refactor malloc's expand_heap to share with __simple_malloc

this extends the brk/stack collision protection added to full malloc
in commit 276904c2 to also protect the
__simple_malloc function used in static-linked programs that don't
reference the free function.

it also extends support for using mmap when brk fails, which full
malloc got in commit 54463033, to
__simple_malloc.

since __simple_malloc may expand the heap by arbitrarily large
increments, the stack collision detection is enhanced to detect
interval overlap rather than just proximity of a single address to the
stack. code size is increased a bit, but this is partly offset by the
sharing of code between the two malloc implementations, which due to
linking semantics, both get linked in a program that needs the full
malloc with realloc/free support.

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;
+}

src/malloc/lite_malloc.c

@@ -4,43 +4,46 @@
 #include <errno.h>
 #include "libc.h"
 
-uintptr_t __brk(uintptr_t);
-
 #define ALIGN 16
 
+void *__expand_heap(size_t *);
+
 void *__simple_malloc(size_t n)
 {
-	static uintptr_t cur, brk;
-	uintptr_t base, new;
+	static char *cur, *end;
 	static volatile int lock[2];
-	size_t align=1;
+	size_t align=1, pad;
+	void *p;
 
 	if (!n) n++;
-	if (n > SIZE_MAX/2) goto toobig;
-
 	while (align<n && align<ALIGN)
 		align += align;
-	n = n + align - 1 & -align;
 
 	LOCK(lock);
-	if (!cur) cur = brk = __brk(0)+16;
-	base = cur + align-1 & -align;
-	if (n > SIZE_MAX - PAGE_SIZE - base) goto fail;
-	if (base+n > brk) {
-		new = base+n + PAGE_SIZE-1 & -PAGE_SIZE;
-		if (__brk(new) != new) goto fail;
-		brk = new;
-	}
-	cur = base+n;
-	UNLOCK(lock);
 
-	return (void *)base;
+	pad = -(uintptr_t)cur & align-1;
+
+	if (n <= SIZE_MAX/2 + ALIGN) n += pad;
+
+	if (n > end-cur) {
+		size_t m = n;
+		char *new = __expand_heap(&m);
+		if (!new) {
+			UNLOCK(lock);
+			return 0;
+		}
+		if (new != end) {
+			cur = new;
+			n -= pad;
+			pad = 0;
+		}
+		end = new + m;
+	}
 
-fail:
+	p = cur + pad;
+	cur += n;
 	UNLOCK(lock);
-toobig:
-	errno = ENOMEM;
-	return 0;
+	return p;
 }
 
 weak_alias(__simple_malloc, malloc);

src/malloc/malloc.c

@@ -13,7 +13,6 @@
 #define inline inline __attribute__((always_inline))
 #endif
 
-uintptr_t __brk(uintptr_t);
 void *__mmap(void *, size_t, int, int, int, off_t);
 int __munmap(void *, size_t);
 void *__mremap(void *, size_t, size_t, int, ...);
@@ -31,13 +30,9 @@ struct bin {
 };
 
 static struct {
-	uintptr_t brk;
-	size_t *heap;
 	volatile uint64_t binmap;
 	struct bin bins[64];
-	volatile int brk_lock[2];
 	volatile int free_lock[2];
-	unsigned mmap_step;
 } mal;
 
 
@@ -152,77 +147,52 @@ void __dump_heap(int x)
 }
 #endif
 
-static int is_near_stack(uintptr_t b)
-{
-	const uintptr_t c = 8<<20;
-	uintptr_t a = (uintptr_t)libc.auxv;
-	uintptr_t d = (uintptr_t)&b;
-	return a-b<=c || d-b<=c;
-}
+void *__expand_heap(size_t *);
 
 static struct chunk *expand_heap(size_t n)
 {
-	static int init;
+	static int heap_lock[2];
+	static void *end;
+	void *p;
 	struct chunk *w;
-	uintptr_t new;
-
-	lock(mal.brk_lock);
 
-	if (!init) {
-		mal.brk = __brk(0);
-#ifdef SHARED
-		mal.brk = mal.brk + PAGE_SIZE-1 & -PAGE_SIZE;
-#endif
-		mal.brk = mal.brk + 2*SIZE_ALIGN-1 & -SIZE_ALIGN;
-		mal.heap = (void *)mal.brk;
-		init = 1;
-	}
+	/* The argument n already accounts for the caller's chunk
+	 * overhead needs, but if the heap can't be extended in-place,
+	 * we need room for an extra zero-sized sentinel chunk. */
+	n += SIZE_ALIGN;
 
-	if (n > SIZE_MAX - mal.brk - 2*PAGE_SIZE) goto fail;
-	new = mal.brk + n + SIZE_ALIGN + PAGE_SIZE - 1 & -PAGE_SIZE;
-	n = new - mal.brk;
+	lock(heap_lock);
 
-	if (is_near_stack(mal.brk) || __brk(new) != new) {
-		size_t min = (size_t)PAGE_SIZE << mal.mmap_step/2;
-		n += -n & PAGE_SIZE-1;
-		if (n < min) n = min;
-		void *area = __mmap(0, n, PROT_READ|PROT_WRITE,
-			MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
-		if (area == MAP_FAILED) goto fail;
+	p = __expand_heap(&n);
+	if (!p) {
+		unlock(heap_lock);
+		return 0;
+	}
 
-		mal.mmap_step++;
-		area = (char *)area + SIZE_ALIGN - OVERHEAD;
-		w = area;
+	/* If not just expanding existing space, we need to make a
+	 * new sentinel chunk below the allocated space. */
+	if (p != end) {
+		/* Valid/safe because of the prologue increment. */
 		n -= SIZE_ALIGN;
+		p = (char *)p + SIZE_ALIGN;
+		w = MEM_TO_CHUNK(p);
 		w->psize = 0 | C_INUSE;
-		w->csize = n | C_INUSE;
-		w = NEXT_CHUNK(w);
-		w->psize = n | C_INUSE;
-		w->csize = 0 | C_INUSE;
-
-		unlock(mal.brk_lock);
-
-		return area;
 	}
 
-	w = MEM_TO_CHUNK(mal.heap);
-	w->psize = 0 | C_INUSE;
-
-	w = MEM_TO_CHUNK(new);
+	/* Record new heap end and fill in footer. */
+	end = (char *)p + n;
+	w = MEM_TO_CHUNK(end);
 	w->psize = n | C_INUSE;
 	w->csize = 0 | C_INUSE;
 
-	w = MEM_TO_CHUNK(mal.brk);
+	/* Fill in header, which may be new or may be replacing a
+	 * zero-size sentinel header at the old end-of-heap. */
+	w = MEM_TO_CHUNK(p);
 	w->csize = n | C_INUSE;
-	mal.brk = new;
-	
-	unlock(mal.brk_lock);
+
+	unlock(heap_lock);
 
 	return w;
-fail:
-	unlock(mal.brk_lock);
-	errno = ENOMEM;
-	return 0;
 }
 
 static int adjust_size(size_t *n)