[PATCH 1/2] cobalt/vfile: seq_file seek index must progress

[Philippe Gerum]

From: Philippe Gerum <rpm at xenomai.org>

The offset field we receive from the kernel in a vfile next() handler
must progress in order for the loop to stop properly, independently
from our own tracking of the end-of-list condition.

Bug is reproducible by running two loops in parallel:

- one continuously spawning an application which creates a few tenths
of threads (10-20 would suffice) before exiting shortly after.

- another one continuously reading from /proc/xenomai/sched/{threads,
  stat, acct}.

At some point, the vfile handler should cause a kernel crash.

Signed-off-by: Philippe Gerum <rpm at xenomai.org>
---
 kernel/cobalt/vfile.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/kernel/cobalt/vfile.c b/kernel/cobalt/vfile.c
index c7b81a704..fae0cc377 100644
--- a/kernel/cobalt/vfile.c
+++ b/kernel/cobalt/vfile.c
@@ -109,11 +109,11 @@ static void *vfile_snapshot_next(struct seq_file *seq, void *v, loff_t *offp)
 	struct xnvfile_snapshot_iterator *it = seq->private;
 	loff_t pos = *offp;
 
+	++*offp;
+
 	if (pos >= it->nrdata)
 		return NULL;
 
-	++*offp;
-
 	return it->databuf + pos * it->vfile->datasz;
 }
 
@@ -452,17 +452,15 @@ static void *vfile_regular_next(struct seq_file *seq, void *v, loff_t *offp)
 	struct xnvfile_regular *vfile = it->vfile;
 	void *data;
 
+	it->pos = ++(*offp);
+
 	if (vfile->ops->next == NULL)
 		return NULL;
 
-	it->pos = *offp + 1;
-
 	data = vfile->ops->next(it);
 	if (data == NULL)
 		return NULL;
 
-	*offp = it->pos;
-
 	return data;
 }
 
-- 
2.31.1