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Date:	Fri, 8 Mar 2019 23:01:38 +0900
From:	Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
To:	cygwin AT cygwin DOT com
Subject:	Re: Logging-in using ssh elevates the user privilege.
Message-Id:	<20190308230138.b7f6b5ac90c9a14cde2647c1@nifty.ne.jp>
In-Reply-To:	<51233666.20190307182445@yandex.ru>
References:	<20190307010000 DOT fc28b73739c2dd66e609982b AT nifty DOT ne DOT jp> <381052629 DOT 20190307141955 AT yandex DOT ru> <51233666 DOT 20190307182445 AT yandex DOT ru>
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Hello,

Thank you for the information.

On Thu, 7 Mar 2019 18:24:45 +0300 Andrey Repin wrote:
> > GNU screen freeze without much of an effort under Cygwin.
> > Try detaching from running screen and then running screen -ls.
> 
> Past discussion
> http://sourceware.org/ml/cygwin/2017-05/msg00448.html
> mid:16810313565 DOT 20170527142723 AT yandex DOT ru

I looked into this problem of GNU screen and found the
cause is very different from that of the problem I had
reported.

The problem I had reported is due to the failure of
sending signal, which is caused by mismatch of tokens
between ssh session and mintty session.

On the other hand, the problem you mentioned is due
to the difference in the behaviour of socket API.

In Linux, connect() in the client returns befor the
server calls accept(). However, in cygwin, connect()
does not return until the server calls accept().

Attached test code clarifies the difference.

[Result in Linux]
Server: Created.
Server: Binded.
Server: Listened.
Client: Created.
Client: Connected.
Client: Written.
Server: Accepted.
10: 1234567890
Server: Read.

[Result in Cygwin]
Server: Created.
Server: Binded.
Server: Listened.
Client: Created.
Server: Accepted.
Client: Connected.
Client: Written.
10: 1234567890
Server: Read.

I am not sure why cygwin behaves differently from
linux.

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <signal.h>

#define SOCKNAME "sock_unix_test"

int main()
{
	int fd;
	struct sockaddr_un sunx;
	pid_t pid;
	ssize_t len;
	char buf[BUFSIZ];
	
	memset(&sunx, 0, sizeof(sunx));
	sunx.sun_family = AF_UNIX;
	strncpy (sunx.sun_path, SOCKNAME, sizeof(sunx.sun_path) -1 );

	pid = fork();
	if (pid) {
		int fd1;
		fd = socket(AF_UNIX, SOCK_STREAM, 0);
		printf("Server: Created.\n");
		if (fd < 0) {
			perror("socket");
			goto end_server;
		}
		if (bind(fd, (struct sockaddr *)&sunx, sizeof(sunx)) < 0) {
			perror("bind");
			goto end_server;
		}
		printf("Server: Binded.\n");
		if (listen(fd, 1) < 0) {
			perror("listen");
			goto end_server;
		}
		printf("Server: Listened.\n");

		usleep(2000000);

		fd1 = accept(fd, 0, 0);
		if (fd1 < 0) {
			perror("accept");
			goto end_server;
		}
		printf("Server: Accepted.\n");
		while ((len = read(fd1, buf, sizeof(buf))) > 0) {
			buf[len] = '\0';
			printf("%d: %s\n", len, buf);
		}
		printf("Server: Read.\n");
		close(fd1);
end_server:
		close(fd);
		kill(pid, SIGTERM);
		wait(NULL);
		if (unlink(SOCKNAME) < 0) {
			perror("unlink");
		}
	} else {
		usleep(1000000);
		fd = socket(AF_UNIX, SOCK_STREAM, 0);
		printf("Client: Created.\n");
		if (fd < 0) {
			perror("socket");
			goto end_client;
		}
		if (connect(fd, (struct sockaddr *)&sunx, sizeof(sunx)) < 0) {
			perror("connect");
			goto end_client;
		}
		printf("Client: Connected.\n");
		write(fd, "1234567890", 10);
		printf("Client: Written.\n");
end_client:
		close(fd);
	}
	
	return 0;
}


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