BUG in 2.4.0: dd if=/dev/random of=out.txt bs=10000 count=100

BUG in 2.4.0: dd if=/dev/random of=out.txt bs=10000 count=100

[Rob Landley]

If I do the dd line in the title under 2.4.0 I get an
out.txt file of 591 bytes.

If I do the same thing from /dev/zero, I get the
expected 1,000,000 byte file.

I've shoehorned 2.4.0 into a fresh red hat 7.0 install
which could quite easily be a bad thing, yes ripped
out their strange gcc and symlinked kgcc->gcc to do
the compile.  But other than this it seems to be
working.  (So far...)

dd says it completes happily even when copying from
random.  0+100 records in, 0+100 records out.  It
takes about thirty seconds to finish on the dual
gigahertz processor intel box I'm using to test it,
which implies it's actually performing the truly
impressive waste of CPU cycles I'm requesting from it.
 I'm just not getting the data in my file.

Am I doing something wrong?

My dd is from fileutils-4.0x-3.  (Straight Red Hat
7.0, I think...)  Didn't see anything about that in
Documentation/Changes...

I'll be happy to try one of the prepatches if anybody
thinks they've addressed this problem already.

Anybody?  Need more debugging info?  Want me to wave a
dead chicken at something specific?  Stick printk's
into the kernel?...

Rob

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Re: BUG in 2.4.0: dd if=/dev/random of=out.txt bs=10000 count=100

[David Ford]

Rob Landley wrote:

> If I do the dd line in the title under 2.4.0 I get an
> out.txt file of 591 bytes.

It isn't broken, you have no more entropy.  You must have some system
activity of various sorts before you regain some entropy.  Moving the mouse
around, hitting keys, etc, will slowly add more entropy.

-d


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Re: BUG in 2.4.0: dd if=/dev/random of=out.txt bs=10000 count=100

[John Heffner]

> dd says it completes happily even when copying from
> random.  0+100 records in, 0+100 records out.  It

This means that dd completed 100 reads, and none of them were of the
requested length (10000 bytes).

> takes about thirty seconds to finish on the dual
> gigahertz processor intel box I'm using to test it,
> which implies it's actually performing the truly
> impressive waste of CPU cycles I'm requesting from it.
>  I'm just not getting the data in my file.

/dev/random generates (hopefully) truly random values, and relies on
receiving interrupts.  It doesn't spend very many CPU cycles.  For most
purposes, /dev/urandom is adequate, and will be much faster for such large
quantities of data.

  -John

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Re: BUG in 2.4.0: dd if=/dev/random of=out.txt bs=10000 count=100

[Rob Landley]

--- David Ford <david@linux.com> wrote:
> Rob Landley wrote:
> 
> > If I do the dd line in the title under 2.4.0 I get
> an
> > out.txt file of 591 bytes.
> 
> It isn't broken, you have no more entropy.  You must
> have some system
> activity of various sorts before you regain some
> entropy.  Moving the mouse
> around, hitting keys, etc, will slowly add more
> entropy.
> 
> -d

I'd wondered what urandom was for.  Thanks.

Rob


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Re: BUG in 2.4.0: dd if=/dev/random of=out.txt bs=10000 count=100

[Bernd Eckenfels]

In article <20010113035314.316.qmail@web5205.mail.yahoo.com> you wrote:
> If I do the dd line in the title under 2.4.0 I get an
> out.txt file of 591 bytes.
/dev/random will only give you as much bytes as are available. and even then
you should not do it cause you drain the random pool. Use /dev/urandom
instead.

Greetings
Bernd
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Re: BUG in 2.4.0: dd if=/dev/random of=out.txt bs=10000 count=100

[David Santinoli]

On Fri, Jan 12, 2001 at 07:53:14PM -0800, Rob Landley wrote:
> If I do the dd line in the title under 2.4.0 I get an
> out.txt file of 591 bytes.
And it's the same under 2.2.x, too.

> dd says it completes happily even when copying from
> random.  0+100 records in, 0+100 records out.  It
It's not a fault of dd, or of the read() system call, either. It's just the way
/dev/random works - you can't read more bytes than those available in the
entropy pool. And if you try, you'll just fail with no error.

Cheers,
 David
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Re: BUG in 2.4.0: dd if=/dev/random of=out.txt bs=10000 count=100

[David Ford]

David Santinoli wrote:

> On Fri, Jan 12, 2001 at 07:53:14PM -0800, Rob Landley wrote:
> > If I do the dd line in the title under 2.4.0 I get an
> > out.txt file of 591 bytes.
> And it's the same under 2.2.x, too.
>
> > dd says it completes happily even when copying from
> > random.  0+100 records in, 0+100 records out.  It
> It's not a fault of dd, or of the read() system call, either. It's just the way
> /dev/random works - you can't read more bytes than those available in the
> entropy pool. And if you try, you'll just fail with no error.

It won't fail, it will block, then continue reading when more bytes are
available.  The application may time out however.

-d

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