Check for negative return value of read, write and lseek instead of -1

The return values are used in ways that assume they are positive.
In practice, it is not possible to have a negative return value
other than -1 due to the size of the buffers being read from or
written to.  Also add overflow checks when updating the buffer len.
Quiets several coverity warnings.

lib/fuzzstub/fuzzstub.c

@@ -104,7 +104,7 @@ main(int argc, char *argv[])
 	}
 	nread = read(fd, buf, filesize);
 	if ((size_t)nread != filesize) {
-	    if (nread == -1)
+	    if (nread < 0)
 		fprintf(stderr, "read %s: %s\n", arg, strerror(errno));
 	    else
 		fprintf(stderr, "read %s: short read\n", arg);

lib/iolog/iolog_nextid.c

@@ -107,7 +107,7 @@ iolog_nextid(const char *iolog_dir, char sessid[7])
     /* Read current seq number (base 36). */
     nread = read(fd, buf, sizeof(buf) - 1);
     if (nread != 0) {
-	if (nread == -1) {
+	if (nread < 0) {
 	    goto done;
 	}
 	if (buf[nread - 1] == '\n')

lib/iolog/regress/iolog_filter/check_iolog_filter.c

@@ -138,7 +138,7 @@ main(int argc, char *argv[])
 
 	    nread = read(fd, tbuf, timing.u.nbytes);
 	    if ((size_t)nread != timing.u.nbytes) {
-		if (nread == -1)
+		if (nread < 0)
 		    sudo_warn("%s/%s", argv[i], name);
 		else
 		    sudo_warnx("%s/%s: short read", argv[i], name);
@@ -155,8 +155,8 @@ main(int argc, char *argv[])
 
 	    if (timing.event == IO_EVENT_TTYIN) {
 		nread = read(ttyin_ok_fd, fbuf, timing.u.nbytes);
-		if (nread == -1) {
+		if ((size_t)nread != timing.u.nbytes) {
-		    if (nread == -1)
+		    if (nread < 0)
 			sudo_warn("%s/ttyin.filtered", argv[i]);
 		    else
 			sudo_warnx("%s/ttyin.filtered: short read", argv[i]);

lib/util/event.c

@@ -154,7 +154,7 @@ signal_pipe_cb(int fd, int what, void *v)
 	sudo_debug_printf(SUDO_DEBUG_INFO,
 	    "%s: received signal %d", __func__, (int)ch);
     }
-    if (nread == -1 && errno != EAGAIN) {
+    if (nread < 0 && errno != EAGAIN) {
 	sudo_debug_printf(SUDO_DEBUG_ERROR|SUDO_DEBUG_LINENO|SUDO_DEBUG_ERRNO,
 	    "%s: error reading from signal pipe fd %d", __func__, fd);
     }

lib/util/regress/fuzz/fuzz_sudo_conf.c

@@ -85,7 +85,7 @@ LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
     if (fd == -1)
 	return 0;
     nwritten = write(fd, data, size);
-    if (nwritten == -1) {
+    if ((size_t)nwritten != size) {
 	close(fd);
 	return 0;
     }

logsrvd/logsrvd.c

@@ -969,13 +969,17 @@ server_msg_cb(int fd, int what, void *v)
     } else
 #endif
     {
-	nwritten = (size_t)write(fd, buf->data + buf->off, buf->len - buf->off);
+	const ssize_t n = write(fd, buf->data + buf->off, buf->len - buf->off);
+	if (n < 0) {
+	    if (errno == EAGAIN || errno == EINTR)
+		debug_return;
+	    sudo_warn("%s: write", closure->ipaddr);
+	    goto finished;
+	}
+	nwritten = (size_t)n;
     }
-
+    if (nwritten > SIZE_MAX - buf->off) {
-    if (nwritten == (size_t)-1) {
+	sudo_warnx(U_("internal error, %s overflow"), __func__);
-	if (errno == EAGAIN || errno == EINTR)
-	    debug_return;
-	sudo_warn("%s: write", closure->ipaddr);
 	goto finished;
     }
     buf->off += nwritten;
@@ -1082,25 +1086,28 @@ client_msg_cb(int fd, int what, void *v)
     } else
 #endif
     {
-        nread = (size_t)read(fd, buf->data + buf->len, buf->size - buf->len);
+	const ssize_t n = read(fd, buf->data + buf->len, buf->size - buf->len);
+	if (n < 0) {
+	    if (errno == EAGAIN || errno == EINTR)
+		debug_return;
+	    sudo_warn("%s: read", closure->ipaddr);
+	    goto close_connection;
+	}
+        nread = (size_t)n;
     }
 
