linux-ext4 - Re: d_off field in struct dirent and 32-on-64 emulation

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Date:   Thu, 27 Dec 2018 19:09:11 +0100
From:   Florian Weimer <fw@...eb.enyo.de>
To:     Adhemerval Zanella <adhemerval.zanella@...aro.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net, libc-alpha@...rceware.org,
        qemu-devel@...gnu.org, ericvh@...il.com, rminnich@...dia.gov,
        lucho@...kov.net, hpa@...or.com, arnd@...db.de
Subject: Re: d_off field in struct dirent and 32-on-64 emulation

* Adhemerval Zanella:

> Also for glibc standpoint, although reverting it back to use getdents 
> syscall for non-LFS mode might fix this issue for architectures that
> provides non-LFS getdents syscall it won't be a fix for architectures 
> that still provides off_t different than off64_t *and* only provides 
> getdents64 syscall.
>
> Currently we only have nios2 and csky (unfortunately).  But since generic 
> definition for off_t and off64_t still assumes non-LFS support, all new
> 32-bits ports potentially might carry the issue.

For csky, we could still change the type of the non-standard d_off
field to long long int.  This way, only telldir would have to fail
when truncation is necessary, as mentioned below:

>> There is another annoying aspect: The standards expose d_off through
>> the telldir function, and that returns long int on all architectures
>> (not off_t, so unchanged by _FILE_OFFSET_BITS).  That's mostly a
>> userspace issue and thus needing different steps to resolve (possibly
>> standards action).