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seq_file: fix problem when seeking mid-record 

If you use lseek or similar (e.g.  pread) to access a location in a
seq_file file that is within a record, rather than at a record boundary,
then the first read will return the remainder of the record, and the
second read will return the whole of that same record (instead of the
next record).  When seeking to a record boundary, the next record is
correctly returned.

This bug was introduced by a recent patch (identified below).  Before
that patch, seq_read() would increment m->index when the last of the
buffer was returned (m->count == 0).  After that patch, we rely on
->next to increment m->index after filling the buffer - but there was
one place where that didn't happen.

Link: https://lkml.kernel.org/lkml/877e7xl029.fsf@notabene.neil.brown.name/
Fixes: 1f4aace60b ("fs/seq_file.c: simplify seq_file iteration code and interface")
Signed-off-by: NeilBrown <neilb@suse.com>
Reported-by: Sergei Turchanov <turchanov@farpost.com>
Tested-by: Sergei Turchanov <turchanov@farpost.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Markus Elfring <Markus.Elfring@web.de>
Cc: <stable@vger.kernel.org>	[4.19+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
NeilBrown 2019-08-13 15:37:44 -07:00 committed by Linus Torvalds
parent ec9f02384f
commit 6a2aeab59e
1 changed files with 1 additions and 1 deletions
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2
fs/seq_file.c
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@ -119,6 +119,7 @@ static int traverse(struct seq_file *m, loff_t offset)
} }
if (seq_has_overflowed(m)) if (seq_has_overflowed(m))
goto Eoverflow; goto Eoverflow;
p = m->op->next(m, p, &m->index);
if (pos + m->count > offset) { if (pos + m->count > offset) {
m->from = offset - pos; m->from = offset - pos;
m->count -= m->from; m->count -= m->from;
@ -126,7 +127,6 @@ static int traverse(struct seq_file *m, loff_t offset)
} }
pos += m->count; pos += m->count;
m->count = 0; m->count = 0;
p = m->op->next(m, p, &m->index);
if (pos == offset) if (pos == offset)
break; break;
} }