[debbugs-tracker] bug#9734: closed ([solaris] `dd if=/dev/urandom of=file bs=1024k count=1' gets a file of 133120 bytes)

[GNU bug Tracking System]

Your message dated Wed, 12 Oct 2011 08:14:15 -0600
with message-id <address@hidden>
and subject line Re: bug#9734: [solaris] `dd if=/dev/urandom of=file bs=1024k 
count=1' gets a file of 133120 bytes
has caused the debbugs.gnu.org bug report #9734,
regarding [solaris] `dd if=/dev/urandom of=file bs=1024k count=1' gets a file 
of 133120 bytes
to be marked as done.

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9734: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=9734
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> Subject: 
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> [solaris] `dd if=/dev/urandom of=file bs=1024k count=1' gets a file	of 133120 bytes
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> Date: 
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> Wed, 12 Oct 2011 16:22:12 +0800
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> 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
> 0+1 records out
> 133120 bytes (133 kB) copied, 0.00290536 s, 45.8 MB/s
> $ ls -l file
> -rw-r--r-- 1 root root 133120 2011-10-12 16:12 file
> $
>
> I'm new to Solaris but I've never seen this problem whe I use Linux so it really suprises me.
>
> 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.
>
> Any idea?
>
> Thanks.
>
> -Clark
>
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> --- Begin Message ---
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> Subject: 
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> Re: bug#9734: [solaris] `dd if=/dev/urandom of=file bs=1024k count=1'	gets a file of 133120 bytes
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> Date: 
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> Wed, 12 Oct 2011 08:14:15 -0600
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> User-agent: 
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> Mozilla/5.0 (X11; U; Linux x86_64; en-US;	rv:1.9.2.23) Gecko/20110928 Fedora/3.1.15-1.fc14	Lightning/1.0b3pre Mnenhy/0.8.4 Thunderbird/3.1.15
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> 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   address@hidden    +1-801-349-2682
> Libvirt virtualization library http://libvirt.org
>
>
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