
+
+        if (n < SIZE_MAX-brk && !traverses_stack_p(brk, brk+n)
+            && __syscall(SYS_brk, brk+n)==brk+n) {
+                *pn = n;
+                brk += n;
+                return (void *)(brk-n);
+        }
+
+        size_t min = (size_t)PAGE_SIZE << mmap_step/2;
+        if (n < min) n = min;
+        void *area = __mmap(0, n, PROT_READ|PROT_WRITE,
+                MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+        if (area == MAP_FAILED) return 0;
+        *pn = n;
+        mmap_step++;
+        return area;
+}