GNU bug report logs - #24874
dd: misleading parsing of hex numbers

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Message #11 received at 24874 <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Stephan Bauroth <der_steffi <at> gmx.de>, 24874 <at> debbugs.gnu.org
Subject: Re: bug#24874: dd: misleading parsing of hex numbers
Date: Fri, 4 Nov 2016 17:03:29 +0000

[Message part 1 (text/plain, inline)]

On 04/11/16 16:20, Pádraig Brady wrote:
> On 04/11/16 11:19, Stephan Bauroth wrote:
>> Dear coreutils team :)
>>
>> I encountered a buglike behaviour of dd when handling skip and count 
>> parameters that are encoded in hex and thus prefixed with 0x.
>>
>> dd is not able to parse them, which is OK but would be great if it would 
>> be, but, worse, reads 0xf00 as 0. It does that silently. While an 
>> enduser will immediately notice this on count, since nothing is copied, 
>> behaviour for skip looks ok. (In fact, I noticed this only because I 
>> hexdumped the result after hours of debugging)
>>
>> While it's OK that dd can't parse these numbers, maybe there should be a 
>> warning that 0x was found and interpreted as 0. Since a char like 'x' is 
>> invalid within a number that by definition has to be decimal, a warning 
>> should be fairly easy to implement.
>>
>> Of course, the ability to parse hex numbers in these parameters would be 
>> awesome :)
>>
>> regards and thanks for your continuing work,
>> Stephan Bauroth
> 
> Ouch. That's a real gotcha.
> Note hex digits after the 0x are diagnosed, but not decimal digits:
> 
>   $ dd skip=0x100 seek=0xf00
>   dd: invalid number: ‘0xf00’
> 
> Disallowing 0x... could definitely break backwards compat though.
> Consider `for rec in 0 1 2; do dd skip=${rec}x1024...`
> 
> I suppose we could output a warning to suggest using
>   $(($rec * $size)) or 0${rec}x${size}
> if that really is the intention?
> 
> Given the warning workaround would be suggested in the message,
> and that it's a relatively rare usage, a warning is probably appropriate here.
> We already warn in dd for various usage.
> 
> I'll fix that for the coming release.

Patch attached.

[dd-0x-warn.patch (text/x-patch, attachment)]

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