GNU bug report logs - #13391
dd silently ignores lseek error

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Pádraig Brady <P <at> draigBrady.com>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#13391: closed (dd silently ignores lseek error)
Date: Wed, 09 Jan 2013 01:15:01 +0000

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Your message dated Wed, 09 Jan 2013 01:14:22 +0000
with message-id <50ECC46E.8030203 <at> draigBrady.com>
and subject line Re: bug#13391: dd silently ignores lseek error
has caused the debbugs.gnu.org bug report #13391,
regarding dd silently ignores lseek error
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


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13391: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13391
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From: Neil Klopfenstein <neil.klopfenstein <at> gmail.com>
To: bug-coreutils <at> gnu.org
Subject: dd silently ignores lseek error
Date: Tue, 8 Jan 2013 15:11:22 -0300

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Hi all,

While trying to diagnose a weird filesystem bug, I found an error in GNU dd
v8.12.

The weird bug is causing lseek() to fail improperly. That's not the problem
I'm reporting, though. I was trying to use dd to demonstrate the lseek
error to my sysadmin. Instead, I found that dd is ignoring the lseek
failure.

Here is the relevant strace output:
$ strace dd if=libdvapp-O.a of=/dev/null bs=33k skip=1
...
open("libdvapp-O.a", O_RDONLY) = 3
dup2(3, 0)                              = 0
close(3)                                = 0
lseek(0, 0, SEEK_CUR)                   = 0
open("/dev/null", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
dup2(3, 1)                              = 1
close(3)                                = 0
clock_gettime(CLOCK_MONOTONIC, {12951065, 389531862}) = 0
ioctl(0, MGSL_IOCSTXIDLE or MTIOCGET or SNDCTL_MIDI_MPUCMD, 0x7fbfffe3f0) =
-1 ENOTTY (Inappropriate ioctl for device)
lseek(0, 33792, SEEK_CUR)               = -1 EINVAL (Invalid argument)
 ^ this is the syscall which should not be failing
ioctl(0, MGSL_IOCSTXIDLE or MTIOCGET or SNDCTL_MIDI_MPUCMD, 0x7fbfffe3f0) =
-1 ENOTTY (Inappropriate ioctl for device)
lseek(0, 0, SEEK_END)                   = -1 EINVAL (Invalid argument)
read(0, "!<arch>\n/               13576110"..., 33792) = 33792
...

Note that it begins reading at the _beginning of the ar file_ -- the 'skip'
argument has failed silently. The output of dd does not indicate any error:

3+1 records in
3+1 records out
103310 bytes (103 kB) copied, 0.000375 s, 275 MB/s

Expected behavior: dd should pass on any unexpected errors reported by
system calls.

I realize this is an old version of coreutils but I don't have a newer
version available to test against the anomaly. Sorry if it's already been
fixed.

Regards,
--Neil

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From: Pádraig Brady <P <at> draigBrady.com>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: Neil Klopfenstein <neil.klopfenstein <at> gmail.com>, 13391-done <at> debbugs.gnu.org
Subject: Re: bug#13391: dd silently ignores lseek error
Date: Wed, 09 Jan 2013 01:14:22 +0000

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On 01/08/2013 08:55 PM, Paul Eggert wrote:
> On 01/08/13 10:11, Neil Klopfenstein wrote:
>> Note that it begins reading at the _beginning of the ar file_ -- the 'skip'
>> argument has failed silently.
>
> But the 'skip' hasn't failed.  It's merely being implemented via 'read'
> rather than via 'lseek'.  The records are being skipped correctly.
>
> It might be useful to give dd a new option, which causes it
> to insist on lseeking rather than reading in cases like these,
> and to report an error if the lseek fails.

I had a look around for a tool to verify
that a file/device supports the seek operation
and couldn't find one.
So this seems like useful functionality.
Worth applying the attached?

thanks,
Pádraig.

[dd-flag-seekable.diff (text/x-patch, attachment)]

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