Re: [PATCH v9 2/2] PCI: Implement custom llseek for sysfs resource entries

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Sep 25, 2023 at 11:40:13AM +0300, 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@vger.kernel.org
> Signed-off-by: Valentine Sinitsyn <valesini@yandex-team.ru>
> Acked-by: Bjorn Helgaas <bhelgaas@google.com>
> ---
>  drivers/pci/pci-sysfs.c | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)

I'll take these now, for 6.7-rc1, but not mark them as fixes or cc:
stable as this is a new functionality, the code has never worked for
lseek on these files so it's not like anything was broken :)

thanks,

greg k-h