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

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Message-ID: <4ee7419d-6537-8683-bf40-175cbdc6b484@linaro.org>
Date:   Fri, 28 Dec 2018 10:21:03 -0200
From:   Adhemerval Zanella <adhemerval.zanella@...aro.org>
To:     Florian Weimer <fw@...eb.enyo.de>
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



On 28/12/2018 10:01, Florian Weimer wrote:
> * 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.
> 

I was referring to all other API that uses off_t as well (pread for
instance), where new ports will have non-LFS variants that will call
only LFS variants.