Re: [PATCH 1/3] tracing: detect the string termination character when parsing user input string

Steven Rostedt
2018-01-10T04:09:23Z

On Wed, 10 Jan 2018 11:01:20 +0800
"Du, Changbin" <changbin.du@intel.com> wrote:

> hi Rostedt,
>
> On Tue, Jan 09, 2018 at 05:54:34PM -0500, Steven Rostedt wrote:
> > On Tue, 9 Jan 2018 17:55:46 +0800
> > changbin.du@intel.com wrote:
> >
> > > From: Changbin Du <changbin.du@intel.com>
> > >
> > > The usersapce can give a '\0' terminated C string or even has '\0' at the
> > > middle of input buffer. We need handle both these two cases correctly.
> >
> > What do you define as correctly. Because I'm not seeing it.
> >
> Soory I don't fully understand your question. What I meant is want to get clear that
> how will tracing parser below strings.
> "", " ", "\0", " \0 ", "aa\0bb"
> The parser may only recognize certain formats, but whatever the behaviour should
> be clear and coherent for all tracing interfaces.

We have a lack of communication here, because now I'm confused by what
exactly you are asking. ;-)

>
> > >
> > > Before this change, trace_get_user() will return a parsed string "\0" in
> > > below case. It is not expected (expects it skip all inputs) and cause the
> > > caller failed.
> > >
> > > open("/sys/kernel/debug/tracing//set_ftrace_pid", O_WRONLY|O_TRUNC) = 3
> > > write(3, " \0", 2) = -1 EINVAL (Invalid argument)
> >
> > That looks more like a feature and not a bug.
> >
> I point this out because I think the parser should take this as an emptry string
> per the comments of trace_get_user().
> /*
> * trace_get_user - reads the user input string separated by space
> * (matched by isspace(ch))
> *
> * For each string found the 'struct trace_parser' is updated,
> * and the function returns.
> *
> * Returns number of bytes read.
> *
> * See kernel/trace/trace.h for 'struct trace_parser' details.
> */

I'm also confused about the inconsistency you see here.

>
> > >
> > > This patch try to make the parser '\0' aware to fix such issue.
> >
> > Why?
> >
>
> > >
> > > Since the caller expects trace_get_user() to parse whole input buffer, so
> > > this patch treat '\0' as a separator as whitespace.
> >
> > It looks more like we are trying to fix a userspace bug via the kernel.
> >
>
> > I'm not liking this. So NACK.

Thinking about this more, I may not be so against it. I'm guessing you
want to do something like:

write(fd, "func1", 6);
write(fd, "func2", 6);

And have it add both func1 and func2, where the '\0' is the separator.

If so, I can see that as a legitimate use case.

-- Steve