linux-ext4 - Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation

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Message-ID: <20181229021157.GG5864@mit.edu>
Date:   Fri, 28 Dec 2018 21:11:57 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Peter Maydell <peter.maydell@...aro.org>
CC:     Andreas Dilger <adilger@...ger.ca>,
        Florian Weimer <fw@...eb.enyo.de>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        <lucho@...kov.net>, <libc-alpha@...rceware.org>,
        Arnd Bergmann <arnd@...db.de>, <ericvh@...il.com>,
        <hpa@...or.com>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>, <rminnich@...dia.gov>,
        <v9fs-developer@...ts.sourceforge.net>
Subject: Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation

On Fri, Dec 28, 2018 at 11:18:18AM +0000, Peter Maydell wrote:
> In general inodes and offsets start from 0 and work up --
> so almost all of the time they don't actually overflow.
> The problem with ext4 directory hash "offsets" is that they
> overflow all the time and immediately, so instead of "works
> unless you have a weird edge case" like all the other filesystems,h
> it's "never works".

Actually, XFS uses the inode number to encode the location of the
inode (it doesn't have a fixed inode table, so it's effectively the
block number shifted left by 3 or 4 bits, with the low bits indicating
the slot in the 4k block).  It has a hack to provide backwards
compatibility for 32-bit API's, but there is a similar, "oh, we're on
a non-paleolithic CPU, let's use the full 64-bits" sort of logic that
ext4 has.

> The problem is that there is no 32-bit API in some cases
> (unless I have misunderstood the kernel code) -- not all
> host architectures implement compat syscalls or allow them
> to be called from 64-bit processes or implement all the older
> syscall variants that had smaller offets. If there was a guaranteed
> "this syscall always exists and always gives me 32-bit offsets"
> we could use it.

Are there going to be cases where a process or a thread will sometimes
want the 64-bit interface, and sometimes want the 32-bit interface?
Or is it always going to be one or the other?  I wonder if we could
simply add a new flag to the process personality(2) flags.

> Yes, that has been suggested, but it seemed a bit dubious
> to bake in knowledge of ext4's internal implementation details.
> Can we rely on this as an ABI promise that will always work
> for all versions of all file systems going forwards?

Yeah, that seems dubious because I'm pretty sure there are other file
systems that may have their own 32/64-bit quirks.

						- Ted