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

[Steven Rostedt]

On Fri, 14 Nov 2014 18:18:28 +0100
Petr Mladek <pmladek@xxxxxxx> wrote:

> 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?

I don't see anything wrong with committing an overflowed state.

Note, seq_commit() allows an overflow number to go in too. But of
course that's where overflow is still on the buffer.

Hmm, I guess I can still make this change. The seq_commit() comment
states that @num needs to be < 0 if the data did not fit, and if it is
in overflow state, then it should be -1 not size + 1 (or size for the
old way).

-- Steve


> 
> 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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