[PATCH AUTOSEL 6.6 16/20] modpost: fix null pointer dereference

[Sasha Levin]

From: Max Kellermann <max.kellermann@xxxxxxxxx>

[ Upstream commit 23dfd914d2bfc4c9938b0084dffd7105de231d98 ]

If the find_fromsym() call fails and returns NULL, the warn() call
will dereference this NULL pointer and cause the program to crash.

This happened when I tried to build with "test_user_copy" module.
With this fix, it prints lots of warnings like this:

 WARNING: modpost: lib/test_user_copy: section mismatch in reference: (unknown)+0x4 (section: .text.fixup) -> (unknown) (section: .init.text)

masahiroy@xxxxxxxxxx:
 The issue is reproduced with ARCH=arm allnoconfig + CONFIG_MODULES=y +
 CONFIG_RUNTIME_TESTING_MENU=y + CONFIG_TEST_USER_COPY=m

Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx>
Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 scripts/mod/modpost.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 5191fdbd3fa23..076f3c9e43fa5 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1153,7 +1153,9 @@ 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->st_value), fromsec, tosym, tosec);
+	     modname, fromsym,
+	     (unsigned int)(faddr - (from ? from->st_value : 0)),
+	     fromsec, tosym, tosec);
 
 	if (mismatch->mismatch == EXTABLE_TO_NON_TEXT) {
 		if (match(tosec, mismatch->bad_tosec))
-- 
2.43.0