[PATCH] kernfs: implement custom llseek method to fix userspace regression

[Valentine Sinitsyn]

Since commit 636b21b50152 ("PCI: Revoke mappings like devmem"),
mmapable sysfs binary attributes have started receiving 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: lseek(fd, 0, SEEK_END)
now returns zero regardless the real sysfs attribute size which stat()
reports. The reason is that kernfs files use generic_file_llseek()
implementation, 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
attribute or kernfs file representing it. This being said, f_inode
remains valid, so stat() which uses it works correctly.

Fixes the regression by implementing a custom llseek fop for kernfs,
which uses an attribute's file inode to get the file size,
just as stat() does.

Fixes: 636b21b50152 ("PCI: Revoke mappings like devmem")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Valentine Sinitsyn <valesini@xxxxxxxxxxxxxx>
---
 fs/kernfs/file.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 180906c36f51..6d81e0c981f3 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -903,6 +903,21 @@ static __poll_t kernfs_fop_poll(struct file *filp, poll_table *wait)
 	return ret;
 }
 
+static loff_t kernfs_fop_llseek(struct file *file, loff_t offset, int whence)
+{
+	/*
+	 * This is almost identical to generic_file_llseek() except it uses
+	 * cached inode value instead of f_mapping->host.
+	 * The reason is that, for PCI resources in sysfs the latter points to
+	 * iomem_inode whose size has nothing to do with the attribute's size.
+	 */
+	struct inode *inode = file_inode(file);
+
+	return generic_file_llseek_size(file, offset, whence,
+					inode->i_sb->s_maxbytes,
+					i_size_read(inode));
+}
+
 static void kernfs_notify_workfn(struct work_struct *work)
 {
 	struct kernfs_node *kn;
@@ -1005,7 +1020,7 @@ EXPORT_SYMBOL_GPL(kernfs_notify);
 const struct file_operations kernfs_file_fops = {
 	.read_iter	= kernfs_fop_read_iter,
 	.write_iter	= kernfs_fop_write_iter,
-	.llseek		= generic_file_llseek,
+	.llseek		= kernfs_fop_llseek,
 	.mmap		= kernfs_fop_mmap,
 	.open		= kernfs_fop_open,
 	.release	= kernfs_fop_release,
-- 
2.34.1