NetBSD/sys/kern/vfs_getcwd.c

/* $NetBSD: vfs_getcwd.c,v 1.59 2020/04/21 21:42:47 ad Exp $ */

/*-
 * Copyright (c) 1999, 2020 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Bill Sommerfeld.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: vfs_getcwd.c,v 1.59 2020/04/21 21:42:47 ad Exp $");

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/namei.h>
#include <sys/filedesc.h>
#include <sys/kernel.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <sys/vnode.h>
#include <sys/mount.h>
#include <sys/proc.h>
#include <sys/uio.h>
#include <sys/kmem.h>
#include <sys/dirent.h>
#include <sys/kauth.h>

#include <ufs/ufs/dir.h>	/* XXX only for DIRBLKSIZ */

#include <sys/syscallargs.h>

/*
 * Vnode variable naming conventions in this file:
 *
 * rvp: the current root we're aiming towards.
 * lvp, *lvpp: the "lower" vnode
 * uvp, *uvpp: the "upper" vnode.
 *
 * Since all the vnodes we're dealing with are directories, and the
 * lookups are going *up* in the filesystem rather than *down*, the
 * usual "pvp" (parent) or "dvp" (directory) naming conventions are
 * too confusing.
 */

/*
 * XXX Will infinite loop in certain cases if a directory read reliably
 *	returns EINVAL on last block.
 * XXX is EINVAL the right thing to return if a directory is malformed?
 */

/*
 * XXX Untested vs. mount -o union; probably does the wrong thing.
 */

/*
 * Find parent vnode of *lvpp, return in *uvpp
 *
 * If we care about the name, scan it looking for name of directory
 * entry pointing at lvp.
 *
 * Place the name in the buffer which starts at bufp, immediately
 * before *bpp, and move bpp backwards to point at the start of it.
 *
 * On entry, *lvpp is a locked vnode reference; on exit, it is vput and NULL'ed
 * On exit, *uvpp is either NULL or is a locked vnode reference.
 */
static int
getcwd_scandir(struct vnode *lvp, struct vnode **uvpp, char **bpp,
    char *bufp, struct lwp *l)
{
	int     error = 0;
	int     eofflag;
	off_t   off;
	int     tries;
	struct uio uio;
	struct iovec iov;
	char   *dirbuf = NULL;
	int	dirbuflen;
	ino_t   fileno;
	struct vattr va;
	struct vnode *uvp = NULL;
	kauth_cred_t cred = l->l_cred;
	struct componentname cn;
	int len, reclen;
	tries = 0;

	/* Need exclusive for UFS VOP_GETATTR (itimes) & VOP_LOOKUP. */
	KASSERT(VOP_ISLOCKED(lvp) == LK_EXCLUSIVE);

	/*
	 * If we want the filename, get some info we need while the
	 * current directory is still locked.
	 */
	if (bufp != NULL) {
		error = VOP_GETATTR(lvp, &va, cred);
		if (error) {
			VOP_UNLOCK(lvp);
			*uvpp = NULL;
			return error;
		}
	}

	/*
	 * Ok, we have to do it the hard way..
	 * Next, get parent vnode using lookup of ..
	 */
	cn.cn_nameiop = LOOKUP;
	cn.cn_flags = ISLASTCN | ISDOTDOT | RDONLY;
	cn.cn_cred = cred;
	cn.cn_nameptr = "..";
	cn.cn_namelen = 2;
	cn.cn_consume = 0;

	/* At this point, lvp is locked  */
	error = VOP_LOOKUP(lvp, uvpp, &cn);
	VOP_UNLOCK(lvp);
	if (error) {
		*uvpp = NULL;
		return error;
	}
	uvp = *uvpp;
	/* If we don't care about the pathname, we're done */
	if (bufp == NULL) {
		return 0;
	}

	fileno = va.va_fileid;

