linux-kernel - Re: [PATCH v2 01/15] Documentation: add newcx initramfs format description

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Date:   Fri, 26 Jan 2018 07:51:21 -0800 (PST)
From:   Victor Kamensky <kamensky@...co.com>
To:     Henrique de Moraes Holschuh <hmh@....eng.br>,
        Rob Landley <rob@...dley.net>
cc:     Arnd Bergmann <arnd@...db.de>,
        Taras Kondratiuk <takondra@...co.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Jonathan Corbet <corbet@....net>,
        James McMechan <james.w.mcmechan@...il.com>,
        initramfs@...r.kernel.org, linux-doc@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        xe-linux-external@...co.com
Subject: Re: [PATCH v2 01/15] Documentation: add newcx initramfs format
 description



On Fri, 26 Jan 2018, Henrique de Moraes Holschuh wrote:

> On Thu, 25 Jan 2018, Rob Landley wrote:
>> That said, I don't think -h newcx should emit (or recognize) the
>> "TRAILER!!!1!" entry. That's kinda silly in-band signaling for 2018:
>> files have a length, pipes provide EOF, and each cpiox entry starts with
>> 6 bytes of c_magic anyway.

My understanding that TRAILER is really used on tape devices,
there is no notion of file end in this case, it is just a stream of bytes
from char device.

Thanks,
Victor

>> (I stopped toybox from producing the TRAILER
>> entry back in june, toybox commit 32550751997d, and the kernel consumes
>> the resulting cpio just fine. All the trailer does is prevent you from
>> concatenating cpio files, which is a feature multiple people asked me for.)
>
> Not in the kernel.  What TRAILER does in the kernel is to act as a
> barrier for the hardlink creation state, which IS a good thing.  You
> could just specify it as such for "newcx".
>
> The kernel will continue reading for more entries after TRAILER, so
> concatenation is not broken by TRAILER.  It is also insensitive to
> NUL-padding length (as long as it is 4-byte aligned), which is another
> nice feature you could specify for "newcx".
>
> Also, the kernel does something nothing in userspace ever tried to,
> AFAIK: it detects compression signatures along with the CPIO header
> signatures, and thus it can take several compressed and uncompressed
> archives concatenater together (and the compressor doesn't need to be
> the same, either).
> --
>  Henrique Holschuh
>