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SubjectRe: [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@tobin.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@tobin.cc>
> > Acked-by: Theodore Ts'o <tytso@mit.edu>
> > Signed-off-by: Tobin C. Harding <me@tobin.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@goodmis.org>

Thanks for the review Steve, will spin again.


Tobin.

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