build: symtab: Add a dummy entry at the end for the binary search

The `symtab_find_symbol_name()` is using an adapted binary
search function to get the entry between 2 addresses, we need
to add a dummy entry at the end so that the search function
can remain simple and straightforward without doing
out-of-bound checks:

   20  \
       |
       |
   50  x
       |
       |
   90  x
    .  |
    .  |
    .  |
dummy  /

Signed-off-by: Yong Cong Sin <ycsin@meta.com>

scripts/build/gen_symtab.py

@@ -55,8 +55,9 @@ def parse_args():
 
 
 class symtab_entry:
-    def __init__(self, addr, offset, name):
+    def __init__(self, addr, size, offset, name):
         self.addr = addr
+        self.size = size
         self.offset = offset
         self.name = name
 
@@ -90,11 +91,12 @@ def main():
         for nsym, symbol in enumerate(symtab.iter_symbols()):  # pylint: disable=unused-variable
             symbol_type = describe_symbol_type(symbol['st_info']['type'])
             symbol_addr = symbol['st_value']
+            symbol_size = symbol['st_size']
 
             if symbol_type == 'FUNC' and symbol_addr != 0:
                 symbol_name = sanitize_func_name(symbol.name)
                 symtab_list.append(symtab_entry(
-                    symbol_addr, symbol_addr, symbol_name))
+                    symbol_addr, symbol_addr, symbol_size, symbol_name))
                 log.debug('%6d: %s %.25s' % (
                     i,
                     hex(symbol_addr),
@@ -115,10 +117,21 @@ def main():
         print("#include <zephyr/debug/symtab.h>", file=wf)
         print("", file=wf)
         print(
-            f"const struct z_symtab_entry z_symtab_entries[{len(symtab_list)}] = {{", file=wf)
+            f"const struct z_symtab_entry z_symtab_entries[{len(symtab_list) + 1}] = {{", file=wf)
         for i, entry in enumerate(symtab_list):
             print(
-                f"\t[{i}] = {{.offset = {hex(entry.offset)}, .name = \"{entry.name}\"}}, /* {hex(entry.addr)} */", file=wf)
+                f"\t/* ADDR: {hex(entry.addr)} SIZE: {hex(entry.size)} */", file=wf)
+            print(
+                f"\t[{i}] = {{.offset = {hex(entry.offset)}, .name = \"{entry.name}\"}},", file=wf)
+
+        # Append a dummy entry at the end to facilitate the binary search
+        if symtab_list[-1].size == 0:
+            dummy_offset = f"{hex(symtab_list[-1].offset)} + sizeof(uintptr_t)"
+        else:
+            dummy_offset = f"{hex(symtab_list[-1].offset + symtab_list[-1].size)}"
+        print("\t/* dummy entry */", file=wf)
+        print(
+            f"\t[{len(symtab_list)}] = {{.offset = {dummy_offset}, .name = \"?\"}},", file=wf)
         print(f"}};\n", file=wf)
 
         print(f"const struct symtab_info z_symtab = {{", file=wf)

subsys/debug/symtab/symtab.c

@@ -5,6 +5,7 @@
  */
 
 #include <string.h>
+#include <stdbool.h>
 
 #include <zephyr/debug/symtab.h>
 
@@ -19,22 +20,25 @@ const char *const symtab_find_symbol_name(uintptr_t addr, uint32_t *offset)
 {
 	const struct symtab_info *const symtab = symtab_get();
 	const uint32_t symbol_offset = addr - symtab->start_addr;
-	uint32_t left = 0, right = symtab->length - 1;
+	uint32_t left = 0, right = symtab->length;
 	uint32_t ret_offset = 0;
 	const char *ret_name = "?";
 
-	while (left <= right) {
+	/* No need to search if the address is out-of-bound */
-		uint32_t mid = left + (right - left) / 2;
+	if (symbol_offset < symtab->entries[symtab->length].offset) {
+		while (left <= right) {
+			uint32_t mid = left + (right - left) / 2;
 
-		if ((symbol_offset >= symtab->entries[mid].offset) &&
+			if ((symbol_offset >= symtab->entries[mid].offset) &&
-		    (symbol_offset < symtab->entries[mid + 1].offset)) {
+			(symbol_offset < symtab->entries[mid + 1].offset)) {
-			ret_offset = symbol_offset - symtab->entries[mid].offset;
+				ret_offset = symbol_offset - symtab->entries[mid].offset;
-			ret_name = symtab->entries[mid].name;
+				ret_name = symtab->entries[mid].name;
-			break;
+				break;
-		} else if (symbol_offset < symtab->entries[mid].offset) {
+			} else if (symbol_offset < symtab->entries[mid].offset) {
-			right = mid - 1;
+				right = mid - 1;
-		} else {
+			} else {
-			left = mid + 1;
+				left = mid + 1;
+			}
 		}
 	}