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Date:   Sun, 20 Sep 2020 19:41:57 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-block@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
        io-uring@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, netdev@...r.kernel.org,
        keyrings@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

On Sun, Sep 20, 2020 at 07:07:42PM +0100, Al Viro wrote:

> /proc/bus/input/devices (fucked bitmap-to-text representation)

To illustrate the, er, beauty of that stuff:

; cat32 /proc/bus/input/devices >/tmp/a
; cat /proc/bus/input/devices >/tmp/b
; diff -u /tmp/a /tmp/b|grep '^[-+]'
--- /tmp/a      2020-09-20 14:28:43.442560691 -0400
+++ /tmp/b      2020-09-20 14:28:49.018543230 -0400
-B: KEY=1100f 2902000 8380307c f910f001 feffffdf ffefffff ffffffff fffffffe
+B: KEY=1100f02902000 8380307cf910f001 feffffdfffefffff fffffffffffffffe
-B: KEY=70000 0 0 0 0 0 0 0 0
+B: KEY=70000 0 0 0 0
-B: KEY=420 0 70000 0 0 0 0 0 0 0 0
+B: KEY=420 70000 0 0 0 0
-B: KEY=100000 0 0 0
+B: KEY=10000000000000 0
-B: KEY=4000 0 0 0 0
+B: KEY=4000 0 0
-B: KEY=8000 0 0 0 0 0 1100b 800 2 200000 0 0 0 0
+B: KEY=800000000000 0 0 1100b00000800 200200000 0 0
-B: KEY=3e000b 0 0 0 0 0 0 0
+B: KEY=3e000b00000000 0 0 0
-B: KEY=ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff fffffffe
+B: KEY=ffffffffffffffff ffffffffffffffff ffffffffffffffff fffffffffffffffe
; 
(cat32 being a 32bit binary of cat)
All the differences are due to homegrown bitmap-to-text conversion.

Note that feeding that into a pipe leaves the recepient with a lovely problem -
you can't go by the width of words (they are not zero-padded) and you can't
go by the number of words either - it varies from device to device.

And there's nothing we can do with that - it's a userland ABI, can't be
changed without breaking stuff.  I would prefer to avoid additional examples
like that, TYVM...

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