Implement CloudABI's readdir().

Summary:
CloudABI's readdir() system call could be thought of as a mixture
between FreeBSD's getdents(2) and pread(). Instead of using the file
descriptor offset, userspace provides a 64-bit cloudabi_dircookie_t
continue reading at a given point. CLOUDABI_DIRCOOKIE_START, having
value 0, can be used to return entries at the start of the directory.

The file descriptor offset is not used to store the cookie for the
reason that in a file descriptor centric environment, it would make
sense to allow concurrent use of a single file descriptor.

The remaining space returned by the system call should be filled with a
partially truncated copy of the next entry. The advantage of doing this
is that it gracefully deals with long filenames. If the C library
provides a buffer that is too small to hold a single entry, it can still
extract the directory entry header, meaning that it can retry the read
with a larger buffer or skip it using the cookie.

Test Plan:
This implementation passes the cloudlibc unit tests at:

	https://github.com/NuxiNL/cloudlibc/tree/master/src/libc/dirent

Reviewers: marcel, kib

Reviewed By: kib

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D3226

1 files changed, 174 insertions, 2 deletions

diff --git a/sys/compat/cloudabi/cloudabi_file.c b/sys/compat/cloudabi/cloudabi_file.c
index 45f6583..cdec170 100644
--- a/sys/compat/cloudabi/cloudabi_file.c
+++ b/sys/compat/cloudabi/cloudabi_file.c
@@ -28,6 +28,7 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
 #include <sys/capsicum.h>
+#include <sys/dirent.h>
 #include <sys/fcntl.h>
 #include <sys/kernel.h>
 #include <sys/malloc.h>
@@ -35,11 +36,15 @@ __FBSDID("$FreeBSD$");
 #include <sys/proc.h>
 #include <sys/stat.h>
 #include <sys/syscallsubr.h>
+#include <sys/uio.h>
+#include <sys/vnode.h>
 
 #include <compat/cloudabi/cloudabi_proto.h>
 #include <compat/cloudabi/cloudabi_syscalldefs.h>
 #include <compat/cloudabi/cloudabi_util.h>
 
+#include <security/mac/mac_framework.h>
+
 static MALLOC_DEFINE(M_CLOUDABI_PATH, "cloudabipath", "CloudABI pathnames");
 
 /*
@@ -197,13 +202,180 @@ cloudabi_sys_file_open(struct thread *td,
 	return (ENOSYS);
 }
 
+/* Converts a FreeBSD directory entry structure and writes it to userspace. */
+static int
+write_dirent(struct dirent *bde, cloudabi_dircookie_t cookie, struct uio *uio)
+{
+	cloudabi_dirent_t cde = {
+		.d_next = cookie,
+		.d_ino = bde->d_fileno,
+		.d_namlen = bde->d_namlen,
+	};
+	size_t len;
+	int error;
+
+	/* Convert file type. */
+	switch (bde->d_type) {
+	case DT_BLK:
+		cde.d_type = CLOUDABI_FILETYPE_BLOCK_DEVICE;
+		break;
+	case DT_CHR:
+		cde.d_type = CLOUDABI_FILETYPE_CHARACTER_DEVICE;
+		break;
+	case DT_DIR:
+		cde.d_type = CLOUDABI_FILETYPE_DIRECTORY;
+		break;
+	case DT_FIFO:
+		cde.d_type = CLOUDABI_FILETYPE_FIFO;
+		break;
+	case DT_LNK:
+		cde.d_type = CLOUDABI_FILETYPE_SYMBOLIC_LINK;
+		break;
+	case DT_REG:
+		cde.d_type = CLOUDABI_FILETYPE_REGULAR_FILE;
+		break;
+	case DT_SOCK:
+		/* The exact socket type cannot be derived. */
+		cde.d_type = CLOUDABI_FILETYPE_SOCKET_STREAM;
+		break;
+	default:
+		cde.d_type = CLOUDABI_FILETYPE_UNKNOWN;
+		break;
+	}
+
+	/* Write directory entry structure. */
+	len = sizeof(cde) < uio->uio_resid ? sizeof(cde) : uio->uio_resid;
+	error = uiomove(&cde, len, uio);
+	if (error != 0)
+		return (error);
+
+	/* Write filename. */
+	len = bde->d_namlen < uio->uio_resid ? bde->d_namlen : uio->uio_resid;
+	return (uiomove(bde->d_name, len, uio));
+}
+
 int
 cloudabi_sys_file_readdir(struct thread *td,
     struct cloudabi_sys_file_readdir_args *uap)
 {
+	struct iovec iov = {
+		.iov_base = uap->buf,
+		.iov_len = uap->nbyte
+	};
+	struct uio uio = {
+		.uio_iov = &iov,
+		.uio_iovcnt = 1,
+		.uio_resid = iov.iov_len,
+		.uio_segflg = UIO_USERSPACE,
+		.uio_rw = UIO_READ,
+		.uio_td = td
+	};
+	struct file *fp;
+	struct vnode *vp;
+	void *readbuf;
+	cap_rights_t rights;
+	cloudabi_dircookie_t offset;
+	int error;
 
