commit ea7423238b85e5227d58b46b6496746d85a672e8
Author: Mateusz Guzik <mjg@FreeBSD.org>
Date:   Sun Dec 19 13:14:37 2021 +0000

    fd: add F_GETPATH

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index d5d22ecfc522..a9da8f83efef 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -480,6 +480,7 @@ kern_fcntl(struct thread *td, int fd, int cmd, intptr_t arg)
 	struct proc *p;
 	struct vnode *vp;
 	struct mount *mp;
+	char *retbuf, *freebuf;
 	struct kinfo_file *kif;
 	int error, flg, kif_sz, seals, tmp;
 	uint64_t bsize;
@@ -891,6 +892,37 @@ kern_fcntl(struct thread *td, int fd, int cmd, intptr_t arg)
 		free(kif, M_TEMP);
 		break;
 
+	case F_GETPATH:
+		/*
+		 * Translate a vnode to a path.
+		 *
+		 * The process is unreliable in the following ways:
+		 * - if the target has more than one name, it is not specified
+		 *   which one will be returned (most notably it may differ
+		 *   from the name used to open it to obtain given fd)
+		 * - there may be no names added to begin with (none were added
+		 *   to the name cache or they got evicted), in which case the
+		 *   routine may fail
+		 *
+		 * See top level comment in kern/vfs_cache.c for more details.
+		 */
+		error = fget_unlocked(fdp, fd, &cap_no_rights, &fp);
+		if (__predict_false(error != 0))
+			break;
+		if (__predict_false(fp->f_type != DTYPE_VNODE)) {
+			fdrop(fp, td);
+			error = EBADF;
+			break;
+		}
+		vp = fp->f_vnode;
+		error = vn_fullpath(vp, &retbuf, &freebuf);
+		fdrop(fp, td);
+		if (__predict_false(error != 0))
+			break;
+		error = copyout(retbuf, (void *)arg, strlen(retbuf) + 1);
+		free(freebuf, M_TEMP);
+		break;
+
 	default:
 		error = EINVAL;
 		break;
diff --git a/sys/sys/fcntl.h b/sys/sys/fcntl.h
index 491b0172a1e6..5264be2d9852 100644
--- a/sys/sys/fcntl.h
+++ b/sys/sys/fcntl.h
@@ -271,6 +271,7 @@ typedef	__pid_t		pid_t;
 #define	F_GET_SEALS	20
 #define	F_ISUNIONSTACK	21		/* Kludge for libc, don't use it. */
 #define	F_KINFO		22		/* Return kinfo_file for this fd */
+#define	F_GETPATH	23		/* Resolve the fd to a path */
 
 /* Seals (F_ADD_SEALS, F_GET_SEALS). */
 #define	F_SEAL_SEAL	0x0001		/* Prevent adding sealings */