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To:	cygwin AT cygwin DOT com
From:	Thomas Zimmermann <tdz AT users DOT sourceforge DOT net>
Subject:	Bug: Synchronous signals broken on Cygwin 64
Openpgp:	preference=signencrypt
Autocrypt:	addr=tdz AT users DOT sourceforge DOT net; prefer-encrypt=mutual; keydata= xsDiBEQ8FzARBAD9199cx5bJZ7nvSuaGA6ZLv1WyPtIeKWXj8mOfpn/tfmKUlzFijUChGBGP 1/FGssV/KnR+1zWdxsgtYuE8bbslJEGRP/fgaOgxisCKjkU8/SKXKKYoxS568m2TwBO512so PTj2mC3yWz4nU7ZTmgoJzf8r8TDZ3GBbHCoO21tUjwCgkjiAm08Kh72NtbMysXW88eOcFfUE ANvt1VrkyDjLD/viv7MHYUMLKLaFdNZUnJi4Rx6NwIqNxLDeGyPPp17/X2ls397Wl9PAZRJB 96/3E5WAOCyaKJc605luoyvv0CJ5+VuAQDVXKGdX/jr7EnhkyXmcm8ChKR/1Lct6jn5Wp922 utdklQcYEkSNMJbRsKReluxXaD6kBAC2qI/mOyRtJLxoszYG2hLf1BAOopJb13J1TCRQv3Df El2zmbPyYOfMQzbnFBrZ5yMve6I5/9hWksMMUWrdtgGDEsBFMa8iFb9FPsbKrDBmR66wpNFf s7GPEcWpIueBGu+vmXtUN8zE4mvUxFixc7iiPx46TCctKFqujGf6ubhlpc0tVGhvbWFzIFpp bW1lcm1hbm4gPHRkekB1c2Vycy5zb3VyY2Vmb3JnZS5uZXQ+wl0EExECAB0FAkQ8FzAGCwkI BwMCBBUCCAMEFgIDAQIeAQIXgAAKCRDZ2tfUTvHfCIydAJ4ndgH+dGp131Bx0su0VkWIiCfg iwCdHB339NSWt0kXY4+/Zhfpenzj9EXOwE0ERDwXMRAEAKcJyo9UpIp4Z4Mer58CL+ErcgwU xWdH5yhRV0++D70R6N5e+Yoa20V5trrNhpCEeAvjLW77VRqCYPO3hn0mmN2jSO8qMdBn7rXS ND9WFrlnSSCGsYQeoKj0z2KlQ8Waxg+r3b23azbMinF30x9303S3ZhrWvSf8wxIgd+u3HeXj AAMGA/9s/AWniacRR8T2ABN0ASjXp1SCSYmezL9Jp8MfSWvLcNrB/0Oi/o3+1HfJiTHg7o8l H6tpqScWsKpPZbYxmo9vb9sVjAOLzGpeV8S6Pe1pcXfiOuZLMJ5OTga6GxWimmP7rrWKTIlP x57wXD+v6c4A4VYW973YXNh1mVXwON3ydMJGBBgRAgAGBQJEPBcxAAoJENna19RO8d8IASEA niQqMp4Yd00/t47D09opsugE9GBHAJ9sJZgtyq9lLB3xCFm16nooejR/XA==
Message-ID:	<bc993d03-a9ac-d1c9-c33a-03cfc810edbe@users.sourceforge.net>
Date:	Thu, 26 Apr 2018 09:57:59 +0200
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Content-Type: multipart/mixed; boundary="wtxaMxR8iwSUN3mw6nddWiunoTLQjB9QC";
 protected-headers="v1"
From: Thomas Zimmermann <tdz AT users DOT sourceforge DOT net>
To: cygwin AT cygwin DOT com
Message-ID: <bc993d03-a9ac-d1c9-c33a-03cfc810edbe AT users DOT sourceforge DOT net>
Subject: Bug: Synchronous signals broken on Cygwin 64


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Hello mailing list,

I noticed that synchronous signals (e.g., SIGSEGV, SIGFPE) appear to be
broken on 64-bit Cygwin systems. I could not find additional information
on the mailing list. If this is not already a known problem, you'll find
some analysis below.


I use Cygwin DLL 2.10 with all packages up-to-date. My test system runs
Windows 7.


The expected behavior is that an installed signal handler runs exactly
once for a signal and the OS terminates the program if the handler
returns. This works on 32-bit Cygwin. From my observation, Cygwin 64
differs in the follow ways:

1) Generating a synchronous signal on the main thread, makes the signal
handler enter an endless loop. The attached test 'syncsig.c' illustrates
this.

