Subject: Re: bug#9734: [solaris] `dd if=/dev/urandom of=file bs=1024k count=1' gets a file of 133120 bytes

From: Eric Blake <[email-masked]>
Date: Wed, 12 Oct 2011 08:14:15 -0600

tag 9734 notabug
thanks

On 10/12/2011 02:22 AM, Clark J. Wang wrote:
> I'm not sure if it's a bug but it's not reasonable to me. On Solaris 11
> (SunOS 5.11 snv_174, i86pc):
>
> $ uname -a
> SunOS sollab-242.cn.oracle.com 5.11 snv_174 i86pc i386 i86pc
> $ pkg list gnu-coreutils
> NAME (PUBLISHER)                                  VERSION
> IFO
> file/gnu-coreutils                                8.5-0.174.0.0.0.0.504
> i--
> $ /usr/gnu/bin/dd if=/dev/urandom of=file bs=1024k count=1
> 0+1 records in

Notice that this means you read a partial record - read() tried to read 
1024k bytes, but the read ended short at only 133120 bytes.

> 0+1 records out

And because you didn't request dd to group multiple short reads before 
doing a full write, you got a single (short) record written.

> I'm new to Solaris but I've never seen this problem whe I use Linux so it
> really suprises me.

Solaris and Linux kernels differ on when you will get short reads, and 
magic files like /dev/urandom are more likely to display the issue than 
regular files.  That said, Linux also has the "problem" of short reads; 
it's especially noticeable when passing the output of dd to a pipe.

You probably wanted to use this GNU extension:

dd if=/dev/urandom of=file bs=1024k count=1 iconv=fullblock

where the iconv flag requests that dd pile together multiple read()s 
until it has a full block, so that you no longer have a partial block 
output.

>
> I found this in the man page of /dev/urandom on Solaris: "The limitation per
> read for /dev/random is 1040 bytes. The limit for /dev/urandom is (128 *
> 1040 = 133120)." That seems to be the reason but I think dd should handle
> that and check the return value of the read() system call and make sure
> 1024k bytes have really been read from /dev/urandom.

Only if the iconv=fullblock flag is specified, since it is a violation 
of POSIX to do more than one read() without an explicit flag requesting 
multiple reads per block.

-- 
Eric Blake   [email-masked]    +1-801-349-2682
Libvirt virtualization library http://libvirt.org