linux-kernel - Re: [PATCH v4 2/3] random: Return nbytes filled from hw RNG

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Date:   Wed, 16 May 2018 07:17:06 +1000
From:   "Tobin C. Harding" <me@...in.cc>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Theodore Ts'o <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] random: Return nbytes filled from hw RNG

On Tue, May 15, 2018 at 09:37:05AM -0400, Steven Rostedt wrote:
> On Tue, 15 May 2018 13:06:25 +1000
> "Tobin C. Harding" <me@...in.cc> wrote:
> 
> > Currently the function get_random_bytes_arch() has return value 'void'.
> > If the hw RNG fails we currently fall back to using get_random_bytes().
> > This defeats the purpose of requesting random material from the hw RNG
> > in the first place.
> > 
> > There are currently no intree users of get_random_bytes_arch().
> > 
> > Only get random bytes from the hw RNG, make function return the number
> > of bytes retrieved from the hw RNG.
> > 
> > Signed-off-by: Tobin C. Harding <me@...in.cc>
> > Acked-by: Theodore Ts'o <tytso@....edu>
> > Signed-off-by: Tobin C. Harding <me@...in.cc>
> > ---
> >  drivers/char/random.c  | 16 +++++++++-------
> >  include/linux/random.h |  2 +-
> >  2 files changed, 10 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/char/random.c b/drivers/char/random.c
> > index 031d18b31e0f..4b0ec597e783 100644
> > --- a/drivers/char/random.c
> > +++ b/drivers/char/random.c
> > @@ -1725,26 +1725,28 @@ EXPORT_SYMBOL(del_random_ready_callback);
> >   * key known by the NSA).  So it's useful if we need the speed, but
> >   * only if we're willing to trust the hardware manufacturer not to
> >   * have put in a back door.
> > + *
> > + * Return number of bytes filled in.
> >   */
> > -void get_random_bytes_arch(void *buf, int nbytes)
> > +int __must_check get_random_bytes_arch(void *buf, int nbytes)
> >  {
> >  	char *p = buf;
> > +	int left = nbytes;
> 
> Just a nit, but I know some kernel devs prefer "upside-down-xmas-tree"
> style of declarations. Which would make the above:
> 
> 	int left = nbytes;
> 	char *p = buf;

Super specific coding style and rigorous code cleanliness is a big part
of why I love kernel dev.  Thanks for pointing this one out. 

While we are on these code lines, whats the typical kernel variable name
for a loop counter that is going to be counted down? 'left',
'remaining', 'to_go', 'still'???

> >  
> > -	trace_get_random_bytes_arch(nbytes, _RET_IP_);
> > -	while (nbytes) {
> > +	trace_get_random_bytes_arch(left, _RET_IP_);
> 
> Nothing to do with this patch series, but I wonder if we should move
> the trace event below, and record how much was done.

I don't fully understand trace events, I just left this line in tact
and hoped for the best :(

/me adds 'trace events' to list of things to learn more about

> > +	while (left) {
> >  		unsigned long v;
> > -		int chunk = min(nbytes, (int)sizeof(unsigned long));
> > +		int chunk = min_t(int, left, (int)sizeof(unsigned long));
> >  
> >  		if (!arch_get_random_long(&v))
> >  			break;
> >  
> >  		memcpy(p, &v, chunk);
> >  		p += chunk;
> > -		nbytes -= chunk;
> > +		left -= chunk;
> >  	}
> >  
> > -	if (nbytes)
> > -		get_random_bytes(p, nbytes);
> > +	return nbytes - left;
> >  }
> >  EXPORT_SYMBOL(get_random_bytes_arch);
> >  
> > diff --git a/include/linux/random.h b/include/linux/random.h
> > index 2ddf13b4281e..f1c9bc5cd231 100644
> > --- a/include/linux/random.h
> > +++ b/include/linux/random.h
> > @@ -38,7 +38,7 @@ extern void get_random_bytes(void *buf, int nbytes);
> >  extern int wait_for_random_bytes(void);
> >  extern int add_random_ready_callback(struct random_ready_callback *rdy);
> >  extern void del_random_ready_callback(struct random_ready_callback *rdy);
> > -extern void get_random_bytes_arch(void *buf, int nbytes);
> > +extern int __must_check get_random_bytes_arch(void *buf, int nbytes);
> >  
> >  #ifndef MODULE
> >  extern const struct file_operations random_fops, urandom_fops;
> 
> Other than that...
> 
> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Thanks for the review Steve, will spin again.


	Tobin.