[PATCH 19/19] avoid -Warray-bounds warning, avoid stack overallocation

[Eric Wong <e@80x24.org>]

Compilers don't like seeing `sl.bt[-1]', so we give backtrace(3)
the same address by another means.  We'll also save allocating
one pointer off the stack by capping the union padding.
---
 mwrap_core.h | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/mwrap_core.h b/mwrap_core.h
index 1b3d98f..3fd67f1 100644
--- a/mwrap_core.h
+++ b/mwrap_core.h
@@ -134,6 +134,7 @@ static void *my_mempcpy(void *dest, const void *src, size_t n)
 #define RETURN_ADDRESS(nr) \
   __builtin_extract_return_addr(__builtin_return_address(nr))
 
+
 #define SRC_LOC_BT(bt) union stk_bt bt; do { \
 	uint32_t depth = locating ? 1 : bt_req_depth; \
 	switch (depth) { \
@@ -142,7 +143,7 @@ static void *my_mempcpy(void *dest, const void *src, size_t n)
 	default: /* skip 1st level of BT since thats our function */ \
 		mwrap_assert(bt_req_depth <= MWRAP_BT_MAX); \
 		++locating; \
-		long n = (long)backtrace(&bt.sl.bt[-1], bt_req_depth); \
+		long n = (long)backtrace(bt_dst(&bt), bt_req_depth); \
 		--locating; \
 		bt.sl.bt_len = n <= 1 ? 0 : (uint32_t)n - 1; \
 		if (n > 1) mwrap_assert(bt.sl.bt[0] == RETURN_ADDRESS(0)); \
@@ -209,9 +210,27 @@ union stk_sf {
 
 union stk_bt {
 	struct src_loc sl;
-	char buf_[sizeof(struct src_loc) + sizeof(void *) * MWRAP_BT_MAX];
+	/* we subtract one level from MWRAP_BT_MAX since we discard one
+	 * level of backtrace(3) (see below for why) */
+	char buf_[sizeof(struct src_loc) + sizeof(void *) * (MWRAP_BT_MAX-1)];
 };
 
+/*
+ * we discard the 1st-level of the backtrace(3) since it's our *alloc
+ * function (and therefore uninteresting), so we want backtrace(3) to
+ * write to bt->sl.bt[-1] so that bt->sl.bt[0] is the first interesting
+ * thing.
+ */
+#ifdef static_assert
+static_assert(offsetof(struct src_loc, lineno) + sizeof(void *) ==
+		offsetof(struct src_loc, bt),
+		"bt lineno is is bt[-1]");
+#endif
+static void **bt_dst(union stk_bt *bt)
+{
+	return (void **)&bt->sl.lineno;
+}
+
 static struct alloc_hdr *ptr2hdr(void *p)
 {
 	return (struct alloc_hdr *)((uintptr_t)p - sizeof(struct alloc_hdr));