-	/* Not implemented. */
-	return (ENOSYS);
+	/* Obtain directory vnode. */
+	error = getvnode(td, uap->fd, cap_rights_init(&rights, CAP_READ), &fp);
+	if (error != 0) {
+		if (error == EINVAL)
+			return (ENOTDIR);
+		return (error);
+	}
+	if ((fp->f_flag & FREAD) == 0) {
+		fdrop(fp, td);
+		return (EBADF);
+	}
+
+	/*
+	 * Call VOP_READDIR() and convert resulting data until the user
+	 * provided buffer is filled.
+	 */
+	readbuf = malloc(MAXBSIZE, M_TEMP, M_WAITOK);
+	offset = uap->cookie;
+	vp = fp->f_vnode;
+	while (uio.uio_resid > 0) {
+		struct iovec readiov = {
+			.iov_base = readbuf,
+			.iov_len = MAXBSIZE
+		};
+		struct uio readuio = {
+			.uio_iov = &readiov,
+			.uio_iovcnt = 1,
+			.uio_rw = UIO_READ,
+			.uio_segflg = UIO_SYSSPACE,
+			.uio_td = td,
+			.uio_resid = MAXBSIZE,
+			.uio_offset = offset
+		};
+		struct dirent *bde;
+		unsigned long *cookies, *cookie;
+		size_t readbuflen;
+		int eof, ncookies;
+
+		/* Validate file type. */
+		vn_lock(vp, LK_SHARED | LK_RETRY);
+		if (vp->v_type != VDIR) {
+			VOP_UNLOCK(vp, 0);
+			error = ENOTDIR;
+			goto done;
+		}
+#ifdef MAC
+		error = mac_vnode_check_readdir(td->td_ucred, vp);
+		if (error != 0) {
+			VOP_UNLOCK(vp, 0);
+			goto done;
+		}
+#endif /* MAC */
+
+		/* Read new directory entries. */
+		cookies = NULL;
+		ncookies = 0;
+		error = VOP_READDIR(vp, &readuio, fp->f_cred, &eof,
+		    &ncookies, &cookies);
+		VOP_UNLOCK(vp, 0);
+		if (error != 0)
+			goto done;
+
+		/* Convert entries to CloudABI's format. */
+		readbuflen = MAXBSIZE - readuio.uio_resid;
+		bde = readbuf;
+		cookie = cookies;
+		while (readbuflen >= offsetof(struct dirent, d_name) &&
+		    uio.uio_resid > 0 && ncookies > 0) {
+			/* Ensure that the returned offset always increases. */
+			if (readbuflen >= bde->d_reclen && bde->d_fileno != 0 &&
+			    *cookie > offset) {
+				error = write_dirent(bde, *cookie, &uio);
+				if (error != 0) {
+					free(cookies, M_TEMP);
+					goto done;
+				}
+			}
+
+			if (offset < *cookie)
+				offset = *cookie;
+			++cookie;
+			--ncookies;
+			readbuflen -= bde->d_reclen;
+			bde = (struct dirent *)((char *)bde + bde->d_reclen);
+		}
+		free(cookies, M_TEMP);
+		if (eof)
+			break;
+	}
+
+done:
+	fdrop(fp, td);
+	free(readbuf, M_TEMP);
+	if (error != 0)
+		return (error);
+
+	/* Return number of bytes copied to userspace. */
+	td->td_retval[0] = uap->nbyte - uio.uio_resid;
+	return (0);
 }
 
 int