From: Kuniyuki Iwashima <kuniyu@xxxxxxxxxxxx>
Date: Thu, 22 Jul 2021 23:16:37 +0900
> From: Martin KaFai Lau <kafai@xxxxxx>
> Date: Thu, 1 Jul 2021 13:05:41 -0700
> > st->bucket stores the current bucket number.
> > st->offset stores the offset within this bucket that is the sk to be
> > seq_show(). Thus, st->offset only makes sense within the same
> > st->bucket.
> >
> > These two variables are an optimization for the common no-lseek case.
> > When resuming the seq_file iteration (i.e. seq_start()),
> > tcp_seek_last_pos() tries to continue from the st->offset
> > at bucket st->bucket.
> >
> > However, it is possible that the bucket pointed by st->bucket
> > has changed and st->offset may end up skipping the whole st->bucket
> > without finding a sk. In this case, tcp_seek_last_pos() currently
> > continues to satisfy the offset condition in the next (and incorrect)
> > bucket. Instead, regardless of the offset value, the first sk of the
> > next bucket should be returned. Thus, "bucket == st->bucket" check is
> > added to tcp_seek_last_pos().
> >
> > The chance of hitting this is small and the issue is a decade old,
> > so targeting for the next tree.
>
> Multiple read()s or lseek()+read() can call tcp_seek_last_pos().
>
> IIUC, the problem happens when the sockets placed before the last shown
> socket in the list are closed between some read()s or lseek() and read().
>
> I think there is still a case where bucket is valid but offset is invalid:
>
> listening_hash[1] -> sk1 -> sk2 -> sk3 -> nulls
> listening_hash[2] -> sk4 -> sk5 -> nulls
>
> read(/proc/net/tcp)
> end up with sk2
>
> close(sk1)
>
> listening_hash[1] -> sk2 -> sk3 -> nulls
> listening_hash[2] -> sk4 -> sk5 -> nulls
>
> read(/proc/net/tcp) (resume)
> offset = 2
>
> listening_get_next() returns sk2
>
> while (offset--)
> 1st loop listening_get_next() returns sk3 (bucket == st->bucket)
> 2nd loop listening_get_next() returns sk4 (bucket != st->bucket)
>
> show() starts from sk4
>
> only is sk3 skipped, but should be shown.
Sorry, this example is wrong.
We can handle this properly by testing bucket != st->bucket.
In the case below, we cannot check if the offset is valid or not by testing
the bucket.
listening_hash[1] -> sk1 -> sk2 -> sk3 -> sk4 -> nulls
read(/proc/net/tcp)
end up with sk2
close(sk1)
listening_hash[1] -> sk2 -> sk3 -> sk4 -> nulls
read(/proc/net/tcp) (resume)
offset = 2
listening_get_first() returns sk2
while (offset--)
1st loop listening_get_next() returns sk3 (bucket == st->bucket)
2nd loop listening_get_next() returns sk4 (bucket == st->bucket)
show() starts from sk4
only is sk3 skipped, but should be shown.
>
> In listening_get_next(), we can check if we passed through sk2, but this
> does not work well if sk2 itself is closed... then there are no way to
> check the offset is valid or not.
>
> Handling this may be too much though, what do you think ?
>
