linux-kernel - [PATCH] mm/uaccess: Use 'unsigned long' to placate UBSAN warnings, again

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Message-Id: <590998aa9cc50f431343f76cae72b2abf8ac1fdd.1608699683.git.jpoimboe@redhat.com>
Date:   Tue, 22 Dec 2020 23:04:54 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: [PATCH] mm/uaccess: Use 'unsigned long' to placate UBSAN warnings, again

GCC 7 has a known bug where UBSAN ignores '-fwrapv' and generates false
signed-overflow-UB warnings.  The type mismatch between 'i' and
'nr_segs' in copy_compat_iovec_from_user() is causing such a warning,
which also happens to violate uaccess rules:

  lib/iov_iter.o: warning: objtool: iovec_from_user()+0x22d: call to __ubsan_handle_add_overflow() with UACCESS enabled

Fix it by making the variable types match.

This is similar to a previous commit:

  29da93fea3ea ("mm/uaccess: Use 'unsigned long' to placate UBSAN warnings on older GCC versions")

Reported-by: Randy Dunlap <rdunlap@...radead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 lib/iov_iter.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 1635111c5bd2..2e6a42f5d1df 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1656,7 +1656,8 @@ static int copy_compat_iovec_from_user(struct iovec *iov,
 {
 	const struct compat_iovec __user *uiov =
 		(const struct compat_iovec __user *)uvec;
-	int ret = -EFAULT, i;
+	int ret = -EFAULT;
+	unsigned long i;
 
 	if (!user_access_begin(uvec, nr_segs * sizeof(*uvec)))
 		return -EFAULT;
-- 
2.29.2