linux-kernel - [PATCH] /dev/zero: fixups for ->read

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Message-Id: <20200907082700.2057137-1-hch@lst.de>
Date:   Mon,  7 Sep 2020 10:27:00 +0200
From:   Christoph Hellwig <hch@....de>
To:     arnd@...db.de, gregkh@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: [PATCH] /dev/zero: fixups for ->read

Reported the cleared bytes in case of a partial clear_user instead
of -EFAULT, and remove a pointless conditional, as cleared must be
non-zero by the time we hit the signal_pending check.

Reported-by: Rasmus Villemoes <linux@...musvillemoes.dk>
Signed-off-by: Christoph Hellwig <hch@....de>
---
 drivers/char/mem.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 1dc99ab158457a..94c2b556cf9728 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -733,14 +733,20 @@ static ssize_t read_zero(struct file *file, char __user *buf,
 
 	while (count) {
 		size_t chunk = min_t(size_t, count, PAGE_SIZE);
+		size_t left;
 
-		if (clear_user(buf + cleared, chunk))
-			return cleared ? cleared : -EFAULT;
+		left = clear_user(buf + cleared, chunk);
+		if (unlikely(left)) {
+			cleared += (chunk - left);
+			if (!cleared)
+				return -EFAULT;
+			break;
+		}
 		cleared += chunk;
 		count -= chunk;
 
 		if (signal_pending(current))
-			return cleared ? cleared : -ERESTARTSYS;
+			break;
 		cond_resched();
 	}
 
-- 
2.28.0