fd: Avoid truncating output buffers for KERN_PROC_{CWD,FILEDESC}

These sysctls failed to return an error if the caller had provided too
short an output buffer.  Change them to return ENOMEM instead, to ensure
that callers can detect truncation in the face of a concurrently
changing fd table.

PR:		228432
Discussed with:	cem, jhb
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D15607

1 files changed, 29 insertions, 25 deletions

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index fb2dc4fdc94e..d5d22ecfc522 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -4290,14 +4290,13 @@ export_kinfo_to_sb(struct export_fd_buf *efbuf)
 
 	kif = &efbuf->kif;
 	if (efbuf->remainder != -1) {
-		if (efbuf->remainder < kif->kf_structsize) {
-			/* Terminate export. */
-			efbuf->remainder = 0;
-			return (0);
-		}
+		if (efbuf->remainder < kif->kf_structsize)
+			return (ENOMEM);
 		efbuf->remainder -= kif->kf_structsize;
 	}
-	return (sbuf_bcat(efbuf->sb, kif, kif->kf_structsize) == 0 ? 0 : ENOMEM);
+	if (sbuf_bcat(efbuf->sb, kif, kif->kf_structsize) != 0)
+		return (sbuf_error(efbuf->sb));
+	return (0);
 }
 
 static int
@@ -4307,7 +4306,7 @@ export_file_to_sb(struct file *fp, int fd, cap_rights_t *rightsp,
 	int error;
 
 	if (efbuf->remainder == 0)
-		return (0);
+		return (ENOMEM);
 	export_file_to_kinfo(fp, fd, rightsp, &efbuf->kif, efbuf->fdp,
 	    efbuf->flags);
 	FILEDESC_SUNLOCK(efbuf->fdp);
@@ -4323,7 +4322,7 @@ export_vnode_to_sb(struct vnode *vp, int fd, int fflags,
 	int error;
 
 	if (efbuf->remainder == 0)
-		return (0);
+		return (ENOMEM);
 	if (efbuf->pdp != NULL)
 		PWDDESC_XUNLOCK(efbuf->pdp);
 	export_vnode_to_kinfo(vp, fd, fflags, &efbuf->kif, efbuf->flags);
@@ -4369,22 +4368,25 @@ kern_proc_filedesc_out(struct proc *p,  struct sbuf *sb, ssize_t maxlen,
 	fdp = fdhold(p);
 	pdp = pdhold(p);
 	PROC_UNLOCK(p);
+
 	efbuf = malloc(sizeof(*efbuf), M_TEMP, M_WAITOK);
 	efbuf->fdp = NULL;
 	efbuf->pdp = NULL;
 	efbuf->sb = sb;
 	efbuf->remainder = maxlen;
 	efbuf->flags = flags;
+
+	error = 0;
 	if (tracevp != NULL)
-		export_vnode_to_sb(tracevp, KF_FD_TYPE_TRACE, FREAD | FWRITE,
+		error = export_vnode_to_sb(tracevp, KF_FD_TYPE_TRACE,
+		    FREAD | FWRITE, efbuf);
+	if (error == 0 && textvp != NULL)
+		error = export_vnode_to_sb(textvp, KF_FD_TYPE_TEXT, FREAD,
 		    efbuf);
-	if (textvp != NULL)
-		export_vnode_to_sb(textvp, KF_FD_TYPE_TEXT, FREAD, efbuf);
-	if (cttyvp != NULL)
-		export_vnode_to_sb(cttyvp, KF_FD_TYPE_CTTY, FREAD | FWRITE,
-		    efbuf);
-	error = 0;
-	if (pdp == NULL || fdp == NULL)
+	if (error == 0 && cttyvp != NULL)
+		error = export_vnode_to_sb(cttyvp, KF_FD_TYPE_CTTY,
+		    FREAD | FWRITE, efbuf);
+	if (error != 0 || pdp == NULL || fdp == NULL)
 		goto fail;
 	efbuf->fdp = fdp;
 	efbuf->pdp = pdp;
@@ -4394,23 +4396,25 @@ kern_proc_filedesc_out(struct proc *p,  struct sbuf *sb, ssize_t maxlen,
 		/* working directory */
 		if (pwd->pwd_cdir != NULL) {
 			vrefact(pwd->pwd_cdir);
-			export_vnode_to_sb(pwd->pwd_cdir, KF_FD_TYPE_CWD,
-			    FREAD, efbuf);
+			error = export_vnode_to_sb(pwd->pwd_cdir,
+			    KF_FD_TYPE_CWD, FREAD, efbuf);
 		}
 		/* root directory */
-		if (pwd->pwd_rdir != NULL) {
+		if (error == 0 && pwd->pwd_rdir != NULL) {
 			vrefact(pwd->pwd_rdir);
-			export_vnode_to_sb(pwd->pwd_rdir, KF_FD_TYPE_ROOT,
-			    FREAD, efbuf);
+			error = export_vnode_to_sb(pwd->pwd_rdir,
+			    KF_FD_TYPE_ROOT, FREAD, efbuf);
 		}
 		/* jail directory */
-		if (pwd->pwd_jdir != NULL) {
+		if (error == 0 && pwd->pwd_jdir != NULL) {
 			vrefact(pwd->pwd_jdir);
-			export_vnode_to_sb(pwd->pwd_jdir, KF_FD_TYPE_JAIL,
-			    FREAD, efbuf);
+			error = export_vnode_to_sb(pwd->pwd_jdir,
+			    KF_FD_TYPE_JAIL, FREAD, efbuf);
 		}
 	}
 	PWDDESC_XUNLOCK(pdp);
+	if (error != 0)
+		goto fail;
 	if (pwd != NULL)
 		pwd_drop(pwd);
 	FILEDESC_SLOCK(fdp);
@@ -4430,7 +4434,7 @@ kern_proc_filedesc_out(struct proc *p,  struct sbuf *sb, ssize_t maxlen,
 		 * loop continues.
 		 */
 		error = export_file_to_sb(fp, i, &rights, efbuf);
-		if (error != 0 || efbuf->remainder == 0)
+		if (error != 0)
 			break;
 	}
 	FILEDESC_SUNLOCK(fdp);