     sudo_debug_printf(SUDO_DEBUG_INFO, "%s: received %zd bytes from client %s",
 	__func__, nread, closure->ipaddr);
-    switch (nread) {
+    if (nread == 0) {
-    case (size_t)-1:
-	if (errno == EAGAIN || errno == EINTR)
-	    debug_return;
-	sudo_warn("%s: read", closure->ipaddr);
-	goto close_connection;
-    case 0:
         if (closure->state != FINISHED) {
             sudo_debug_printf(SUDO_DEBUG_WARN|SUDO_DEBUG_LINENO,
                 "unexpected EOF");
         }
         goto close_connection;
-    default:
+    }
-	break;
+    if (nread > SIZE_MAX - buf->len) {
+	sudo_warnx(U_("internal error, %s overflow"), __func__);
+        goto close_connection;
     }
     buf->len += nread;
 

logsrvd/logsrvd_local.c

@@ -376,7 +376,7 @@ store_exit_info_json(int dfd, struct eventlog *evlog)
     iov[1].iov_len = sudo_json_get_len(&jsonc);
     iov[2].iov_base = (char *)"\n}\n";
     iov[2].iov_len = 3;
-    if (writev(fd, iov, 3) == -1) {
+    if (writev(fd, iov, 3) < 0) {
 	sudo_debug_printf(SUDO_DEBUG_ERROR|SUDO_DEBUG_LINENO|SUDO_DEBUG_ERRNO,
 	    "unable to write %s/log.json", evlog->iolog_path);
 	/* Back up and try to restore to original state. */

logsrvd/logsrvd_relay.c

@@ -798,23 +798,26 @@ relay_server_msg_cb(int fd, int what, void *v)
     } else
 #endif
     {
+	ssize_t n;
+
 	sudo_debug_printf(SUDO_DEBUG_INFO,
 	    "%s: ServerMessage from relay %s (%s)", __func__,
 	    relay_closure->relay_name.name, relay_closure->relay_name.ipaddr);
-	nread = (size_t)read(fd, buf->data + buf->len, buf->size - buf->len);
+	n = read(fd, buf->data + buf->len, buf->size - buf->len);
+	if (n < 0) {
+	    if (errno == EAGAIN || errno == EINTR)
+		debug_return;
+	    sudo_warn("%s: read", relay_closure->relay_name.ipaddr);
+	    closure->errstr = _("error reading from relay");
+	    goto send_error;
+	}
+	nread = (size_t)n;
     }
 
     sudo_debug_printf(SUDO_DEBUG_INFO,
 	"%s: received %zd bytes from relay %s (%s)", __func__, nread,
 	relay_closure->relay_name.name, relay_closure->relay_name.ipaddr);
-    switch (nread) {
+    if (nread == 0) {
-    case (size_t)-1:
-	if (errno == EAGAIN || errno == EINTR)
-	    debug_return;
-	sudo_warn("%s: read", relay_closure->relay_name.ipaddr);
-	closure->errstr = _("unable to read from relay");
-	goto send_error;
-    case 0:
 	/* EOF from relay server, close the socket. */
 	shutdown(relay_closure->sock, SHUT_RDWR);
 	close(relay_closure->sock);
@@ -833,8 +836,11 @@ relay_server_msg_cb(int fd, int what, void *v)
 	if (closure->sock == -1)
 	    connection_close(closure);
 	debug_return;
-    default:
+    }
-	break;
+    if (nread > SIZE_MAX - buf->len) {
+	sudo_warnx(U_("internal error, %s overflow"), __func__);
+	closure->errstr = _("error reading from relay");
+	goto send_error;
     }
     buf->len += nread;
 
