libdevctl: Add devctl_getpath

Helper routine to call the kernel to get a path to the named device.
Different path enumeration methods (called locators) can be used
for different path types depending on what the kernel implements.

Sponsored by:		Netflix
Reviewed by:		jhb
Differential Revision:	https://reviews.freebsd.org/D32746

lib/libdevctl/devctl.3

@@ -36,6 +36,7 @@
 .Nm devctl_disable ,
 .Nm devctl_enable ,
 .Nm devctl_freeze ,
+.Nm devctl_getpath ,
 .Nm devctl_rescan ,
 .Nm devctl_reset ,
 .Nm devctl_resume ,
@@ -62,6 +63,8 @@
 .Ft int
 .Fn devctl_freeze "void"
 .Ft int
+.Fn devctl_getpath "const char *device" "const char *locator" "char **buffer"
+.Ft int
 .Fn devctl_rescan "const char *device"
 .Ft int
 .Fn devctl_reset "const char *device" "bool detach"
@@ -199,6 +202,18 @@ function rescans a bus device checking for devices that have been added or
 removed.
 .Pp
 The
+.Fn devctl_getpath
+retrieves the path to the
+.Fa device
+from the kernel using the
+.Fa locator
+method to construct the path.
+The
+.Fa buffer
+pointer is updated with an allocated buffer that must be freed with
+.Xr free .
+.Pp
+The
 .Fn devctl_freeze
 function freezes probe and attach processing initiated in response to
 drivers being loaded.

lib/libdevctl/devctl.c

@@ -30,6 +30,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/bus.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <stdlib.h>
 #include <string.h>
 #include "devctl.h"
 
@@ -166,3 +167,43 @@ devctl_reset(const char *device, bool detach)
 	return (devctl_simple_request(DEV_RESET, device, detach ?
 	    DEVF_RESET_DETACH : 0));
 }
+
+#define BUFLEN 1024
+
+int
+devctl_getpath(const char *device, const char *locator, char **buffer)
+{
+	struct devreq req;
+	int serrno;
+
+	memset(&req, 0, sizeof(req));
+	if (strlcpy(req.dr_name, device, sizeof(req.dr_name)) >=
+	    sizeof(req.dr_name)) {
+		errno = EINVAL;
+		*buffer = NULL;
+		return (-1);
+	}
+
+	/*
+	 * Maybe do the request twice. Once to get the length, and then again to
+	 * get the string if BUFLEN bytes is insufficient.
+	 */
+	req.dr_flags = 0;
+	req.dr_buffer.length = BUFLEN;
+again:
+	req.dr_buffer.buffer = malloc(req.dr_buffer.length);
+	strlcpy(req.dr_buffer.buffer, locator, req.dr_buffer.length);
+	if (devctl_request(DEV_GET_PATH, &req) == 0) {
+		*buffer = req.dr_buffer.buffer;
+		return (0);
+	}
+	if (errno == ENAMETOOLONG && req.dr_buffer.length != BUFLEN) {
+		free(req.dr_buffer.buffer);
+		goto again;
+	}
+	serrno = errno;
+	free(req.dr_buffer.buffer);
+	errno = serrno;
+	*buffer = NULL;
+	return (-1);
+}

lib/libdevctl/devctl.h

@@ -44,6 +44,7 @@ int	devctl_delete(const char *device, bool force);
 int	devctl_freeze(void);
 int	devctl_thaw(void);
 int	devctl_reset(const char *device, bool detach);
+int	devctl_getpath(const char *device, const char *locator, char **buffer);
 __END_DECLS
 
 #endif /* !__DEVCTL_H__ */