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Date:   Sun, 20 Sep 2020 20:01:59 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Al Viro <viro@...iv.linux.org.uk>
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:
> 	2) a few drivers are really fucked in head.  They use different
> *DATA* layouts for reads/writes, depending upon the calling process.
> IOW, if you fork/exec a 32bit binary and your stdin is one of those,
> reads from stdin in parent and child will yield different data layouts.
> On the same struct file.
> 	That's what Christoph worries about (/dev/sg he'd mentioned is
> one of those).
> 
> 	IMO we should simply have that dozen or so of pathological files
> marked with FMODE_SHITTY_ABI; it's not about how they'd been opened -
> it describes the userland ABI provided by those.  And it's cast in stone.
> 
> 	Any in_compat_syscall() in ->read()/->write() instances is an ABI
> bug, plain and simple.  Some are unfixable for compatibility reasons, but
> any new caller like that should be a big red flag.

So an IOCB_COMPAT flag would let us know whether the caller is expecting
a 32-bit or 64-bit layout?  And io_uring could set it based on the
ctx->compat flag.

> 	Current list of those turds:
> /dev/sg (pointer-chasing, generally insane)
> /sys/firmware/efi/vars/*/raw_var (fucked binary structure)
> /sys/firmware/efi/vars/new_var (fucked binary structure)
> /sys/firmware/efi/vars/del_var (fucked binary structure)
> /dev/uhid	(pointer-chasing for one obsolete command)
> /dev/input/event* (timestamps)
> /dev/uinput (timestamps)
> /proc/bus/input/devices (fucked bitmap-to-text representation)
> /sys/class/input/*/capabilities/* (fucked bitmap-to-text representation)

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