Fix setjmp/longjmp table matching to survive raw jmp_buf save/restore

src/glibc/setjmp/wasm_eh_setjmp.c

@@ -13,13 +13,26 @@
  *     __wasm_longjmp(buf, val)   →   throw __c_longjmp(&buf[2])
  *
  * jmp_buf layout (as used by this implementation):
- *   buf[0]  self-reference set by saveSetjmp (used by testSetjmp)
+ *   buf[0]  registration token set by saveSetjmp (used by testSetjmp)
  *   buf[1]  unused (reserved)
  *   buf[2]  self-reference set by __wasm_longjmp (payload for throw)
  *   buf[3]  normalised return value set by __wasm_longjmp
  *
  * The jmp_buf typedef is long int[8] (32 bytes on wasm32), which is
  * large enough for all four 4-byte slots used here.
+ *
+ * buf[0] holds a per-call unique token rather than buf's own address.  This
+ * matters for jmp_bufs that live in a fixed-address global and get saved and
+ * restored as raw bytes by their caller (a common unwind-protect idiom built
+ * on setjmp/longjmp: snapshot sizeof(jmp_buf), memcpy it back later to
+ * restore an outer context): using buf's address as the token would make
+ * every registration on that global indistinguishable, since the address
+ * never changes.  A fresh token per saveSetjmp() call means a caller that
+ * restores an *older* snapshot of the buffer (reverting buf[0] to an outer,
+ * still-live registration's token) correctly falls through this frame's
+ * table and resolves in whichever outer frame's table still holds that
+ * token — matching what raw stack/register snapshot restoration does
+ * natively.  See testSetjmp() for the failure mode this avoids.
  */
 
 #include <stdint.h>
@@ -32,6 +45,10 @@
  * caller via getTempRet0 (wasm has no multi-value imported functions yet). */
 static __thread int g_tempRet0;
 
+/* Thread-local monotonic counter used to mint a unique, never-zero token for
+ * each saveSetjmp() registration (see buf[0] discussion above). */
+static __thread int g_setjmp_token;
+
 __attribute__((visibility("default"))) void setTempRet0(int val)
 {
     g_tempRet0 = val;
@@ -45,22 +62,24 @@ __attribute__((visibility("default"))) int getTempRet0(void)
 /*
  * saveSetjmp(buf, label_id, table, size) → new_table
  *
- * Registers the (buf, label_id) pair in *table* and stores buf's own address
- * in buf[0] so testSetjmp can identify the frame later.  Grows the table
- * (via realloc) when full.  Communicates the (possibly new) table size via
- * setTempRet0().
+ * Registers the (token, label_id) pair in *table* and stores a fresh,
+ * never-zero token in buf[0] so testSetjmp can identify the frame later
+ * (see the file header comment for why this is a per-call token rather than
+ * buf's own address).  Grows the table (via realloc) when full.
+ * Communicates the (possibly new) table size via setTempRet0().
  *
  * The initial table is allocated by the caller (malloc(40), size=4).
  */
 __attribute__((visibility("default"))) int *saveSetjmp(int *buf, int label_id, int *table, int size)
 {
-    /* Store self-reference so testSetjmp can match on buf[0]. */
-    buf[0] = (int)(uintptr_t)buf;
+    int token = ++g_setjmp_token;
+    if (token == 0) token = ++g_setjmp_token; /* skip the 0 sentinel on wraparound */
+    buf[0] = token;
 
     /* Find an empty slot. */
     for (int i = 0; i < size; i++) {
         if (table[2 * i] == 0) {
-            table[2 * i]     = (int)(uintptr_t)buf;
+            table[2 * i]     = token;
             table[2 * i + 1] = label_id;
             setTempRet0(size);
             return table;
@@ -79,7 +98,7 @@ __attribute__((visibility("default"))) int *saveSetjmp(int *buf, int label_id, i
     /* Zero the freshly allocated half. */
     memset(new_table + size * 2, 0, (size_t)size * 2 * sizeof(int));
 
-    new_table[size * 2]     = (int)(uintptr_t)buf;
+    new_table[size * 2]     = token;
     new_table[size * 2 + 1] = label_id;
 
     setTempRet0(new_size);
@@ -89,16 +108,43 @@ __attribute__((visibility("default"))) int *saveSetjmp(int *buf, int label_id, i
 /*
  * testSetjmp(env, table, size) → label_id or 0
  *
- * Called inside the EH catch block.  Searches *table* for a buf whose
- * address matches *env* (== buf[0] set by saveSetjmp).  Returns the
- * label_id of the matching setjmp frame, or 0 if not found (meaning the
- * exception belongs to an outer frame and must be re-thrown).
+ * Called inside the EH catch block.  Searches *table* for a registration
+ * whose token matches *env* (== buf[0], read fresh from the jmp_buf at catch
+ * time).  Returns the label_id of the matching setjmp frame, or 0 if not
+ * found (meaning the exception belongs to an outer frame and must be
+ * re-thrown).
+ *
+ * *table* is shared by every setjmp() call site within one function
+ * invocation (see saveSetjmp), so it can hold more than one entry: e.g. when
+ * a function calls setjmp() on the same jmp_buf more than once (a loop, or
+ * two call sites merged into one function by inlining), each call appends a
+ * new entry with a fresh token rather than replacing the old one.  Because
+ * tokens are unique per call, at most one entry can ever match a given env
+ * value, so search order doesn't matter for correctness; newest-first is
+ * kept simply because the live registration is usually the most recently
+ * appended one, making the common case a single-iteration scan.
+ *
+ * Entries are intentionally left in the table after a match (not cleared).
+ * buf[0]/env reflects whatever a caller's raw jmp_buf save/restore (e.g. an
+ * unwind-protect helper built on memcpy(&saved, &buf, sizeof(buf))) currently
+ * holds, which may be an older, still-live outer registration's token rather
+ * than this frame's own — see the file header comment. In that case env
+ * simply won't match anything in *this* table and correctly falls through to
+ * be re-thrown to the enclosing frame, which still holds that token in its
+ * own table.  This is what actually resolves an unbounded catch/rethrow loop
+ * that used to occur when buf[0] was the jmp_buf's own (fixed,
+ * restore-invariant) address instead of a token: a caller's post-catch
+ * re-throw to the same buffer kept re-matching this frame's own entry
+ * forever, exhausting the null-collected GC heap one exception object at a
+ * time until it crashed with "allocation size too large" instead of
+ * terminating.
  */
 __attribute__((visibility("default"))) int testSetjmp(int env, int *table, int size)
 {
-    for (int i = 0; i < size; i++) {
-        if (table[2 * i] == env)
+    for (int i = size - 1; i >= 0; i--) {
+        if (table[2 * i] == env) {
             return table[2 * i + 1];
+        }
     }
     return 0;
 }