fuse: allow kernel to access "direct_io" files

Allow the kernel read and write on "direct_io" files.  This is
necessary for nfs export and execute support.

The implementation is simple: if an access from the kernel is
detected, don't perform get_user_pages(), just use the kernel address
provided by the requester to copy from/to the userspace filesystem.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/dir.c

@@ -1032,6 +1032,7 @@ static int fuse_readdir(struct file *file, void *dstbuf, filldir_t filldir)
 		fuse_put_request(fc, req);
 		return -ENOMEM;
 	}
+	req->out.argpages = 1;
 	req->num_pages = 1;
 	req->pages[0] = page;
 	fuse_read_fill(req, file, inode, file->f_pos, PAGE_SIZE, FUSE_READDIR);

fs/fuse/file.c

@@ -386,7 +386,6 @@ void fuse_read_fill(struct fuse_req *req, struct file *file,
 	req->in.numargs = 1;
 	req->in.args[0].size = sizeof(struct fuse_read_in);
 	req->in.args[0].value = inarg;
-	req->out.argpages = 1;
 	req->out.argvar = 1;
 	req->out.numargs = 1;
 	req->out.args[0].size = count;
@@ -453,6 +452,7 @@ static int fuse_readpage(struct file *file, struct page *page)
 	attr_ver = fuse_get_attr_version(fc);
 
 	req->out.page_zeroing = 1;
+	req->out.argpages = 1;
 	req->num_pages = 1;
 	req->pages[0] = page;
 	num_read = fuse_send_read(req, file, inode, pos, count, NULL);
@@ -510,6 +510,8 @@ static void fuse_send_readpages(struct fuse_req *req, struct file *file,
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	loff_t pos = page_offset(req->pages[0]);
 	size_t count = req->num_pages << PAGE_CACHE_SHIFT;
+
+	req->out.argpages = 1;
 	req->out.page_zeroing = 1;
 	fuse_read_fill(req, file, inode, pos, count, FUSE_READ);
 	req->misc.read.attr_ver = fuse_get_attr_version(fc);
@@ -621,7 +623,6 @@ static void fuse_write_fill(struct fuse_req *req, struct file *file,
 	inarg->flags = file ? file->f_flags : 0;
 	req->in.h.opcode = FUSE_WRITE;
 	req->in.h.nodeid = get_node_id(inode);
-	req->in.argpages = 1;
 	req->in.numargs = 2;
 	if (fc->minor < 9)
 		req->in.args[0].size = FUSE_COMPAT_WRITE_IN_SIZE;
@@ -695,6 +696,7 @@ static int fuse_buffered_write(struct file *file, struct inode *inode,
 	if (IS_ERR(req))
 		return PTR_ERR(req);
 
+	req->in.argpages = 1;
 	req->num_pages = 1;
 	req->pages[0] = page;
 	req->page_offset = offset;
@@ -771,6 +773,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
 	size_t count = 0;
 	int err;
 
+	req->in.argpages = 1;
 	req->page_offset = offset;
 
 	do {
@@ -935,28 +938,44 @@ static void fuse_release_user_pages(struct fuse_req *req, int write)
 }
 
 static int fuse_get_user_pages(struct fuse_req *req, const char __user *buf,
-			       unsigned nbytes, int write)
+			       unsigned *nbytesp, int write)
 {
+	unsigned nbytes = *nbytesp;
 	unsigned long user_addr = (unsigned long) buf;
 	unsigned offset = user_addr & ~PAGE_MASK;
 	int npages;
 
-	/* This doesn't work with nfsd */
-	if (!current->mm)
-		return -EPERM;
+	/* Special case for kernel I/O: can copy directly into the buffer */
+	if (segment_eq(get_fs(), KERNEL_DS)) {
+		if (write)
+			req->in.args[1].value = (void *) user_addr;
+		else
+			req->out.args[0].value = (void *) user_addr;
+
+		return 0;
+	}
 
 	nbytes = min(nbytes, (unsigned) FUSE_MAX_PAGES_PER_REQ << PAGE_SHIFT);
 	npages = (nbytes + offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	npages = clamp(npages, 1, FUSE_MAX_PAGES_PER_REQ);
 	down_read(&current->mm->mmap_sem);
-	npages = get_user_pages(current, current->mm, user_addr, npages, write,
+	npages = get_user_pages(current, current->mm, user_addr, npages, !write,
 				0, req->pages, NULL);
 	up_read(&current->mm->mmap_sem);
 	if (npages < 0)
 		return npages;
 
 	req->num_pages = npages;
 	req->page_offset = offset;
+
+	if (write)
+		req->in.argpages = 1;
+	else
+		req->out.argpages = 1;
+
+	nbytes = (req->num_pages << PAGE_SHIFT) - req->page_offset;
+	*nbytesp = min(*nbytesp, nbytes);
+
 	return 0;
 }
 
@@ -979,15 +998,13 @@ static ssize_t fuse_direct_io(struct file *file, const char __user *buf,
 
 	while (count) {
 		size_t nres;
-		size_t nbytes_limit = min(count, nmax);
-		size_t nbytes;
-		int err = fuse_get_user_pages(req, buf, nbytes_limit, !write);
+		size_t nbytes = min(count, nmax);
+		int err = fuse_get_user_pages(req, buf, &nbytes, write);
 		if (err) {
 			res = err;
 			break;
 		}
-		nbytes = (req->num_pages << PAGE_SHIFT) - req->page_offset;
-		nbytes = min(nbytes_limit, nbytes);
+
 		if (write)
 			nres = fuse_send_write(req, file, inode, pos, nbytes,
 					       current->files);
@@ -1163,6 +1180,7 @@ static int fuse_writepage_locked(struct page *page)
 	fuse_write_fill(req, NULL, ff, inode, page_offset(page), 0, 1);
 
 	copy_highpage(tmp_page, page);
+	req->in.argpages = 1;
 	req->num_pages = 1;
 	req->pages[0] = tmp_page;
 	req->page_offset = 0;