linux-kernel - Re: [PATCH] mtd: mtd_oobtest: fix verify errors due to incorrect use of prandom_bytes

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Message-ID: <20140122164115.211a819d@ipc1.ka-ro>
Date:	Wed, 22 Jan 2014 16:41:15 +0100
From:	Lothar Waßmann <LW@...O-electronics.de>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Brian Norris <computersforpeace@...il.com>,
	Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Huang Shijie <b32955@...escale.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mtd: mtd_oobtest: fix verify errors due to incorrect
 use of prandom_bytes_state()

Hi,

Akinobu Mita wrote:
> 2014/1/22 Lothar Waßmann <LW@...o-electronics.de>:
> > Hi,
> >
> > Is anyone taking care of this?
> >
> > Lothar Waßmann wrote:
> >> When using prandom_bytes_state() it is critical to use the same block
> >> size in all invocations that are to produce the same random sequence.
> >> Otherwise the state of the PRNG will be out of sync if the blocksize
> >> is not divisible by 4.
> >> This leads to bogus verification errors in several tests which use
> >> different block sizes to initialize the buffer for writing and
> >> comparison.
> >>
> >> Signed-off-by: Lothar Waßmann <LW@...O-electronics.de>
> >> ---
> >>  drivers/mtd/tests/oobtest.c |   14 ++++++++++++--
> >>  1 files changed, 12 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/mtd/tests/oobtest.c b/drivers/mtd/tests/oobtest.c
> >> index 2e9e2d1..72c7359 100644
> >> --- a/drivers/mtd/tests/oobtest.c
> >> +++ b/drivers/mtd/tests/oobtest.c
> >> @@ -213,8 +213,15 @@ static int verify_eraseblock_in_one_go(int ebnum)
> >>       int err = 0;
> >>       loff_t addr = ebnum * mtd->erasesize;
> >>       size_t len = mtd->ecclayout->oobavail * pgcnt;
> >> +     int i;
> >> +
> >> +     for (i = 0; i < pgcnt; i++)
> >> +             prandom_bytes_state(&rnd_state, &writebuf[i * use_len],
> >> +                             use_len);
> >> +     if (len % use_len)
> >> +             prandom_bytes_state(&rnd_state, &writebuf[i * use_len],
> >> +                             len % use_len);
> >>
> >> -     prandom_bytes_state(&rnd_state, writebuf, len);
> >>       ops.mode      = MTD_OPS_AUTO_OOB;
> >>       ops.len       = 0;
> >>       ops.retlen    = 0;
> 
> I would rather fix the use of prandom_bytes_state() in write_eraseblock()
> than fix in verify_eraseblock_in_one_go().
> 
Why and how?
write_whole_device() (which calls write_eraseblock()) is used multiple
times with different verification methods (all blocks in one go or each
block individually).
If prandom_state_bytes() in write_eraseblock() would be changed, that
function would have to know, how the block are going to be checked
lateron to know how to set up the writebuffer.

Dto. for mtd_write_oob()


Lothar Waßmann
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