Re: [RFC][PATCH 07/12 v3] tracing: Have seq_buf use full buffer

Petr Mladek
2014-11-06T15:31:13Z

On Wed 2014-11-05 16:06:18, Steven Rostedt wrote:
> On Wed, 5 Nov 2014 15:21:30 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > >
> > > I wonder if we want this change at all. It means that we are not able to
> > > detect overflow in some functions. It is pity because the users
> > > might want to increase the buffer size and try again if the print
> > > was incomplete.
> >
> > What do you mean we can't detect overflow? That's what
> > seq_buf_has_overflowed() does.
> >
>
> Although I'm looking at the seq_file versions of the bitmap code, which
> does only return the len of what was written and not what would have
> been written, and it does have this issue.
>
> I hate to go back to the -1 of the size of buffer as that causes
> inconsistencies within the functions themselves, as proved with the
> seq_file code.

Yeah, the -1 and the unused byte is strange and it would be great to
avoid it.

On the other hand, I am slightly afraid of the "len = size + 1" that
signalizes the buffer overflow. It might be prone for creating security
bugs. If people forget to check seq_buf_has_overflowed() before
reading or if there is a race, they might read outside of the buffer.

> What I might do as just have the bitmap calls not be allowed to fill
> the buffer and keep the logic the same. That is, if the bitmap calls
> fill the rest of the length, assume we overflowed, otherwise we are
> fine.
>
> I'm going to change seq_buf to do that instead of my new update with
> the bitmask code.

I like the idea of having the exception only in the bitmap code and filling
the whole buffer in other cases.

I am now in doubts about the overflow state. A solution would
be to add an "overflow" flag to struct seq_bug. I agree that it
is ugly but it looks more secure then "len = size + 1".

Well, I do not have that strong opinion about it. What do you think?

Best Regards,
Petr