@@ -979,14 +985,20 @@ relay_client_msg_cb(int fd, int what, void *v)
     } else
 #endif
     {
-	nwritten = (size_t)write(fd, buf->data + buf->off, buf->len - buf->off);
+	const ssize_t n = write(fd, buf->data + buf->off, buf->len - buf->off);
-	if (nwritten == (size_t)-1) {
+	if (n < 0) {
 	    if (errno == EAGAIN || errno == EINTR)
 		debug_return;
 	    sudo_warn("%s: write", relay_closure->relay_name.ipaddr);
 	    closure->errstr = _("error writing to relay");
 	    goto send_error;
 	}
+	nwritten = (size_t)n;
+    }
+    if (nwritten > SIZE_MAX - buf->off) {
+	sudo_warnx(U_("internal error, %s overflow"), __func__);
+	closure->errstr = _("error writing to relay");
+	goto send_error;
     }
     buf->off += nwritten;
 

logsrvd/regress/fuzz/fuzz_logsrvd_conf.c

@@ -193,7 +193,7 @@ LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
     if (fd == -1)
 	return 0;
     nwritten = write(fd, data, size);
-    if (nwritten == -1) {
+    if ((size_t)nwritten != size) {
 	close(fd);
 	return 0;
     }

logsrvd/sendlog.c

@@ -1370,23 +1370,28 @@ server_msg_cb(int fd, int what, void *v)
     } else
 #endif
     {
+	ssize_t n;
+
 	sudo_debug_printf(SUDO_DEBUG_INFO, "%s: reading ServerMessage", __func__);
-	nread = (size_t)read(fd, buf->data + buf->len, buf->size - buf->len);
+	n = read(fd, buf->data + buf->len, buf->size - buf->len);
+	if (n < 0) {
+	    if (errno == EAGAIN || errno == EINTR)
+		debug_return;
+	    sudo_warn("read");
+	    goto bad;
+	}
+	nread = (size_t)n;
     }
     sudo_debug_printf(SUDO_DEBUG_INFO, "%s: received %zd bytes from server",
 	__func__, nread);
-    switch (nread) {
+    if (nread == 0) {
-    case (size_t)-1:
-	if (errno == EAGAIN || errno == EINTR)
-	    debug_return;
-	sudo_warn("read");
-	goto bad;
-    case 0:
 	if (closure->state != FINISHED)
 	    sudo_warnx("%s", U_("premature EOF"));
 	goto bad;
-    default:
+    }
-	break;
+    if (nread > SIZE_MAX - buf->len) {
+	sudo_warnx(U_("internal error, %s overflow"), __func__);
+	goto bad;
     }
     buf->len += nread;
 
@@ -1496,12 +1501,17 @@ client_msg_cb(int fd, int what, void *v)
     } else
 #endif
     {
-	nwritten = (size_t)write(fd, buf->data + buf->off, buf->len - buf->off);
+	const ssize_t n = write(fd, buf->data + buf->off, buf->len - buf->off);
+	if (n < 0) {
+	    if (errno == EAGAIN || errno == EINTR)
+		debug_return;
+	    sudo_warn("write");
+	    goto bad;
+	}
+	nwritten = (size_t)n;
     }
-    if (nwritten == (size_t)-1) {
+    if (nwritten > SIZE_MAX - buf->off) {
-	if (errno == EAGAIN || errno == EINTR)
+	sudo_warnx(U_("internal error, %s overflow"), __func__);
-	    debug_return;
-	sudo_warn("write");
 	goto bad;
     }
     buf->off += nwritten;

plugins/sudoers/ldap.c

@@ -1293,21 +1293,27 @@ sudo_krb5_copy_cc_file(struct sudoers_context *ctx)
 			do {
 			    nwritten = write(nfd, buf + off,
 				(size_t)(nread - off));
-			    if (nwritten == -1) {
+			    if (nwritten < 0) {
 				sudo_warn("error writing to %s", new_ccname);
 				goto write_error;
 			    }
+			    if (nwritten > SSIZE_MAX - off) {
+				sudo_warnx(U_("internal error, %s overflow"),
+				    __func__);
+				nwritten = -1;
+				goto write_error;
+			    }
 			    off += nwritten;
 			} while (off < nread);
 		    }
-		    if (nread == -1)
+		    if (nread < 0)
 			sudo_warn("unable to read %s", new_ccname);
 write_error:
 		    close(nfd);
-		    if (nread != -1 && nwritten != -1) {
+		    if (nread < 0 || nwritten < 0) {
-			ret = new_ccname;	/* success! */
-		    } else {
 			unlink(new_ccname);	/* failed */
+		    } else {
+			ret = new_ccname;	/* success! */
 		    }
 		} else {
 		    sudo_warn("unable to create temp file %s", new_ccname);

