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

[NeilBrown]

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@xxxxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
 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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