2) Generating a synchronous signal on an additional POSIX thread ignores
the installed signal handler entirely. The handler does not run and the
program gets terminated immediately. The attached test 'syncsig_mt.c'
illustrates this.


As mentioned, both cases work correctly on 32-bit installations. I
cannot point to the exact location of the problem, but it appears that
the Cygwin exception-handler code is not set up by 64-bit code.

In case 2) I can see the Windows function KiUserExceptionDispatcher()
being executed for the synchronous signal on the affected thread. This
should lead to a call to the Cygwin exception handler. It does for
32-bit, but not for 64-bit code.

=46rom what I've seen, the exception handler is installed in the c'tor
exception::exception(). [exception.cc:138] The only invocation I found
was in SetThreadName(). [miscfuncs.cc:951] This set-up code is 32-bit-only.


BTW, is there a bug tracker for Cygwin? I'd open a bug report there, or
add the information to an existing one.


Best regards
Thomas


[exception.cc:138]
https://cygwin.com/git/gitweb.cgi?p=3Dnewlib-cygwin.git;a=3Dblob;f=3Dwinsup=
/cygwin/exception.h;h=3Dffff7464ba11b5526fcf9d13e32334912a30a3b0;hb=3D4c73a=
d6b20378e4b74355fcdb2005f2aac489c9f#l138

[miscfuncs.cc:951]
https://cygwin.com/git/gitweb.cgi?p=3Dnewlib-cygwin.git;a=3Dblob;f=3Dwinsup=
/cygwin/miscfuncs.cc;h=3D3ad658739ad9e85868ca74ce2e86aa6ddbdbfb37;hb=3D4c73=
ad6b20378e4b74355fcdb2005f2aac489c9f#l951

--=20
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--------------2835FB10DBC0098E1855BDF8
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 name="syncsig.c"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: attachment;
 filename="syncsig.c"


/*
 * This test program works on 32-bit and 64-bit Cygwin. On 32-bit=20
 * systems, the signal handler runs *exactly* once, which is the correct
 * behaviour. On 64-bit systems, the exception code enters an endless=20
 * loop and the signal handler runs over and over.
 *
 * Compile with
 *
 * 	gcc -Wall -O0 -osyncsig ./syncsig.c
 *
 * Run with
 *
 * 	./syncsig
 *
 * Expected Result
 *
 * 	Program prints "signal handler running" once
 *
 * Actual Result
 *
 * 	On 64-bit systems, the handler runs in an endless loop
 * */

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>

static void
signal_handler(int signum, siginfo_t* info, void* ucontext)
{
	fprintf(stderr, "signal handler running\n"); // unsafe in real code!
}

int
main(int argc, char* argv[])
{
	struct sigaction act =3D {
		.sa_sigaction =3D signal_handler,
		.sa_flags =3D SA_SIGINFO,
	};
	sigemptyset(&act.sa_mask);

	sigaction(SIGFPE, &act, NULL);

	/* Trigger Windows Exception for division by zero */
	argc /=3D !argc;

	fprintf(stderr, "no exception generated\n");

	return EXIT_SUCCESS;
}

--------------2835FB10DBC0098E1855BDF8
Content-Type: text/x-csrc;
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Content-Transfer-Encoding: quoted-printable
Content-Disposition: attachment;
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/*
 * This test program works on 32-bit Cygwin, but fails on 64-bit systems.
 * Running a signal handler for SIGFPE on the created POSIX thread does
 * not work.
 *
 * Compile with
 *
 * 	gcc -Wall -O0 -pthread -osyncsig_mt ./syncsig_mt.c
 *
 * Run with
 *
 * 	./syncsig_mt
 *
 * Expected Result
 *
 * 	Program prints "signal handler running"
 *
 * Actual Result
 *
 * 	No output on 64-bit systems, signal handler does not run
 * */

#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>

static void
signal_handler(int signum, siginfo_t* info, void* ucontext)
{
	fprintf(stderr, "signal handler running\n"); // unsafe in real code!
}

static void*
thread_start(void* arg)
{
	int argc =3D *(int*)arg;

	/* Trigger Windows Exception for division by zero */
	argc /=3D !argc;

	fprintf(stderr, "no exception generated\n");

	return NULL;
}

int
main(int argc, char* argv[])
{
	struct sigaction act =3D {
		.sa_sigaction =3D signal_handler,
		.sa_flags =3D SA_SIGINFO,
	};
	sigemptyset(&act.sa_mask);

	sigaction(SIGFPE, &act, NULL);

	pthread_t thread;
	pthread_create(&thread, NULL, thread_start, &argc);
	pthread_join(thread, NULL);

	return EXIT_SUCCESS;
}

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