[PATCH mm] VFS: seq_file: ensure ->from is valid.

From: NeilBrown <neilb@suse.com>
To: Jann Horn <jannh@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	linux-doc@vger.kernel.org,
	kernel list <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>
Subject: [PATCH mm] VFS: seq_file: ensure ->from is valid.
Date: Sat, 07 Jul 2018 13:29:41 +1000	[thread overview]
Message-ID: <87601ryb8a.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <878t6nybj7.fsf@notabene.neil.brown.name>

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Previous patch ("VFS: simplify seq_file iteration code and interface")
removed code to set ->from to zero when ->count is zero, as ->from is
dead at that time.  However it didn't ensure ->from was set properly
whenever ->count becomes non-zero.
This can only happen when ->show() is called.  Of the three places it
is called one already has ->from set to zero.  The other two are
fixed by setting from to zero after fully flushing the buffer (at which
point ->count will also be zero).

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: NeilBrown <neilb@suse.com>
---
 fs/seq_file.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index fd82585ab50f..1dea7a8a5255 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -220,6 +220,7 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 			goto Done;
 	}
 	/* we need at least one record in buffer */
+	m->from = 0;
 	p = m->op->start(m, &m->index);
 	while (1) {
 		err = PTR_ERR(p);
-- 
2.14.0.rc0.dirty


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