	/* I guess UFS_DIRBLKSIZ is a good guess at a good size to use? */
	dirbuflen = UFS_DIRBLKSIZ;
	if (dirbuflen < va.va_blocksize)
		dirbuflen = va.va_blocksize;
	dirbuf = kmem_alloc(dirbuflen, KM_SLEEP);

	/* Now lvp is unlocked, try to lock uvp */
	error = vn_lock(uvp, LK_SHARED);
	if (error) {
		vrele(uvp);
		*uvpp = NULL;
		return error;
	}

#if 0
unionread:
#endif
	off = 0;
	do {
		/* call VOP_READDIR of parent */
		iov.iov_base = dirbuf;
		iov.iov_len = dirbuflen;

		uio.uio_iov = &iov;
		uio.uio_iovcnt = 1;
		uio.uio_offset = off;
		uio.uio_resid = dirbuflen;
		uio.uio_rw = UIO_READ;
		UIO_SETUP_SYSSPACE(&uio);

		eofflag = 0;

		error = VOP_READDIR(uvp, &uio, cred, &eofflag, 0, 0);

		off = uio.uio_offset;

		/*
		 * Try again if NFS tosses its cookies.
		 * XXX this can still loop forever if the directory is busted
		 * such that the second or subsequent page of it always
		 * returns EINVAL
		 */
		if ((error == EINVAL) && (tries < 3)) {
			off = 0;
			tries++;
			continue;	/* once more, with feeling */
		}

		if (!error) {
			char   *cpos;
			struct dirent *dp;

			cpos = dirbuf;
			tries = 0;

			/* scan directory page looking for matching vnode */
			for (len = (dirbuflen - uio.uio_resid); len > 0;
			    len -= reclen) {
				dp = (struct dirent *) cpos;
				reclen = dp->d_reclen;

				/* check for malformed directory.. */
				if (reclen < _DIRENT_MINSIZE(dp) ||
				    reclen > len) {
					error = EINVAL;
					goto out;
				}
				/*
				 * XXX should perhaps do VOP_LOOKUP to
				 * check that we got back to the right place,
				 * but getting the locking games for that
				 * right would be heinous.
				 */
				if ((dp->d_type != DT_WHT) &&
				    (dp->d_fileno == fileno)) {
					char *bp = *bpp;

					bp -= dp->d_namlen;
					if (bp <= bufp) {
						error = ERANGE;
						goto out;
					}
					memcpy(bp, dp->d_name, dp->d_namlen);
					error = 0;
					*bpp = bp;
					goto out;
				}
				cpos += reclen;
			}
		} else
			goto out;
	} while (!eofflag);
#if 0
	/*
	 * Deal with mount -o union, which unions only the
	 * root directory of the mount.
	 */
	if ((uvp->v_vflag & VV_ROOT) &&
	    (uvp->v_mount->mnt_flag & MNT_UNION)) {
		struct vnode *tvp = uvp;

		uvp = uvp->v_mount->mnt_vnodecovered;
		vput(tvp);
		vref(uvp);
		*uvpp = uvp;
		vn_lock(uvp, LK_SHARED | LK_RETRY);
		goto unionread;
	}
#endif
	error = ENOENT;

out:
	VOP_UNLOCK(uvp);
	kmem_free(dirbuf, dirbuflen);
	return error;
}

/*
 * common routine shared by sys___getcwd() and vn_isunder()
 */
int
getcwd_common(struct vnode *lvp, struct vnode *rvp, char **bpp, char *bufp,
    int limit, int flags, struct lwp *l)
{
	struct cwdinfo *cwdi = l->l_proc->p_cwdi;
	kauth_cred_t cred = l->l_cred;
	struct vnode *uvp = NULL;
	char *bp = NULL;
	int error;
	int perms = VEXEC;

	error = 0;
	if (rvp == NULL) {
		rvp = cwdi->cwdi_rdir;
		if (rvp == NULL)
			rvp = rootvnode;
	}

	vref(rvp);
	vref(lvp);

