Re: [GIT PULL] tracing: Prevent trace_marker being bigger than unsigned short

[Mathieu Desnoyers]

On 2024-03-04 19:43, Steven Rostedt wrote:
> On Mon, 4 Mar 2024 16:17:15 -0800
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> > On Mon, 4 Mar 2024 at 15:50, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > > But this still isn't fixing anything. It's just adding a limit.
> >
> > Limiting things to a common maximum size is a good thing. The kernel
> > limits much  more important things for very good reasons.
> >
> > The kernel really shouldn't have big strings. EVER.  And it literally
> > shows in our kernel infrastructure. It showed in that vsnprintf
> > precision thing. It shows in our implementation choices, where we tend
> > to have simplistic implementations because doing things a byte at a
> > time is simple and cheap when the strings are limited in size (and we
> > don't want fancy and can't use vector state anyway).
> >
> > If something as core as a pathname can be limited to 4kB, then
> > something as unimportant as a trace string had better be limited too.
> > Because we simply DO NOT WANT to have to deal with longer strings in
> > the kernel.
>
> So I made three patches that do basically what you want. And as a bonus,
> it's not really an arbitrary limit but based on trace_seq size.
>
> The first patch will be removing the precision check, as that's not needed.
>
> The second patch is to remove the dependency between trace_seq and
> PAGE_SIZE, as its size really can just be 8K for all architectures. Which
> has the side effect of limiting the size of trace_marker, as its size is
> limited by the trace_seq size.
>
> Finally, because the trace_seq defines the max output that a trace_event
> can write (for all its fields), the extra data of a print event could
> possibly overflow that, which will cause the event not to print, and just
> an "OVERFLOW" output would show in the trace buffer. So I used the
> TRACE_SEQ_SIZE / 2 as the max size that trace_marker can read, which
> happens to be 4K.

Steven, see my other reply. This is backwards.

You can leave the trace_seq as is if you want. It's the trace marking
input size that should be #define to 4kB, not defined as
half-the-size-of-an-internal-buffer-that-happens-to-be-8k.

Then add a BUILD_BUG_ON() in the output code to make sure the output
buffer is always large enough.

Thanks,

Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com