linux-kernel - Re: [PATCH v1 2/4] initramfs: Refactor to use hex2bin() instead of custom approach

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Message-ID: <aW_xA0wbC2bf981d@smile.fi.intel.com>
Date: Tue, 20 Jan 2026 23:17:55 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: David Disseldorp <ddiss@...e.de>
Cc: Christian Brauner <brauner@...nel.org>, Petr Mladek <pmladek@...e.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1 2/4] initramfs: Refactor to use hex2bin() instead of
 custom approach

On Wed, Jan 21, 2026 at 08:00:15AM +1100, David Disseldorp wrote:
> On Tue, 20 Jan 2026 22:34:45 +0200, Andy Shevchenko wrote:
> > On Wed, Jan 21, 2026 at 07:12:50AM +1100, David Disseldorp wrote:
> > > On Mon, 19 Jan 2026 21:38:39 +0100, Andy Shevchenko wrote:

...

> > > > +	ret = hex2bin((u8 *)header, s + 6, sizeof(header));
> > > > +	if (ret)
> > > > +		error("damaged header");  
> > > 
> > > The changes look reasonable to me on first glance, but I think we really
> > > should improve the error handling to abort the state machine on
> > > malformed header here.
> > > 
> > > One further issue that we have is simple_strntoul()'s acceptance of
> > > "0x" prefixes for the hex strings - any initramfs which carries such
> > > prefixes will now result in an error.
> > > It's a pretty obscure corner case, but cpio is really easy to generate
> > > from printf(), so maybe there are some images out there which rely on
> > > this.
> > > 
> > > I've written an initramfs_test regression test for the "0x" prefix
> > > handling. I'll send it to the list.  
> > 
> > Is it specified?
> > 
> > The standard refers to octal numbers, we seem to use hexadecimal.
> > I don't believe the 0x will ever appear here.
> > 
> > Otherwise, please point out to the specifications.
> 
> The kernel initramfs specification is at
> Documentation/driver-api/early-userspace/buffer-format.rst :

Thanks!

>   The structure of the cpio_header is as follows (all fields contain
>   hexadecimal ASCII numbers fully padded with '0' on the left to the
>   full width of the field, for example, the integer 4780 is represented
>   by the ASCII string "000012ac"):
>   ...
> 
> I.e. a "0x" isn't specified as valid prefix. I don't feel strongly
> regarding diverging from existing behaviour,

> but it should still be
> considered (and documented) as a potentially user-visible regression.

I disagree, this is not specified and should not be used. The CPIO archive in
the original form doesn't specify leading 0 for octals (at least how I read it,
please correct me, if I'm wrong).

https://pubs.opengroup.org/onlinepubs/007908799/xcu/pax.html

-- 
With Best Regards,
Andy Shevchenko