plugins/sudoers/log_client.c

@@ -1791,21 +1791,24 @@ server_msg_cb(int fd, int what, void *v)
     } else
 #endif /* HAVE_OPENSSL */
     {
-        nread = (size_t)read(fd, buf->data + buf->len, buf->size - buf->len);
+	const ssize_t n = read(fd, buf->data + buf->len, buf->size - buf->len);
+	if (n < 0) {
+	    if (errno == EAGAIN)
+		debug_return;
+	    sudo_warn("read");
+	    goto bad;
+	}
+	nread = (size_t)n;
     }
     sudo_debug_printf(SUDO_DEBUG_INFO, "%s: received %zd bytes from server",
 	__func__, nread);
-    switch (nread) {
+    if (nread == 0) {
-    case (size_t)-1:
-	if (errno == EAGAIN)
-	    debug_return;
-	sudo_warn("read");
-	goto bad;
-    case 0:
 	sudo_warnx("%s", U_("lost connection to log server"));
 	goto bad;
-    default:
+    }
-	break;
+    if (nread > SIZE_MAX - buf->len) {
+	sudo_warnx(U_("internal error, %s overflow"), __func__);
+	goto bad;
     }
     buf->len += nread;
 
@@ -1928,11 +1931,15 @@ client_msg_cb(int fd, int what, void *v)
     } else
 #endif /* HAVE_OPENSSL */
     {
-        nwritten = (size_t)write(fd, buf->data + buf->off, buf->len - buf->off);
+	const ssize_t n = write(fd, buf->data + buf->off, buf->len - buf->off);
+	if (n < 0) {
+	    sudo_warn("send");
+	    goto bad;
+	}
+	nwritten = (size_t)n;
     }
-
+    if (nwritten > SIZE_MAX - buf->off) {
-    if (nwritten == (size_t)-1) {
+	sudo_warnx(U_("internal error, %s overflow"), __func__);
-	sudo_warn("send");
 	goto bad;
     }
     buf->off += nwritten;

plugins/sudoers/timestamp.c

@@ -348,7 +348,7 @@ ts_write(const struct sudoers_context *ctx, int fd, const char *fname,
 	nwritten = pwrite(fd, entry, entry->size, offset);
     }
     if ((size_t)nwritten != entry->size) {
-	if (nwritten == -1) {
+	if (nwritten < 0) {
 	    log_warning(ctx, SLOG_SEND_MAIL,
 		N_("unable to write to %s"), fname);
 	} else {

plugins/sudoers/tsdump.c

@@ -147,7 +147,7 @@ main(int argc, char *argv[])
 
 	if ((nread = read(fd, &cur, sizeof(cur))) == 0)
 	    break;
-	if (nread == -1)
+	if (nread < 0)
 	    sudo_fatal(U_("unable to read %s"), fname);
 
 	valid = valid_entry(&cur, pos);

plugins/sudoers/visudo.c

@@ -510,7 +510,7 @@ edit_sudoers(struct sudoersfile *sp, char *editor, int editor_argc,
 
 	    (void) lseek(sp->fd, (off_t)0, SEEK_SET);
 	    while ((nread = read(sp->fd, buf, sizeof(buf))) > 0) {
-		if (write(tfd, buf, (size_t)nread) == -1)
+		if (write(tfd, buf, (size_t)nread) != nread)
 		    sudo_fatal("%s", U_("write error"));
 		lastch = buf[nread - 1];
 	    }
@@ -518,7 +518,7 @@ edit_sudoers(struct sudoersfile *sp, char *editor, int editor_argc,
 	    /* Add missing newline at EOF if needed. */
 	    if (lastch != '\n') {
 		lastch = '\n';
-		if (write(tfd, &lastch, 1) == -1)
+		if (write(tfd, &lastch, 1) != 1)
 		    sudo_fatal("%s", U_("write error"));
 	    }
 	}

src/conversation.c

@@ -113,8 +113,8 @@ sudo_conversation(int num_msgs, const struct sudo_conv_message msgs[],
 		    }
 		    if (ttyfd != -1) {
 			/* Try writing to tty but fall back to fp on error. */
-			if ((len == 0 || write(ttyfd, msg->msg, len) != -1) &&
+			if ((len == 0 || write(ttyfd, msg->msg, len) > 0) &&
-				(crnl == NULL || write(ttyfd, crnl, 2) != -1)) {
+				(crnl == NULL || write(ttyfd, crnl, 2) > 0)) {
 			    written = true;
 			}
 			close(ttyfd);

src/copy_file.c

@@ -43,38 +43,42 @@ sudo_extend_file(int fd, const char *name, off_t new_size)
 {
     off_t old_size, size;
     ssize_t nwritten;
+    int serrno;
     char zeroes[BUFSIZ] = { '\0' };
     debug_decl(sudo_extend_file, SUDO_DEBUG_UTIL);
 
