fd: move vnodes out of filedesc into a dedicated structure

The new structure is copy-on-write. With the assumption that path lookups are
significantly more frequent than chdirs and chrooting this is a win.

This provides stable root and jail root vnodes without the need to reference
them on lookup, which in turn means less work on globally shared structures.
Note this also happens to fix a bug where jail vnode was never referenced,
meaning subsequent access on lookup could run into use-after-free.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D23884

1 files changed, 6 insertions, 4 deletions

diff --git a/sys/ufs/ffs/ffs_alloc.c b/sys/ufs/ffs/ffs_alloc.c
index ea441404f477..fb270b11d912 100644
--- a/sys/ufs/ffs/ffs_alloc.c
+++ b/sys/ufs/ffs/ffs_alloc.c
@@ -3190,6 +3190,7 @@ sysctl_ffs_fsck(SYSCTL_HANDLER_ARGS)
 	struct inode *ip, *dp;
 	struct mount *mp;
 	struct fs *fs;
+	struct pwd *pwd;
 	ufs2_daddr_t blkno;
 	long blkcnt, blksize;
 	u_long key;
@@ -3448,11 +3449,11 @@ sysctl_ffs_fsck(SYSCTL_HANDLER_ARGS)
 		/*
 		 * Now we get and lock the child directory containing "..".
 		 */
-		FILEDESC_SLOCK(td->td_proc->p_fd);
-		dvp = td->td_proc->p_fd->fd_cdir;
-		FILEDESC_SUNLOCK(td->td_proc->p_fd);
+		pwd = pwd_hold(td);
+		dvp = pwd->pwd_cdir;
 		if ((error = vget(dvp, LK_EXCLUSIVE, td)) != 0) {
 			vput(fdvp);
+			pwd_drop(pwd);
 			break;
 		}
 		dp = VTOI(dvp);
@@ -3463,6 +3464,7 @@ sysctl_ffs_fsck(SYSCTL_HANDLER_ARGS)
 		cache_purge(dvp);
 		vput(dvp);
 		vput(fdvp);
+		pwd_drop(pwd);
 		break;
 
 	case FFS_UNLINK:
@@ -3607,7 +3609,7 @@ buffered_write(fp, uio, active_cred, flags, td)
 		return (EINVAL);
 	fdp = td->td_proc->p_fd;
 	FILEDESC_SLOCK(fdp);
-	vp = fdp->fd_cdir;
+	vp = fdp->fd_pwd->pwd_cdir;
 	vref(vp);
 	FILEDESC_SUNLOCK(fdp);
 	vn_lock(vp, LK_SHARED | LK_RETRY);