linux-kernel - Re: [PATCH] kernfs: implement custom llseek method to fix userspace regression

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Message-ID: <8d48c4aa-1bf7-3b25-99fa-3dd76cec070e@yandex-team.ru>
Date:   Thu, 17 Aug 2023 23:53:57 +0500
From:   Valentin Sinitsyn <valesini@...dex-team.ru>
To:     Dan Williams <dan.j.williams@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernfs: implement custom llseek method to fix userspace
 regression

On 15.08.2023 20:48, Dan Williams wrote:
> Valentin Sinitsyn wrote:
> [..]
>>> My only concern is whether there are any scenarios where this is not
>>> appropriate. I.e. do a bit more work to define a kernfs_ops instance
>>> specifically for overriding lseek() in this scenario.
>>
>> Not sure I'm getting you here: do you mean something like this?
>>
>> struct inode *inode = is_f_mapping_redefined(file) ? file_inode(file) :
>> file->f_mapping->host;
> 
> I meant something like the patch below (incomplete, but shows the idea).
Understood, thanks. I believe the change can be localized even further, 
up to the point where battr->f_mapping is initialized with 
iomem_get_mapping, as it is the only override known to break 
generic_file_llseek().

I'll send the updated patch in a few days.

Best,
Valentin