-    if ((old_size = lseek(fd, 0, SEEK_END)) == -1) {
+    if ((old_size = lseek(fd, 0, SEEK_END)) < 0) {
 	sudo_warn("lseek");
 	debug_return_int(-1);
     }
     sudo_debug_printf(SUDO_DEBUG_INFO, "%s: extending %s from %lld to %lld",
 	__func__, name, (long long)old_size, (long long)new_size);
 
-    for (size = old_size; size < new_size; size += nwritten) {
+    for (size = old_size; size < new_size; ) {
 	off_t len = new_size - size;
 	if (len > ssizeof(zeroes))
 	    len = ssizeof(zeroes);
 	nwritten = write(fd, zeroes, (size_t)len);
-	if (nwritten == -1) {
+	if (nwritten < 0 || nwritten > OFF_T_MAX - size) {
-	    int serrno = errno;
+	    goto fail;
-	    if (ftruncate(fd, old_size) == -1) {
-		sudo_debug_printf(
-		    SUDO_DEBUG_ERROR|SUDO_DEBUG_LINENO|SUDO_DEBUG_ERRNO,
-		    "unable to truncate %s to %lld", name, (long long)old_size);
-	    }
-	    errno = serrno;
-	    debug_return_int(-1);
 	}
+	size += nwritten;
     }
-    if (lseek(fd, 0, SEEK_SET) == -1) {
+    if (lseek(fd, 0, SEEK_SET) < 0) {
 	sudo_warn("lseek");
 	debug_return_int(-1);
     }
 
     debug_return_int(0);
+fail:
+    serrno = errno;
+    if (ftruncate(fd, old_size) != 0) {
+	sudo_debug_printf(
+	    SUDO_DEBUG_ERROR|SUDO_DEBUG_LINENO|SUDO_DEBUG_ERRNO,
+	    "unable to truncate %s to %lld", name, (long long)old_size);
+    }
+    errno = serrno;
+    debug_return_int(-1);
 }
 
 /*
@@ -107,24 +111,27 @@ sudo_copy_file(const char *src, int src_fd, off_t src_len, const char *dst,
     }
 
     /* Overwrite the old file with the new contents. */
-    while ((nread = read(src_fd, buf, sizeof(buf))) > 0) {
+    for (;;) {
 	ssize_t off = 0;
-	do {
+	nread = read(src_fd, buf, sizeof(buf));
+	if (nread <= 0) {
+	    if (nread == 0)
+		break;
+	    sudo_warn(U_("unable to read from %s"), src);
+	    debug_return_int(-1);
+	}
+	while (nread > off) {
 	    nwritten = write(dst_fd, buf + off, (size_t)(nread - off));
-	    if (nwritten == -1)
+	    if (nwritten < 0 || nwritten > SSIZE_MAX - off)
 		goto write_error;
 	    off += nwritten;
-	} while (nread > off);
+	}
-    }
-    if (nread == -1) {
-	sudo_warn(U_("unable to read from %s"), src);
-	debug_return_int(-1);
     }
 
     /* Did the file shrink? */
     if (src_len < dst_len) {
 	/* We don't open with O_TRUNC so must truncate manually. */
-	if (ftruncate(dst_fd, src_len) == -1) {
+	if (ftruncate(dst_fd, src_len) != 0) {
 	    sudo_debug_printf(
 		SUDO_DEBUG_ERROR|SUDO_DEBUG_LINENO|SUDO_DEBUG_ERRNO,
 		"unable to truncate %s to %lld", dst, (long long)src_len);

