Re: [RFC][PATCH 18/23 v4] tracing: Add seq_buf_get_buf() and seq_buf_commit() helper functions

[Petr Mladek]

On Thu 2014-11-13 20:13:02, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
> 
> Add two helper functions; seq_buf_get_buf() and seq_buf_commit() that
> are used by seq_buf_path(). This makes the code similar to the
> seq_file: seq_path() function, and will help to be able to consolidate
> the functions between seq_file and trace_seq.
> 
> Link: http://lkml.kernel.org/r/20141104160222.644881406@xxxxxxxxxxx
> 
> Tested-by: Jiri Kosina <jkosina@xxxxxxx>
> Acked-by: Jiri Kosina <jkosina@xxxxxxx>
> Reviewed-by: Petr Mladek <pmladek@xxxxxxx>
> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> ---
>  include/linux/seq_buf.h | 40 ++++++++++++++++++++++++++++++++++++++++
>  kernel/trace/seq_buf.c  |  7 +++----
>  2 files changed, 43 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
> index 581c1fc733c3..3e550d214187 100644
> --- a/include/linux/seq_buf.h
> +++ b/include/linux/seq_buf.h
> @@ -64,6 +64,46 @@ seq_buf_buffer_left(struct seq_buf *s)
>  	return s->size - s->len;
>  }
>  
> +/**
> + * seq_buf_get_buf - get buffer to write arbitrary data to
> + * @s: the seq_buf handle
> + * @bufp: the beginning of the buffer is stored here
> + *
> + * Return the number of bytes available in the buffer, or zero if
> + * there's no space.
> + */
> +static inline size_t seq_buf_get_buf(struct seq_buf *s, char **bufp)
> +{
> +	WARN_ON(s->len > s->size + 1);
> +
> +	if (s->len < s->size) {
> +		*bufp = s->buffer + s->len;
> +		return s->size - s->len;
> +	}
> +
> +	*bufp = NULL;
> +	return 0;
> +}
> +
> +/**
> + * seq_buf_commit - commit data to the buffer
> + * @s: the seq_buf handle
> + * @num: the number of bytes to commit
> + *
> + * Commit @num bytes of data written to a buffer previously acquired
> + * by seq_buf_get.  To signal an error condition, or that the data
> + * didn't fit in the available space, pass a negative @num value.
> + */
> +static inline void seq_buf_commit(struct seq_buf *s, int num)
> +{
> +	if (num < 0) {
> +		seq_buf_set_overflow(s);
> +	} else {
> +		BUG_ON(s->len + num > s->size + 1);

I thought about it more and we should probably do

		BUG_ON(s->len + num > s->size);

"size + 1" signalizes that there was an overflow. It is a valid
value for the "len" variable. But it is not valid to commit "size + 1"
bytes into the buffer. It would mean access outside of the buffer.

What do you think, please?

Best Regards,
Petr

PS: Sigh, I wish, I saw all this problems during the first review and not
triggering another patchset version.


> +		s->len += num;
> +	}
> +}
> +
>  extern __printf(2, 3)
>  int seq_buf_printf(struct seq_buf *s, const char *fmt, ...);
>  extern __printf(2, 0)
> diff --git a/kernel/trace/seq_buf.c b/kernel/trace/seq_buf.c
> index 9d3bb64dca31..4f35a783b82f 100644
> --- a/kernel/trace/seq_buf.c
> +++ b/kernel/trace/seq_buf.c
> @@ -283,8 +283,8 @@ int seq_buf_putmem_hex(struct seq_buf *s, const void *mem,
>   */
>  int seq_buf_path(struct seq_buf *s, const struct path *path, const char *esc)
>  {
> -	char *buf = s->buffer + s->len;
> -	size_t size = seq_buf_buffer_left(s);
> +	char *buf;
> +	size_t size = seq_buf_get_buf(s, &buf);
>  	int res = -1;
>  
>  	WARN_ON(s->size == 0);
> @@ -297,8 +297,7 @@ int seq_buf_path(struct seq_buf *s, const struct path *path, const char *esc)
>  				res = end - buf;
>  		}
>  	}
> -	if (res > 0)
> -		s->len += res;
> +	seq_buf_commit(s, res);
>  
>  	return res;
>  }
> -- 
> 2.1.1
> 
> 
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