linux-hardening - Re: [PATCH v2] seq_buf: Introduce DECLARE_SEQ_BUF and seq_buf

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Date: Fri, 27 Oct 2023 08:49:28 -0700
From: Kees Cook <keescook@...omium.org>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Christoph Hellwig <hch@....de>,
	Justin Stitt <justinstitt@...gle.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Petr Mladek <pmladek@...e.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arnd Bergmann <arnd@...db.de>, Jonathan Corbet <corbet@....net>,
	Yun Zhou <yun.zhou@...driver.com>,
	Jacob Keller <jacob.e.keller@...el.com>,
	Zhen Lei <thunder.leizhen@...wei.com>,
	linux-trace-kernel@...r.kernel.org,
	Yosry Ahmed <yosryahmed@...gle.com>, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] seq_buf: Introduce DECLARE_SEQ_BUF and seq_buf_str()

On Thu, Oct 26, 2023 at 11:20:15PM +0300, Andy Shevchenko wrote:
> On Thu, Oct 26, 2023 at 12:40:37PM -0700, Kees Cook wrote:
> > Solve two ergonomic issues with struct seq_buf;
> > 
> > 1) Too much boilerplate is required to initialize:
> > 
> > 	struct seq_buf s;
> > 	char buf[32];
> > 
> > 	seq_buf_init(s, buf, sizeof(buf));
> > 
> > Instead, we can build this directly on the stack. Provide
> > DECLARE_SEQ_BUF() macro to do this:
> > 
> > 	DECLARE_SEQ_BUF(s, 32);
> > 
> > 2) %NUL termination is fragile and requires 2 steps to get a valid
> >    C String (and is a layering violation exposing the "internals" of
> >    seq_buf):
> > 
> > 	seq_buf_terminate(s);
> > 	do_something(s->buffer);
> > 
> > Instead, we can just return s->buffer direction after terminating it
> > in refactored seq_buf_terminate(), now known as seq_buf_str():
> > 
> > 	do_soemthing(seq_buf_str(s));
> 
> ...
> 
> > +#define DECLARE_SEQ_BUF(NAME, SIZE)					\
> > +	char __ ## NAME ## _buffer[SIZE] = "";				\
> > +	struct seq_buf NAME = { .buffer = &__ ## NAME ## _buffer,	\
> > +				.size = SIZE }
> 
> Hmm... Wouldn't be more readable to have it as
> 
> #define DECLARE_SEQ_BUF(NAME, SIZE)			\
> 	char __ ## NAME ## _buffer[SIZE] = "";		\
> 	struct seq_buf NAME = {				\
> 		.buffer = &__ ## NAME ## _buffer,	\
> 		.size = SIZE,				\
> 	}
> 
> ?

Yes, I don't know why I did it the smooshed way. Fixed for v3.

> > +static inline char *seq_buf_str(struct seq_buf *s)
> >  {
> >  	if (WARN_ON(s->size == 0))
> > -		return;
> > +		return "";
> 
> I'm wondering why it's a problem to have an empty string?

Well, it's a pathological case where "size" is 0 -- it shouldn't happen
(hence the warn), but it's more robust to return an empty .data string
pointer than a NULL s->buffer or an s->buffer that isn't intended to be
used (i.e. the size == 0).

-- 
Kees Cook