modpost: improve the section mismatch warning format

This commit improves the section mismatch warning format when there is
no suitable symbol name to print.

The section mismatch warning prints the reference source in the form
of <symbol_name>+<offset> and the reference destination in the form
of <symbol_name>.

However, there are some corner cases where <symbol_name> becomes
"(unknown)", as reported in commit 23dfd914 ("modpost: fix null
pointer dereference").

In such cases, it is better to print the symbol address.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/mod/modpost.c

@@ -697,10 +697,7 @@ static char *get_modinfo(struct elf_info *info, const char *tag)
 
 static const char *sym_name(struct elf_info *elf, Elf_Sym *sym)
 {
-	if (sym)
-		return elf->strtab + sym->st_name;
-	else
-		return "(unknown)";
+	return sym ? elf->strtab + sym->st_name : "";
 }
 
 /*
@@ -1013,6 +1010,7 @@ static void default_mismatch_handler(const char *modname, struct elf_info *elf,
 	Elf_Sym *from;
 	const char *tosym;
 	const char *fromsym;
+	char taddr_str[16];
 
 	from = find_fromsym(elf, faddr, fsecndx);
 	fromsym = sym_name(elf, from);
@@ -1026,10 +1024,17 @@ static void default_mismatch_handler(const char *modname, struct elf_info *elf,
 
 	sec_mismatch_count++;
 
-	warn("%s: section mismatch in reference: %s+0x%x (section: %s) -> %s (section: %s)\n",
-	     modname, fromsym,
-	     (unsigned int)(faddr - (from ? from->st_value : 0)),
-	     fromsec, tosym, tosec);
+	if (!tosym[0])
+		snprintf(taddr_str, sizeof(taddr_str), "0x%x", (unsigned int)taddr);
+
+	/*
+	 * The format for the reference source:      <symbol_name>+<offset> or <address>
+	 * The format for the reference destination: <symbol_name>          or <address>
+	 */
+	warn("%s: section mismatch in reference: %s%s0x%x (section: %s) -> %s (section: %s)\n",
+	     modname, fromsym, fromsym[0] ? "+" : "",
+	     (unsigned int)(faddr - (fromsym[0] ? from->st_value : 0)),
+	     fromsec, tosym[0] ? tosym : taddr_str, tosec);
 
 	if (mismatch->mismatch == EXTABLE_TO_NON_TEXT) {
 		if (match(tosec, mismatch->bad_tosec))