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Re: Cannot pass a single backslash in multiboot cmdline

From: Jakub Jermář
Subject: Re: Cannot pass a single backslash in multiboot cmdline
Date: Mon, 26 Dec 2016 19:12:53 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

Hi Andrei,

On 12/26/2016 06:59 PM, Andrei Borzenkov wrote:
> 26.12.2016 17:44, Jakub Jermář пишет:
>> Hi,
>> I am observing a strange behavior when passing boot arguments with a
>> backslash to the kernel (the multiboot cmd_line via the multiboot
>> command in grub.cfg). I would like to pass foo\bar to the kernel, but to
>> no avail. I tried:
> Which kernel? What do you load?

The kernel is a modified version of HelenOS, but IMHO this issue is
kernel agnostic. The cmd_line is already wrong when picked up from the
multiboot info and printed out (so there is no processing on it from the
HelenOS side). The argument I am trying to pass is actually a HelenOS
path to the serial console service, something like:


The backslashes are part of the service's name.

>> foo\bar gets passed as foobar
>> foo\\bar gets passed as foo\\bar
>> 'foo\bar' gets passed as foo\\bar
>> "foo\bar" gets passed as foo\\bar
>> Note that the backslash gets doubled when I try to escape it.
>> I am using grub 2.02~beta2, revision
>> bc220962e366b1b46769ed6f9fa5be603ba58ab5.
>> How does one pass foo\bar so that the back slash does not get eaten or
>> doubled?
> You can't currently. I do not know what was intended when grub cmdline
> was written, but the way it quotes string is definitely not compatible
> with linux kernel (which only recognizes `"' as valid quote character).

I think I understand why it is eating the single backslash (expected
behavior), so the issue mainly is with why is it duplicating the escaped
(unexpected behavior) backslashes.

> Could you please open formal bug report on savannah, so we could discuss
> its priority.

Ok, will do.

> And yes, this is not the first report. Actually there could be similar
> bug already.

Sorry, I was searching for it, but the bug tracking system on savannah
is not very user friendly and I simply got no results.


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