From: Joe Perches <joe@perches.com>
To: David Laight <David.Laight@ACULAB.COM>,
"Jason A. Donenfeld" <Jason@zx2c4.com>,
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Subject: Re: [PATCH v1 3/5] treewide: use get_random_u32() when possible
Date: Wed, 12 Oct 2022 18:37:11 -0700 [thread overview]
Message-ID: <3f527ec95a12135eb40f5f2d156a2954feb7fbfe.camel@perches.com> (raw)
In-Reply-To: <d45bd258e033453b85a137112e7694e1@AcuMS.aculab.com>
On Wed, 2022-10-12 at 21:29 +0000, David Laight wrote:
> From: Joe Perches
> > Sent: 12 October 2022 20:17
> >
> > On Wed, 2022-10-05 at 23:48 +0200, Jason A. Donenfeld wrote:
> > > The prandom_u32() function has been a deprecated inline wrapper around
> > > get_random_u32() for several releases now, and compiles down to the
> > > exact same code. Replace the deprecated wrapper with a direct call to
> > > the real function.
> > []
> > > diff --git a/drivers/infiniband/hw/cxgb4/cm.c b/drivers/infiniband/hw/cxgb4/cm.c
> > []
> > > @@ -734,7 +734,7 @@ static int send_connect(struct c4iw_ep *ep)
> > > &ep->com.remote_addr;
> > > int ret;
> > > enum chip_type adapter_type = ep->com.dev->rdev.lldi.adapter_type;
> > > - u32 isn = (prandom_u32() & ~7UL) - 1;
> > > + u32 isn = (get_random_u32() & ~7UL) - 1;
> >
> > trivia:
> >
> > There are somewhat odd size mismatches here.
> >
> > I had to think a tiny bit if random() returned a value from 0 to 7
> > and was promoted to a 64 bit value then truncated to 32 bit.
> >
> > Perhaps these would be clearer as ~7U and not ~7UL
>
> That makes no difference - the compiler will generate the same code.
True, more or less. It's more a question for the reader.
> The real question is WTF is the code doing?
True.
> The '& ~7u' clears the bottom 3 bits.
> The '- 1' then sets the bottom 3 bits and decrements the
> (random) high bits.
Right.
> So is the same as get_random_u32() | 7.
True, it's effectively the same as the upper 29 bits are random
anyway and the bottom 3 bits are always set.
> But I bet the coder had something else in mind.
Likely.
And it was also likely copy/pasted a few times.
next prev parent reply other threads:[~2022-10-13 1:43 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-05 21:48 [PATCH v1 0/5] treewide cleanup of random integer usage Jason A. Donenfeld
2022-10-05 21:48 ` [PATCH v1 1/5] treewide: use prandom_u32_max() when possible Jason A. Donenfeld
2022-10-05 21:48 ` [PATCH v1 2/5] treewide: use get_random_{u8,u16}() " Jason A. Donenfeld
2022-10-05 21:48 ` [PATCH v1 3/5] treewide: use get_random_u32() " Jason A. Donenfeld
[not found] ` <20221006084331.4bdktc2zlvbaszym@quack3>
[not found] ` <Yz7LCyIAHC6l5mG9@zx2c4.com>
2022-10-06 13:01 ` [f2fs-dev] " Andy Shevchenko
2022-10-06 13:07 ` Jason A. Donenfeld
[not found] ` <Yz7OdfKZeGkpZSKb@ziepe.ca>
[not found] ` <CAHmME9r_vNRFFjUvqx8QkBddg_kQU=FMgpk9TqOVZdvX6zXHNg@mail.gmail.com>
2022-10-06 13:15 ` Jason Gunthorpe
2022-10-06 13:20 ` Andy Shevchenko
2022-10-12 19:16 ` Joe Perches
2022-10-12 21:29 ` David Laight
2022-10-13 1:37 ` Joe Perches [this message]
2022-10-05 21:48 ` [PATCH v1 4/5] treewide: use get_random_bytes " Jason A. Donenfeld
2022-10-05 21:48 ` [PATCH v1 5/5] prandom: remove unused functions Jason A. Donenfeld
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