modpost: fix potential mmap'ed file overrun in get_src_version()

I do not know how reliably this function works, but it looks dangerous
to me.

    strchr(sources, '\n');

... continues searching until it finds '\n' or it reaches the '\0'
terminator. In other words, 'sources' should be a null-terminated
string.

However, grab_file() just mmaps a file, so 'sources' is not terminated
with null byte. If the file does not contain '\n' at all, strchr() will
go beyond the mmap'ed memory.

Use read_text_file(), which loads the file content into a malloc'ed
buffer, appending null byte.

Here we are interested only in the first line of *.mod files. Use
get_line() helper to get the first line.

This also makes missing *.mod file a fatal error.

Commit 4be40e2223 ("kbuild: do not emit src version warning for
non-modules") ignored missing *.mod files.

I do not fully understand what that commit addressed, but commit
91341d4b2c ("kbuild: introduce new option to enhance section mismatch
analysis") introduced partial section checks by using modpost. built-in.o
was parsed by modpost. Even modules had a problem because *.mod files
were created after the modpost check.

Commit b7dca6dd1e ("kbuild: create *.mod with full directory path and
remove MODVERDIR") stopped doing that. Now that modpost is only invoked
after the directory descend, *.mod files should always exist at the
modpost stage.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/mod/sumversion.c

@@ -392,40 +392,34 @@ out:
 /* Calc and record src checksum. */
 void get_src_version(const char *modname, char sum[], unsigned sumlen)
 {
-	void *file;
-	unsigned long len;
+	char *buf, *pos, *firstline;
 	struct md4_ctx md;
-	char *sources, *end, *fname;
+	char *fname;
 	char filelist[PATH_MAX + 1];
 
 	/* objects for a module are listed in the first line of *.mod file. */
 	snprintf(filelist, sizeof(filelist), "%.*smod",
 		 (int)strlen(modname) - 1, modname);
 
-	file = grab_file(filelist, &len);
-	if (!file)
-		/* not a module or .mod file missing - ignore */
-		return;
+	buf = read_text_file(filelist);
 
-	sources = file;
-
-	end = strchr(sources, '\n');
-	if (!end) {
+	pos = buf;
+	firstline = get_line(&pos);
+	if (!firstline) {
 		warn("bad ending versions file for %s\n", modname);
-		goto release;
+		goto free;
 	}
-	*end = '\0';
 
 	md4_init(&md);
-	while ((fname = strsep(&sources, " ")) != NULL) {
+	while ((fname = strsep(&firstline, " "))) {
 		if (!*fname)
 			continue;
 		if (!(is_static_library(fname)) &&
 				!parse_source_files(fname, &md))
-			goto release;
+			goto free;
 	}
 
 	md4_final_ascii(&md, sum, sumlen);
-release:
-	release_file(file, len);
+free:
+	free(buf);
 }