linux-kernel - Re: string_escape_mem ESCAPE

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Date:	Thu, 23 Jul 2015 15:57:31 -0700
From:	Kees Cook <keescook@...gle.com>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Joe Perches <joe@...ches.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: string_escape_mem ESCAPE_SPACE

On Thu, Jul 23, 2015 at 1:50 PM, Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
> On Thu, 2015-07-23 at 13:36 -0700, Kees Cook wrote:
>> On Thu, Jul 23, 2015 at 1:27 PM, Andy Shevchenko
>> <andriy.shevchenko@...ux.intel.com> wrote:
>> > On Thu, 2015-07-23 at 12:59 -0700, Kees Cook wrote:
>> > > Hi,
>> > >
>> > > I'm curious why ESCAPE_SPACE doesn't escape spaces (0x20)?
>> >
>> > Space is a printable character.
>> > You perhaps wants something like ESCAPE_SPACE | ESCAPE_HEX.
>>
>> Yeah, I can get the effect I want with:
>>
>> flags = ESCAPE_SPACE | ESCAPE_SPECIAL | ESCAPE_NULL | ESCAPE_HEX;
>> esc = "\f\n\r\t\v\\\a\e\0 ";
>
> esc can't contain '\0' in the middle.

Ah, yes, of course.

> So, you would like to convert only space to hex and leave everything
> else printable as is?

That was one idea I was having, yes.

>> This isn't reachable via kasprintf, though (it always has a NULL
>> esc).
>> I will consider some options and send patches.
>
> Before doing this, describe your use case in detail, please.

Sure. I'd like to be able to hex-escape everything <0x20, >0x7f, and
". (I'm working on building a quotable string that is safe to log.) I
think I've settled for a subset of this as:

string_escape_mem(src, slen, dst, 0, ESCAPE_HEX, "\f\n\r\t\v\a\e\\\"")

>> > >  That is
>> > > surprising to me, especially since things like isspace() include
>> > > 0x20.
>> >
>> > Moreover, there are test cases in test-string_helpers.c module and
>> > they
>> > are based on the real use cases (before helpers were introduced and
>> > users were converted). So, there is no user which expects hex
>> > conversio
>> > n of the printable character if not asked explicitly.
>>
>> Yeah, I saw it was testing for space to be excluded. I guess I just
>> think the name "ESCAPE_SPACE" is misleading. :)
>
> For sake of name shortness I suppose. The idea is to escape *special*
> spaces by this.

Makes sense. I'll send a patch with some clarifications on the comments.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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