linux-kernel - [PATCH] kernel/kcov: unproxify debugfs file's fops

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Message-Id: <1464011147-31836-1-git-send-email-nicstange@gmail.com>
Date:	Mon, 23 May 2016 15:45:47 +0200
From:	Nicolai Stange <nicstange@...il.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Nicolai Stange <nicstange@...il.com>,
	James Morse <james.morse@....com>, linux-kernel@...r.kernel.org
Subject: [PATCH] kernel/kcov: unproxify debugfs file's fops

Since commit 49d200deaa68 ("debugfs: prevent access to removed files'
private data"), a debugfs file's file_operations methods get proxied
through lifetime aware wrappers.

However, only a certain subset of the file_operations members is supported
by debugfs and ->mmap isn't among them -- it appears to be NULL from the
VFS layer's perspective.

This behaviour breaks the /sys/kernel/debug/kcov file introduced
concurrently with commit 5c9a8750a640 ("kernel: add kcov code coverage").

Since that file never gets removed, there is no file removal race and thus,
a lifetime checking proxy isn't needed.

Avoid the proxying for /sys/kernel/debug/kcov by creating it via
debugfs_create_file_unsafe() rather than debugfs_create_file().

Fixes: 49d200deaa68 ("debugfs: prevent access to removed files' private
                      data")
Fixes: 5c9a8750a640 ("kernel: add kcov code coverage")
Signed-off-by: Nicolai Stange <nicstange@...il.com>
---
 Applicable to linux-next 20160523.
 In particular, it depends on
 - c64688081490 ("debugfs: add support for self-protecting attribute file
                  fops")
 - 5c9a8750a640 ("kernel: add kcov code coverage")

 This issue has been debugged and reported by
 Sasha Levin <sasha.levin@...cle.com>:
 http://lkml.kernel.org/g/573F4200.3080208@oracle.com

 kernel/kcov.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index a02f2dd..4c349dd 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -264,7 +264,7 @@ static const struct file_operations kcov_fops = {

 static int __init kcov_init(void)
 {
-	if (!debugfs_create_file("kcov", 0600, NULL, NULL, &kcov_fops)) {
+	if (!debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops)) {
 		pr_err("failed to create kcov in debugfs\n");
 		return -ENOMEM;
 	}
-- 
2.8.2