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

fusl/src/malloc/expand_heap.c

Index: fusl/src/malloc/expand_heap.c
diff --git a/fusl/src/malloc/expand_heap.c b/fusl/src/malloc/expand_heap.c
new file mode 100644
index 0000000000000000000000000000000000000000..d8c0be742b69f1739ad54b9d3df56849a983ef2b
--- /dev/null
+++ b/fusl/src/malloc/expand_heap.c
@@ -0,0 +1,72 @@
+#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;
+}