61c3357ce1
It is required to prevent deadlocks in single-threaded select-based IO
programs (namely: qrexec). POSIX API doesn't support checking how much
can be written to pipe/socket without blocking, so to prevent blocking
application must use O_NONBLOCK mode, and somehow deal with non-written
data (buffer it).
QubesOS/qubes-issues#1347
(cherry picked from commit 6a44eaeb09)
120 lines
2.9 KiB
C
120 lines
2.9 KiB
C
/*
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* The Qubes OS Project, http://www.qubes-os.org
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*
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* Copyright (C) 2010 Rafal Wojtczuk <rafal@invisiblethingslab.com>
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version 2
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* of the License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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*
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*/
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#include <stdio.h>
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#include <unistd.h>
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#include <errno.h>
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#include <fcntl.h>
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#include <stdlib.h>
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#include <libvchan.h>
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#include "qrexec.h"
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#include "libqrexec-utils.h"
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/*
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There is buffered data in "buffer" for client and select()
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reports that "fd" is writable. Write as much as possible to fd.
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*/
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int flush_client_data(int fd, struct buffer *buffer)
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{
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int ret;
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int len;
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for (;;) {
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len = buffer_len(buffer);
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if (!len) {
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return WRITE_STDIN_OK;
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}
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if (len > MAX_DATA_CHUNK)
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len = MAX_DATA_CHUNK;
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ret = write(fd, buffer_data(buffer), len);
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if (ret == -1) {
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if (errno != EAGAIN) {
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return WRITE_STDIN_ERROR;
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} else
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return WRITE_STDIN_BUFFERED;
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}
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// we previously called buffer_remove(buffer, len)
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// it will be wrong if we change MAX_DATA_CHUNK to something large
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// as pipes writes are atomic only to PIPE_MAX limit
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buffer_remove(buffer, ret);
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}
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}
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/*
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Write "len" bytes from "data" to "fd". If not all written, buffer the rest
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to "buffer".
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*/
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int write_stdin(int fd, const char *data, int len, struct buffer *buffer)
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{
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int ret;
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int written = 0;
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if (buffer_len(buffer)) {
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buffer_append(buffer, data, len);
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return WRITE_STDIN_BUFFERED;
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}
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while (written < len) {
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ret = write(fd, data + written, len - written);
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if (ret == 0) {
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perror("write_stdin: write returns 0 ???");
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exit(1);
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}
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if (ret == -1) {
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if (errno != EAGAIN)
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return WRITE_STDIN_ERROR;
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buffer_append(buffer, data + written,
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len - written);
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return WRITE_STDIN_BUFFERED;
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}
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written += ret;
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}
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return WRITE_STDIN_OK;
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}
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/*
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Data feed process has exited, so we need to clear all control structures for
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the client. However, if we have buffered data for the client (which is rare btw),
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fire&forget a separate process to flush them.
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*/
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int fork_and_flush_stdin(int fd, struct buffer *buffer)
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{
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int i;
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if (!buffer_len(buffer))
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return 0;
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switch (fork()) {
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case -1:
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perror("fork");
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exit(1);
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case 0:
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break;
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default:
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return 1;
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}
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for (i = 0; i < MAX_FDS; i++)
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if (i != fd && i != 2)
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close(i);
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set_block(fd);
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write_all(fd, buffer_data(buffer), buffer_len(buffer));
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_exit(0);
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}
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