syscalls.c: Bugfix _sbrk() implementation.

Fix _sbrk() implementation so it properly rejects negative arguments
which would send the program break below the heap start.  Fix
incorrect check against argument causing heap overflow.  Also set
errno properly to ENOMEM when the call fails.

Beginning and end of the heap are now determined by HEAP_START and
HEAP_END macros.  Their current values seem to work OK for heaps on
the internal SRAM, but they'll need to get generalized for Maple
Native.

1 files changed, 31 insertions, 16 deletions

diff --git a/libmaple/syscalls.c b/libmaple/syscalls.c
index ae80010..f28202c 100644
--- a/libmaple/syscalls.c
+++ b/libmaple/syscalls.c
@@ -25,32 +25,47 @@
  *****************************************************************************/
 
 #include "libmaple.h"
+
 #include <sys/stat.h>
+#include <errno.h>
 
-/* _end is set in the linker command file */
-extern caddr_t _end;
+/* Set by the linker script */
+extern int _end;
 
+/* FIXME these should be determined by the linker script.
+ *
+ * Doing so will allow the heap to be configured on a per-board basis.
+ * Current values are just stopgaps for a heap in built-in SRAM.
+ *
+ * STACK_RESERVED_BYTES is just a hack to ensure a minimum stack size.
+ * It should probably go away as well. */
+#define STACK_RESERVED_BYTES 1024
+#define HEAP_START           ((caddr_t)&_end)
+#define HEAP_END             ((caddr_t)((uint32)STM32_SRAM_END -        \
+                                        STACK_RESERVED_BYTES))
 
 /*
- * sbrk -- changes heap size size. Get nbytes more
- *         RAM. We just increment a pointer in what's
- *         left of memory on the board.
+ * _sbrk -- Increment the program break.
+ *
+ * Get incr bytes more RAM (for use by the heap).  malloc() and
+ * friends call this function behind the scenes.
  */
-caddr_t _sbrk(int nbytes) {
-    static caddr_t heap_ptr = NULL;
-    caddr_t        base;
+caddr_t _sbrk(int incr) {
+    static caddr_t pbreak = NULL; /* current program break */
+    caddr_t ret;
 
-    if (heap_ptr == NULL) {
-        heap_ptr = (caddr_t)&_end;
+    if (pbreak == NULL) {
+        pbreak = HEAP_START;
     }
 
-    if ((STACK_TOP - (unsigned int)heap_ptr) >= 0) {
-        base = heap_ptr;
-        heap_ptr += nbytes;
-        return (base);
-    } else {
-        return ((caddr_t)-1);
+    if ((HEAP_END - pbreak < incr) || (pbreak - HEAP_START < -incr)) {
+        errno = ENOMEM;
+        return (caddr_t)-1;
     }
+
+    ret = pbreak;
+    pbreak += incr;
+    return ret;
 }
 
 int _open(const char *path, int flags, ...) {