	/*
	 * Error handling invariant:
	 * Before a `goto out':
	 *	lvp is either NULL, or held.
	 *	uvp is either NULL, or held.
	 */

	if (bufp)
		bp = *bpp;

	/*
	 * this loop will terminate when one of the following happens:
	 *	- we hit the root
	 *	- getdirentries or lookup fails
	 *	- we run out of space in the buffer.
	 */
	if (lvp == rvp) {
		if (bp)
			*(--bp) = '/';
		goto out;
	}
	do {
		/*
		 * access check here is optional, depending on
		 * whether or not caller cares.
		 */
		int chkaccess = (flags & GETCWD_CHECK_ACCESS);
		bool locked = false;

		/*
		 * step up if we're a covered vnode..
		 * check access on the first vnode only.
		 */
		if (lvp->v_vflag & VV_ROOT) {
			vn_lock(lvp, LK_SHARED | LK_RETRY);
			if (chkaccess) {
				error = VOP_ACCESS(lvp, perms, cred);
				if (error) {
					VOP_UNLOCK(lvp);
					goto out;
				}
				chkaccess = 0;
			}
			while (lvp->v_vflag & VV_ROOT) {
				struct vnode *tvp;

				if (lvp == rvp) {
					VOP_UNLOCK(lvp);
					goto out;
				}

				tvp = lvp->v_mount->mnt_vnodecovered;
				/*
				 * hodie natus est radici frater
				 */
				if (tvp == NULL) {
					VOP_UNLOCK(lvp);
					error = ENOENT;
					goto out;
				}
				vref(tvp);
				vput(lvp);
				lvp = tvp;
				if (lvp->v_vflag & VV_ROOT)
					vn_lock(lvp, LK_SHARED | LK_RETRY);
			}
		}

		/* Do we need to check access to the directory? */
		if (chkaccess && !cache_have_id(lvp)) {
			/* Need exclusive for UFS VOP_GETATTR (itimes) & VOP_LOOKUP. */
			vn_lock(lvp, LK_EXCLUSIVE | LK_RETRY);
			error = VOP_ACCESS(lvp, perms, cred);
			if (error) {
				VOP_UNLOCK(lvp);
				goto out;
			}
			chkaccess = 0;
			locked = true;
		}

		/*
		 * Look in the name cache; if that fails, look in the
		 * directory..
		 */
		error = cache_revlookup(lvp, &uvp, &bp, bufp, chkaccess,
		    perms);
		if (error == -1) {
			if (!locked) {
				locked = true;
				vn_lock(lvp, LK_EXCLUSIVE | LK_RETRY);
			}
			if (lvp->v_type != VDIR) {
				VOP_UNLOCK(lvp);
				error = ENOTDIR;
				goto out;
			}
			error = getcwd_scandir(lvp, &uvp, &bp, bufp, l);
			/* lvp now unlocked */
		} else if (locked) {
			VOP_UNLOCK(lvp);
		}
		if (error)
			goto out;
#if DIAGNOSTIC
		if (bufp && (bp <= bufp)) {
			panic("getcwd: oops, went back too far");
		}
#endif
		perms = VEXEC | VREAD;
		if (bp)
			*(--bp) = '/';
		vrele(lvp);
		lvp = uvp;
		uvp = NULL;
		limit--;
	} while ((lvp != rvp) && (limit > 0));

out:
	if (bpp)
		*bpp = bp;
	if (uvp)
		vrele(uvp);
	if (lvp)
		vrele(lvp);
	vrele(rvp);
	return error;
}

