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Date:   Thu, 26 Oct 2023 23:20:15 +0300
From:   Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Christoph Hellwig <hch@lst.de>,
        Justin Stitt <justinstitt@google.com>,
        Kent Overstreet <kent.overstreet@linux.dev>,
        Petr Mladek <pmladek@suse.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Jonathan Corbet <corbet@lwn.net>,
        Yun Zhou <yun.zhou@windriver.com>,
        Jacob Keller <jacob.e.keller@intel.com>,
        Zhen Lei <thunder.leizhen@huawei.com>,
        linux-trace-kernel@vger.kernel.org,
        Yosry Ahmed <yosryahmed@google.com>,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH v2] seq_buf: Introduce DECLARE_SEQ_BUF and seq_buf_str()
Message-ID: <ZTrJ/5Jrzz5D62hh@smile.fi.intel.com>
References: <20231026194033.it.702-kees@kernel.org>
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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,				\
	}

?

...

> +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?

>  	if (seq_buf_buffer_left(s))
>  		s->buffer[s->len] = 0;
>  	else
>  		s->buffer[s->size - 1] = 0;
> +
> +	return s->buffer;
>  }

-- 
With Best Regards,
Andy Shevchenko