Re: [RFC][PATCH 07/12 v3] tracing: Have seq_buf use full buffer

[Petr Mladek <pmladek@suse.cz>]

On Tue 2014-11-04 10:52:44, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
> 
> Currently seq_buf is full when all but one byte of the buffer is
> filled. Change it so that the seq_buf is full when all of the
> buffer is filled.
> 
> Some of the functions would fill the buffer completely and report
> everything was fine. This was inconsistent with the max of size - 1.
> Changing this to be max of size makes all functions consistent.
> 
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
>  include/linux/seq_buf.h |  4 ++--
>  kernel/trace/seq_buf.c  | 18 +++++++++---------
>  2 files changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
> index 064a8604ad33..3cd25038cb5e 100644
> --- a/include/linux/seq_buf.h
> +++ b/include/linux/seq_buf.h
> @@ -46,13 +46,13 @@ seq_buf_init(struct seq_buf *s, unsigned char *buf, unsigned int size)
>  static inline bool
>  seq_buf_has_overflowed(struct seq_buf *s)
>  {
> -	return s->len == s->size;
> +	return s->len > s->size;
>  }
>  
>  static inline void
>  seq_buf_set_overflow(struct seq_buf *s)
>  {
> -	s->len = s->size;
> +	s->len = s->size + 1;
>  }
>  
>  extern __printf(2, 3)
> diff --git a/kernel/trace/seq_buf.c b/kernel/trace/seq_buf.c
> index 243123b12d16..06fd1833e692 100644
> --- a/kernel/trace/seq_buf.c
> +++ b/kernel/trace/seq_buf.c
> @@ -11,17 +11,17 @@
>   * This will set up the counters within the descriptor. You can call
>   * seq_buf_init() more than once to reset the seq_buf to start
>   * from scratch.
> - * 
> + *
>   */
>  #include <linux/uaccess.h>
>  #include <linux/seq_file.h>
>  #include <linux/seq_buf.h>
>  
>  /* How much buffer is left on the seq_buf? */
> -#define SEQ_BUF_LEFT(s) (((s)->size - 1) - (s)->len)
> +#define SEQ_BUF_LEFT(s) ((s)->size - (s)->len)
>  
>  /* How much buffer is written? */
> -#define SEQ_BUF_USED(s) min((s)->len, (s)->size - 1)
> +#define SEQ_BUF_USED(s) min((s)->len, (s)->size)
>  
>  /**
>   * seq_buf_print_seq - move the contents of seq_buf into a seq_file
> @@ -55,7 +55,7 @@ int seq_buf_vprintf(struct seq_buf *s, const char *fmt, va_list args)
>  
>  	if (s->len < s->size) {
>  		len = vsnprintf(s->buffer + s->len, s->size - s->len, fmt, args);
> -		if (s->len + len < s->size) {
> +		if (s->len + len <= s->size) {

This is always true because we limit vsnprintf() to write (s->size -
s->len) bytes. Similar problem is also in the other parts of this
patch.

I wonder if we want this change at all. It means that we are not able to
detect overflow in some functions. It is pity because the users
might want to increase the buffer size and try again if the print
was incomplete.

I think that we need to leave the one byte for the overflow detection
if we want to detect it properly.

Best Regards,
Petr

>  			s->len += len;
>  			return 0;
>  		}
> @@ -105,7 +105,7 @@ int seq_buf_bitmask(struct seq_buf *s, const unsigned long *maskp,
>  
>  	if (s->len < s->size) {
>  		ret = bitmap_scnprintf(s->buffer, len, maskp, nmaskbits);
> -		if (s->len + ret < s->size) {
> +		if (s->len + ret <= s->size) {
>  			s->len += ret;
>  			return 0;
>  		}