/*
 * Check if one directory can be found inside another in the directory
 * hierarchy.
 *
 * Intended to be used in chroot, chdir, fchdir, etc., to ensure that
 * chroot() actually means something.
 */
int
vn_isunder(struct vnode *lvp, struct vnode *rvp, struct lwp *l)
{
	int error;

	error = getcwd_common(lvp, rvp, NULL, NULL, MAXPATHLEN / 2, 0, l);

	if (!error)
		return 1;
	else
		return 0;
}

/*
 * Returns true if proc p1's root directory equal to or under p2's
 * root directory.
 *
 * Intended to be used from ptrace/procfs sorts of things.
 */

int
proc_isunder(struct proc *p1, struct lwp *l2)
{
	struct vnode *r1 = p1->p_cwdi->cwdi_rdir;
	struct vnode *r2 = l2->l_proc->p_cwdi->cwdi_rdir;

	if (r1 == NULL)
		return (r2 == NULL);
	else if (r2 == NULL)
		return 1;
	else
		return vn_isunder(r1, r2, l2);
}

/*
 * Find pathname of process's current directory.
 *
 * Use vfs vnode-to-name reverse cache; if that fails, fall back
 * to reading directory contents.
 */

int
sys___getcwd(struct lwp *l, const struct sys___getcwd_args *uap, register_t *retval)
{
	/* {
		syscallarg(char *) bufp;
		syscallarg(size_t) length;
	} */

	int     error;
	char   *path;
	char   *bp, *bend;
	int     len = SCARG(uap, length);
	int	lenused;
	struct	cwdinfo *cwdi;

	if (len > MAXPATHLEN * 4)
		len = MAXPATHLEN * 4;
	else if (len < 2)
		return ERANGE;

	path = kmem_alloc(len, KM_SLEEP);
	bp = &path[len];
	bend = bp;
	*(--bp) = '\0';

	/*
	 * 5th argument here is "max number of vnodes to traverse".
	 * Since each entry takes up at least 2 bytes in the output buffer,
	 * limit it to N/2 vnodes for an N byte buffer.
	 */
	cwdi = l->l_proc->p_cwdi;
	rw_enter(&cwdi->cwdi_lock, RW_READER);
	error = getcwd_common(cwdi->cwdi_cdir, NULL, &bp, path, 
	    len/2, GETCWD_CHECK_ACCESS, l);
	rw_exit(&cwdi->cwdi_lock);

	if (error)
		goto out;
	lenused = bend - bp;
	*retval = lenused;
	/* put the result into user buffer */
	error = copyout(bp, SCARG(uap, bufp), lenused);

out:
	kmem_free(path, len);
	return error;
}

/*
 * Try to find a pathname for a vnode.  Since there is no mapping vnode ->
 * parent directory, this needs the namecache to succeed.  Caller holds a
 * reference to the vnode.
 */
int
vnode_to_path(char *path, size_t len, struct vnode *vp, struct lwp *curl,
    struct proc *p)
{
	struct proc *curp = curl->l_proc;
	int error, lenused, elen;
	char *bp, *bend;
	struct vnode *dvp;

	KASSERT(vrefcnt(vp) > 0);

	bp = bend = &path[len];
	*(--bp) = '\0';

	error = cache_revlookup(vp, &dvp, &bp, path, false, 0);
	if (error != 0)
		return (error == -1 ? ENOENT : error);

	*(--bp) = '/';
	error = getcwd_common(dvp, NULL, &bp, path, len / 2,
	    GETCWD_CHECK_ACCESS, curl);
	vrele(dvp);
	if (error != 0)
		return error;

	/*
	 * Strip off emulation path for emulated processes looking at
	 * the maps file of a process of the same emulation. (Won't
	 * work if /emul/xxx is a symlink..)
	 */
	if (curp->p_emul == p->p_emul && curp->p_emul->e_path != NULL) {
		elen = strlen(curp->p_emul->e_path);
		if (!strncmp(bp, curp->p_emul->e_path, elen))
			bp = &bp[elen];
	}

	lenused = bend - bp;

	memcpy(path, bp, lenused);
	path[lenused] = '\0';

	return 0;
}