ANDROID: fuse-bpf: add bpf solution for splice read

filemap_splice_read is implemented through readahead and read_folio,
which results in sending fuse-bpf files to FuseDaemon.

When a backing file exists, redirect the splice read to the lowlevel
filesystem.

Bug: 337714402
Signed-off-by: Ed Tsai <ed.tsai@mediatek.com>
Change-Id: Ic22ad18a770fdf6911b2b13483b099da51edab10

fs/fuse/backing.c

@@ -10,6 +10,7 @@
 #include <linux/filelock.h>
 #include <linux/filter.h>
 #include <linux/fs_stack.h>
+#include <linux/splice.h>
 #include <linux/namei.h>
 
 #include "../internal.h"
@@ -988,6 +989,18 @@ void *fuse_file_write_iter_finalize(struct fuse_bpf_args *fa,
 	return ERR_PTR(fwio->ret);
 }
 
+ssize_t fuse_splice_read_backing(struct file *in, loff_t *ppos,
+		struct pipe_inode_info *pipe, size_t len, unsigned long flags)
+{
+	struct fuse_file *ff = in->private_data;
+	ssize_t ret;
+
+	ret = vfs_splice_read(ff->backing_file, ppos, pipe, len, flags);
+	fuse_file_accessed(in, ff->backing_file);
+
+	return ret;
+}
+
 long fuse_backing_ioctl(struct file *file, unsigned int command, unsigned long arg, int flags)
 {
 	struct fuse_file *ff = file->private_data;

fs/fuse/file.c

@@ -1845,6 +1845,12 @@ static ssize_t fuse_splice_read(struct file *in, loff_t *ppos,
 {
 	struct fuse_file *ff = in->private_data;
 
+#ifdef CONFIG_FUSE_BPF
+	/* TODO - this is simply passthrough, not a proper BPF filter */
+	if (ff->backing_file)
+		return fuse_splice_read_backing(in, ppos, pipe, len, flags);
+#endif
+
 	/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
 	if (fuse_file_passthrough(ff) && !(ff->open_flags & FOPEN_DIRECT_IO))
 		return fuse_passthrough_splice_read(in, ppos, pipe, len, flags);

fs/fuse/fuse_i.h

@@ -1813,6 +1813,9 @@ int fuse_file_write_iter_backing(struct fuse_bpf_args *fa,
 void *fuse_file_write_iter_finalize(struct fuse_bpf_args *fa,
 		struct kiocb *iocb, struct iov_iter *from);
 
+ssize_t fuse_splice_read_backing(struct file *in, loff_t *ppos,
+		struct pipe_inode_info *pipe, size_t len, unsigned long flags);
+
 long fuse_backing_ioctl(struct file *file, unsigned int command, unsigned long arg, int flags);
 
 int fuse_file_flock_backing(struct file *file, int cmd, struct file_lock *fl);