cached operations sometimes need to do invalid operations (e.g. read on a write only file) Historic fscache had added a "writeback fid", a special handle opened RW as root, for this. The conversion to new fscache missed that bit. This commit reinstates a slightly lesser variant of the original code that uses the writeback fid for partial pages backfills if the regular user fid had been open as WRONLY, and thus would lack read permissions. Link: https://lkml.kernel.org/r/20220614033802.1606738-1-asmadeus@xxxxxxxxxxxxx Fixes: eb497943fa21 ("9p: Convert to using the netfs helper lib to do reads and caching") Cc: stable@xxxxxxxxxxxxxxx Cc: David Howells <dhowells@xxxxxxxxxx> Reported-By: Christian Schoenebeck <linux_oss@xxxxxxxxxxxxx> Reviewed-by: Christian Schoenebeck <linux_oss@xxxxxxxxxxxxx> Tested-by: Christian Schoenebeck <linux_oss@xxxxxxxxxxxxx> Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx> --- v3: use the least permissive version of the patch that only uses writeback fid when really required If no problem shows up by then I'll post this patch around Wed 23 (next week) with the other stable fixes. fs/9p/vfs_addr.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c index a8f512b44a85..d0833fa69faf 100644 --- a/fs/9p/vfs_addr.c +++ b/fs/9p/vfs_addr.c @@ -58,8 +58,21 @@ static void v9fs_issue_read(struct netfs_io_subrequest *subreq) */ static int v9fs_init_request(struct netfs_io_request *rreq, struct file *file) { + struct inode *inode = file_inode(file); + struct v9fs_inode *v9inode = V9FS_I(inode); struct p9_fid *fid = file->private_data; + BUG_ON(!fid); + + /* we might need to read from a fid that was opened write-only + * for read-modify-write of page cache, use the writeback fid + * for that */ + if (rreq->origin == NETFS_READ_FOR_WRITE && + (fid->mode & O_ACCMODE) == O_WRONLY) { + fid = v9inode->writeback_fid; + BUG_ON(!fid); + } + refcount_inc(&fid->count); rreq->netfs_priv = fid; return 0; -- 2.35.1
