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

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Date:   Fri, 28 Dec 2018 13:01:43 +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, lucho@...kov.net,
        hpa@...or.com, arnd@...db.de
Subject: Re: d_off field in struct dirent and 32-on-64 emulation

* Florian Weimer:

> * Adhemerval Zanella:
>
>> On 27/12/2018 16:09, Florian Weimer wrote:
>>> * 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:
>>
>> I think it makes no sense to continue making non-LFS as default for
>> newer 32 bits ports, the support will be emulated with LFS syscalls.
>
> Sorry, I don't see how this matters.  seekdir and telldir are NOT
> affected by LFS.

Ah, right.  If struct dirent is 64-bit only, then the d_off member
will be 64 bits as well.  But it is unclear whether you can use that
with lseek (probably yes, in its 64-bit variant), and it's unlikely
it's going to work with seekdir because of the POSIX-required long int
type.