Implement malloc_usable_size(); ok millert@ deraadt@ and jmc@ for the man page

src/include/stdlib.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: stdlib.h,v 1.73 2018/11/05 08:23:40 otto Exp $	*/
+/*	$OpenBSD: stdlib.h,v 1.74 2018/11/18 16:15:18 otto Exp $	*/
 /*	$NetBSD: stdlib.h,v 1.25 1995/12/27 21:19:08 jtc Exp $	*/
 
 /*-
@@ -112,6 +112,7 @@ long	 labs(long);
 ldiv_t	 ldiv(long, long);
 void	*malloc(size_t);
 #if __BSD_VISIBLE
+size_t	 malloc_usable_size(void *);
 void	freezero(void *, size_t)
 		 __attribute__ ((__bounded__(__buffer__,1,2)));
 void	*reallocarray(void *, size_t, size_t);

src/lib/libc/stdlib/malloc.3

@@ -30,9 +30,9 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"	$OpenBSD: malloc.3,v 1.118 2018/11/08 05:58:21 otto Exp $
+.\"	$OpenBSD: malloc.3,v 1.119 2018/11/18 16:15:18 otto Exp $
 .\"
-.Dd $Mdocdate: November 8 2018 $
+.Dd $Mdocdate: November 18 2018 $
 .Dt MALLOC 3
 .Os
 .Sh NAME
@@ -43,7 +43,8 @@
 .Nm reallocarray ,
 .Nm recallocarray ,
 .Nm freezero ,
-.Nm aligned_alloc
+.Nm aligned_alloc ,
+.Nm malloc_usable_size
 .Nd memory allocation and deallocation
 .Sh SYNOPSIS
 .In stdlib.h
@@ -63,6 +64,8 @@
 .Fn freezero "void *ptr" "size_t size"
 .Ft void *
 .Fn aligned_alloc "size_t alignment" "size_t size"
+.Ft size_t
+.Fn malloc_usable_size "void *ptr"
 .Vt char *malloc_options ;
 .Sh DESCRIPTION
 The standard functions
@@ -191,7 +194,7 @@ must be a value such that
 .Fa size
 is the size of the earlier allocation that returned
 .Fa ptr ,
-otherwise the behaviour is undefined.
+otherwise the behavior is undefined.
 .Pp
 The
 .Fn freezero
@@ -233,6 +236,25 @@ If
 is not a multiple of
 .Fa alignment ,
 behavior is undefined.
+.Pp
+The
+.Fn malloc_usable_size
+function returns the actual size of the allocated memory pointed to by
+.Va ptr .
+If
+.Va ptr
+is
+.Dv NULL ,
+it returns 0.
+If
+.Va ptr
+was never returned by an allocation function or freed before,
+the behavior is undefined.
+This function should not be relied upon since it exposes some of the internal
+workings of the
+.Fn malloc
+family of functions.
+Writing beyond the requested size introduces undefined behavior.
 .Sh RETURN VALUES
 Upon successful completion, the allocation functions
 return a pointer to the allocated space; otherwise,
@@ -618,7 +640,9 @@ function appeared in
 .Ox 6.2 .
 The
 .Fn aligned_alloc
-function appeared in
+and
+.Fn malloc_usable_size
+functions appeared in
 .Ox 6.5 .
 .Sh CAVEATS
 When using

src/lib/libc/stdlib/malloc.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: malloc.c,v 1.251 2018/11/06 08:01:43 otto Exp $	*/
+/*	$OpenBSD: malloc.c,v 1.252 2018/11/18 16:15:18 otto Exp $	*/
 /*
  * Copyright (c) 2008, 2010, 2011, 2016 Otto Moerbeek <otto@drijf.net>
  * Copyright (c) 2012 Matthew Dempsky <matthew@openbsd.org>
@@ -1466,6 +1466,84 @@ freezero(void *ptr, size_t sz)
 }
 DEF_WEAK(freezero);
 
+static size_t
+osize(struct dir_info *argpool, void *p)
+{
+	struct dir_info *pool;
+	struct region_info *r;
+	char *saved_function;
+	size_t sz;
+	int i;
+
+	pool = argpool;
+	r = find(pool, p);
+	if (r == NULL) {
+		if (mopts.malloc_mt)  {
+			for (i = 0; i < _MALLOC_MUTEXES; i++) {
+				if (i == argpool->mutex)
+					continue;
+				pool->active--;
+				_MALLOC_UNLOCK(pool->mutex);
+				pool = mopts.malloc_pool[i];
+				_MALLOC_LOCK(pool->mutex);
+				pool->active++;
+				r = find(pool, p);
+				if (r != NULL) {
+					saved_function = pool->func;
+					pool->func = argpool->func;
+					break;
+				}
+			}
+		}
+		if (r == NULL)
+			wrterror(argpool, "bogus pointer (double free?) %p", p);
+	}
+
+	REALSIZE(sz, r);
+	if (sz > MALLOC_MAXCHUNK) {
+		if (MALLOC_MOVE_COND(sz))
+			sz = MALLOC_PAGESIZE - ((char *)p - (char *)r->p);
+		else
+			sz = PAGEROUND(sz);
+	}
+	if (argpool != pool) {
+		pool->active--;
+		pool->func = saved_function;
+		_MALLOC_UNLOCK(pool->mutex);
+		_MALLOC_LOCK(argpool->mutex);
+		argpool->active++;
+	}
+	return sz;
+}
+
+size_t
+malloc_usable_size(void *ptr)
+{
+	struct dir_info *d;
+	int saved_errno = errno;
+	size_t sz;
+
+	/* This is legal. */
+	if (ptr == NULL)
+		return 0;
+
+	d = getpool();
+	if (d == NULL)
+		wrterror(d, "malloc_usable_size() called before allocation");
+	_MALLOC_LOCK(d->mutex);
+	d->func = "malloc_usable_size";
+	if (d->active++) {
+		malloc_recurse(d);
+		return 0;
+	}
+	sz = osize(d, ptr);
+	d->active--;
+	_MALLOC_UNLOCK(d->mutex);
+	errno = saved_errno;
+	return sz;
+}
+DEF_WEAK(malloc_usable_size);
+
 static void *
 orealloc(struct dir_info *argpool, void *p, size_t newsz, void *f)
 {