linux-kernel - RE: [PATCH v9 2/2] PCI: Implement custom llseek for sysfs resource entries

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Date:   Thu, 5 Oct 2023 11:16:11 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Valentine Sinitsyn <valesini@...dex-team.ru>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
CC:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Dan Williams <dan.j.williams@...el.com>,
        <linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>
Subject: RE: [PATCH v9 2/2] PCI: Implement custom llseek for sysfs resource
 entries

Valentine Sinitsyn wrote:
> Since commit 636b21b50152 ("PCI: Revoke mappings like devmem"), mmappable
> sysfs entries have started to receive their f_mapping from the iomem
> pseudo filesystem, so that CONFIG_IO_STRICT_DEVMEM is honored in sysfs
> (and procfs) as well as in /dev/[k]mem.
> 
> This resulted in a userspace-visible regression:
> 
> 1. Open a sysfs PCI resource file (eg. /sys/bus/pci/devices/*/resource0)
> 2. Use lseek(fd, 0, SEEK_END) to determine its size
> 
> Expected result: a PCI region size is returned.
> Actual result: 0 is returned.
> 
> The reason is that PCI resource files residing in sysfs use
> generic_file_llseek(), which relies on f_mapping->host inode to get the
> file size. As f_mapping is now redefined, f_mapping->host points to an
> anonymous zero-sized iomem_inode which has nothing to do with sysfs file
> in question.
> 
> Implement a custom llseek method for sysfs PCI resources, which is
> almost the same as proc_bus_pci_lseek() used for procfs entries.
> 
> This makes sysfs and procfs entries consistent with regards to seeking,
> but also introduces userspace-visible changes to seeking PCI resources
> in sysfs:
> 
> - SEEK_DATA and SEEK_HOLE are no longer supported;
> - Seeking past the end of the file is prohibited while previously
>   offsets up to MAX_NON_LFS were accepted (reading from these offsets
>   was always invalid).
> 
> Fixes: 636b21b50152 ("PCI: Revoke mappings like devmem")
> Cc: stable@...r.kernel.org
> Signed-off-by: Valentine Sinitsyn <valesini@...dex-team.ru>
> Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>

Thanks for doing this reorganization and the follow-up, looks good to me:

Reviewed-by: Dan Williams <dan.j.williams@...el.com>