FUSE passthrough: handle unstable kernel interface

The interface for FUSE passthrough is still under discussion upstream,
thus this change fixes the issue of being able to support multiple
interfaces for passthrough, depending on what the kernel provides.

This implementation tries to explore the different FUSE passthrough
interface implementation and updates an index to the one that has
been found working.

Bug: 167695973
Test: Manual read/write of passthrough files with extra printks
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: I74e0e6c0691b37160a00af77fb18eda5342630f5

2 files changed, 60 insertions, 9 deletions

diff --git a/include/fuse_kernel.h b/include/fuse_kernel.h
index 8bd7f0d..55d8e2c 100644
--- a/include/fuse_kernel.h
+++ b/include/fuse_kernel.h
@@ -575,7 +575,7 @@ struct fuse_read_in {
 	uint32_t	padding;
 };
 
-struct fuse_passthrough_out {
+struct fuse_passthrough_out_v0 {
 	uint32_t	fd;
 	/* For future implementation */
 	uint32_t	len;
@@ -834,7 +834,8 @@ struct fuse_notify_retrieve_in {
 
 /* Device ioctls: */
 #define FUSE_DEV_IOC_CLONE	      _IOR(229, 0, uint32_t)
-#define FUSE_DEV_IOC_PASSTHROUGH_OPEN _IOW(229, 1, struct fuse_passthrough_out)
+#define FUSE_DEV_IOC_PASSTHROUGH_OPEN_V0 _IOW(229, 1, struct fuse_passthrough_out_v0)
+#define FUSE_DEV_IOC_PASSTHROUGH_OPEN_V1 _IOW(229, 127, struct fuse_passthrough_out_v0)
 
 struct fuse_lseek_in {
 	uint64_t	fh;
diff --git a/lib/fuse_lowlevel.c b/lib/fuse_lowlevel.c
index c7efc3d..baa6d2b 100644
--- a/lib/fuse_lowlevel.c
+++ b/lib/fuse_lowlevel.c
@@ -459,17 +459,67 @@ int fuse_reply_canonical_path(fuse_req_t req, const char *path)
 	return send_reply_ok(req, path, strlen(path) + 1);
 }
 
+enum {
+	FUSE_PASSTHROUGH_API_UNAVAILABLE,
+	FUSE_PASSTHROUGH_API_V0,
+	FUSE_PASSTHROUGH_API_V1,
+	FUSE_PASSTHROUGH_API_STABLE,
+};
+
+/*
+ * Requests the FUSE passthrough feature to be enabled on a specific file
+ * through the passed fd.
+ * This function returns an identifier that must be used as passthrough_fh
+ * when the open/create_open request reply is sent back to /dev/fuse.
+ * As for the current FUSE passthrough implementation, passthrough_fh values
+ * are only valid if > 0, so in case the FUSE passthrough open ioctl returns
+ * a value <= 0, this must be considered an error and is returned as-is by
+ * this function.
+ */
 int fuse_passthrough_enable(fuse_req_t req, unsigned int fd) {
-    struct fuse_passthrough_out out = {};
-    int ret;
+	static sig_atomic_t passthrough_version = FUSE_PASSTHROUGH_API_STABLE;
+	int ret = 0; /* values <= 0 represent errors in FUSE passthrough */
 
-    out.fd = fd;
+	/*
+	 * The interface of FUSE passthrough is still unstable in the kernel,
+	 * so the following solution is to search for the most updated API
+	 * version and, if not found, fall back to an older one.
+	 * This happens when ioctl() returns -1 and errno is set to ENOTTY,
+	 * an error code that corresponds to the lack of a specific ioctl.
+	 */
+	switch (passthrough_version) {
+	case FUSE_PASSTHROUGH_API_STABLE:
+		/* There is not a stable API yet */
+		passthrough_version = FUSE_PASSTHROUGH_API_V1;
+	case FUSE_PASSTHROUGH_API_V1: {
+		struct fuse_passthrough_out_v0 out = {};
+		out.fd = fd;
+
+		ret = ioctl(req->se->fd, FUSE_DEV_IOC_PASSTHROUGH_OPEN_V1, &out);
+		if (ret == -1 && errno == ENOTTY)
+			passthrough_version = FUSE_PASSTHROUGH_API_V0;
+		else
+			break;
+	}
+	case FUSE_PASSTHROUGH_API_V0: {
+		struct fuse_passthrough_out_v0 out = {};
+		out.fd = fd;
 
-    ret = ioctl(req->se->fd, FUSE_DEV_IOC_PASSTHROUGH_OPEN, &out);
-    if (ret <= 0)
-        fuse_log(FUSE_LOG_ERR, "fuse: passthrough_enable: %s\n", strerror(errno));
+		ret = ioctl(req->se->fd, FUSE_DEV_IOC_PASSTHROUGH_OPEN_V0, &out);
+		if (ret == -1 && errno == ENOTTY)
+			passthrough_version = FUSE_PASSTHROUGH_API_UNAVAILABLE;
+		else
+			break;
+	}
+	default:
+		fuse_log(FUSE_LOG_ERR, "fuse: passthrough_enable no valid API\n");
+		return -ENOTTY;
+	}
+
+	if (ret <= 0)
+		fuse_log(FUSE_LOG_ERR, "fuse: passthrough_enable: %s\n", strerror(errno));
 
-    return ret;
+	return ret;
 }
 
 int fuse_reply_open(fuse_req_t req, const struct fuse_file_info *f)