PCI: Implement custom llseek for sysfs resource entries

Since commit 636b21b5 ("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).
Signed-off-by: Valentine Sinitsyn <valesini@yandex-team.ru>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20230925084013.309399-2-valesini@yandex-team.ruSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/pci/pci-sysfs.c

@@ -835,6 +835,19 @@ static const struct attribute_group pci_dev_config_attr_group = {
 	.is_bin_visible = pci_dev_config_attr_is_visible,
 };
 
+/*
+ * llseek operation for mmappable PCI resources.
+ * May be left unused if the arch doesn't provide them.
+ */
+static __maybe_unused loff_t
+pci_llseek_resource(struct file *filep,
+		    struct kobject *kobj __always_unused,
+		    struct bin_attribute *attr,
+		    loff_t offset, int whence)
+{
+	return fixed_size_llseek(filep, offset, whence, attr->size);
+}
+
 #ifdef HAVE_PCI_LEGACY
 /**
  * pci_read_legacy_io - read byte(s) from legacy I/O port space
@@ -967,6 +980,8 @@ void pci_create_legacy_files(struct pci_bus *b)
 	b->legacy_io->attr.mode = 0600;
 	b->legacy_io->read = pci_read_legacy_io;
 	b->legacy_io->write = pci_write_legacy_io;
+	/* See pci_create_attr() for motivation */
+	b->legacy_io->llseek = pci_llseek_resource;
 	b->legacy_io->mmap = pci_mmap_legacy_io;
 	b->legacy_io->f_mapping = iomem_get_mapping;
 	pci_adjust_legacy_attr(b, pci_mmap_io);
@@ -981,6 +996,8 @@ void pci_create_legacy_files(struct pci_bus *b)
 	b->legacy_mem->size = 1024*1024;
 	b->legacy_mem->attr.mode = 0600;
 	b->legacy_mem->mmap = pci_mmap_legacy_mem;
+	/* See pci_create_attr() for motivation */
+	b->legacy_mem->llseek = pci_llseek_resource;
 	b->legacy_mem->f_mapping = iomem_get_mapping;
 	pci_adjust_legacy_attr(b, pci_mmap_mem);
 	error = device_create_bin_file(&b->dev, b->legacy_mem);
@@ -1199,8 +1216,15 @@ static int pci_create_attr(struct pci_dev *pdev, int num, int write_combine)
 			res_attr->mmap = pci_mmap_resource_uc;
 		}
 	}
-	if (res_attr->mmap)
+	if (res_attr->mmap) {
 		res_attr->f_mapping = iomem_get_mapping;
+		/*
+		 * generic_file_llseek() consults f_mapping->host to determine
+		 * the file size. As iomem_inode knows nothing about the
+		 * attribute, it's not going to work, so override it as well.
+		 */
+		res_attr->llseek = pci_llseek_resource;
+	}
 	res_attr->attr.name = res_attr_name;
 	res_attr->attr.mode = 0600;
 	res_attr->size = pci_resource_len(pdev, num);