linux-kernel - gcov: NULL pointer dereference with gcc 9.3.1

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Date:   Tue, 1 Jun 2021 16:56:50 +0100
From:   Luis Henriques <lhenriques@...e.de>
To:     Peter Oberparleiter <oberpar@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org
Subject: gcov: NULL pointer dereference with gcc 9.3.1

Hi!

Maybe this is a known (gcc?) issue, but I'm seeing a NULL pointer splat if
I instrument my kernel (or a module, more specifically) using gcc 9.3.1.

It looks like, during initialization in __gcov_init(), gcov_info struct is
invalid: the filename seems to be correct but ->function is NULL and
->n_functions contains garbage.  This will result in the NULL pointer if,
for example, I run:

  # echo 1 > /sys/kernel/debug/gcov/reset

Other installed gcc versions (7.5.0 and 10.3.0) work fine.

Is this a know issue with this specific gcc versions?  Anyway, it may
be worth having something like (not tested!):

diff --git a/kernel/gcov/gcc_base.c b/kernel/gcov/gcc_base.c
index 3cf736b9f880..de3ea47f4f62 100644
--- a/kernel/gcov/gcc_base.c
+++ b/kernel/gcov/gcc_base.c
@@ -13,6 +13,11 @@ void __gcov_init(struct gcov_info *info)
 {
 	static unsigned int gcov_version;

+	if (!info || !info->functions) {
+		pr_err_once("Invalid gcov_info structure\n");
+		return;
+	}
+
 	mutex_lock(&gcov_lock);
 	if (gcov_version == 0) {
 		gcov_version = gcov_info_version(info);

And also something similar in the clang code, I guess.

I can send a real (tested) patch if you wish, but I just wanted to know if
I'm missing something first.

Cheers,
--
Luis