src/exec_monitor.c

@@ -279,8 +279,7 @@ mon_errsock_cb(int fd, int what, void *v)
      * Note that the error socket is *blocking*.
      */
     nread = read(fd, &errval, sizeof(errval));
-    switch (nread) {
+    if (nread < 0) {
-    case -1:
 	if (errno != EAGAIN && errno != EINTR) {
 	    if (mc->cstat->val == CMD_INVALID) {
 		/* XXX - need a way to distinguish non-exec error. */
@@ -291,22 +290,22 @@ mon_errsock_cb(int fd, int what, void *v)
 		"%s: failed to read error socket", __func__);
 	    sudo_ev_loopbreak(mc->evbase);
 	}
-	break;
+	debug_return;
-    default:
-	if (nread == 0) {
-	    /* The error socket closes when the command is executed. */
-	    sudo_debug_printf(SUDO_DEBUG_INFO, "EOF on error socket");
-	} else {
-	    /* Errno value when child is unable to execute command. */
-	    sudo_debug_printf(SUDO_DEBUG_INFO, "errno from child: %s",
-		strerror(errval));
-	    mc->cstat->type = CMD_ERRNO;
-	    mc->cstat->val = errval;
-	}
-	sudo_ev_del(mc->evbase, mc->errsock_event);
-	close(fd);
-	break;
     }
+
+    if (nread == 0) {
+	/* The error socket closes when the command is executed. */
+	sudo_debug_printf(SUDO_DEBUG_INFO, "EOF on error socket");
+    } else {
+	/* Errno value when child is unable to execute command. */
+	sudo_debug_printf(SUDO_DEBUG_INFO, "errno from child: %s",
+	    strerror(errval));
+	mc->cstat->type = CMD_ERRNO;
+	mc->cstat->val = errval;
+    }
+    sudo_ev_del(mc->evbase, mc->errsock_event);
+    close(fd);
+
     debug_return;
 }
 

src/exec_ptrace.c

@@ -1443,7 +1443,7 @@ proc_read_vec(pid_t pid, const char *name, int *countp, char ***vecp,
     /* Read in strings until EOF. */
     do {
 	nread = read(fd, strtab, remainder);
-	if (nread == -1) {
+	if (nread < 0) {
 	    sudo_debug_printf(SUDO_DEBUG_ERROR|SUDO_DEBUG_ERRNO,
 		"%s: unable to read %s", __func__, path);
 	    close(fd);

src/sudo_intercept_common.c

@@ -80,7 +80,7 @@ send_req(int sock, const void *buf, size_t len)
 
     do {
 	nwritten = send(sock, cp, len, 0);
-	if (nwritten == -1) {
+	if (nwritten < 0) {
 	    if (errno == EINTR)
 		continue;
 	    debug_return_bool(false);

src/tgetpass.c

@@ -218,11 +218,11 @@ restart:
 
     if (ISSET(flags, TGP_BELL) && output != STDERR_FILENO) {
 	/* Ring the bell if requested and there is a tty. */
-	if (write(output, "\a", 1) == -1)
+	if (write(output, "\a", 1) != 1)
 	    goto restore;
     }
     if (prompt) {
-	if (write(output, prompt, strlen(prompt)) == -1)
+	if (write(output, prompt, strlen(prompt)) < 0)
 	    goto restore;
     }
 
@@ -233,7 +233,7 @@ restart:
     save_errno = errno;
 
     if (neednl || pass == NULL) {
-	if (write(output, "\n", 1) == -1)
+	if (write(output, "\n", 1) != 1)
 	    goto restore;
     }
     tgetpass_display_error(errval);
@@ -403,7 +403,7 @@ getln(int fd, char *buf, size_t bufsiz, bool feedback,
 		break;
 	    } else if (c == sudo_term_kill) {
 		while (cp > buf) {
-		    if (write(fd, "\b \b", 3) == -1)
+		    if (write(fd, "\b \b", 3) != 3)
 			break;
 		    cp--;
 		}
@@ -426,7 +426,7 @@ getln(int fd, char *buf, size_t bufsiz, bool feedback,
     if (feedback) {
 	/* erase stars */
 	while (cp > buf) {
-	    if (write(fd, "\b \b", 3) == -1)
+	    if (write(fd, "\b \b", 3) != 3)
 		break;
 	    --cp;
 	}

src/ttyname.c

@@ -248,7 +248,7 @@ get_process_ttyname(char *name, size_t namelen)
     if ((fd = open(path, O_RDONLY | O_NOFOLLOW)) != -1) {
 	cp = buf;
 	while ((nread = read(fd, cp, sizeof(buf) - (size_t)(cp - buf))) != 0) {
-	    if (nread == -1) {
+	    if (nread < 0) {
 		if (errno == EAGAIN || errno == EINTR)
 		    continue;
 		break;