[CC Ben, please keep him on the CC in replies. Thank you]
Hi Ken, On Jun 26 12:55, Ken Brown wrote: > Hi Corinna, > > On 6/26/2015 11:36 AM, Corinna Vinschen wrote: > >Thanks. Another question: How does emacs compute stack_bottom? > > Very near the beginning of main() it does the following: > > char stack_bottom_variable; > [...] > /* Record (approximately) where the stack begins. */ > stack_bottom = &stack_bottom_variable; Thank you. I created an STC with your code snippets and it now works for me (attached for reference). First problem was the return value of getrlimit(RLIMIT_STACK). Second problem is emacs. The check for an offset of the offending address in si_addr being less than 16K (STACK_DANGER_ZONE) is non-portable, putting it mildly. This might work on 32 bit Cygwin (I didn't test that), but the value is too low for 64 bit Cygwin. With STACK_DANGER_ZONE == 32K the handler works as desired on 64 bit Cygwin. Part of the reason is probably the _cygtls area of 12K reserved on each thread's stack, which moves the address of &stack_bottom_variable to a pretty low value right from the start. Another the size of the guard page area on the main thread (16K). I had a brief email exchange with a collegue of mine. Ben allowed me to quote him, so here are the important snippets of his replies: - Rlimits are an old way of doing a job and they were to a certain extent tied up in the pre-thread world of unix processes. rlimits have never been fully implemented on linux with a way that reproduces the unix way in the pre-thread era. rlimits have become a bit of a historical legacy and are there for posix compliance and code compatibility. The posix language was designed to be vague enough that all implementations could be made to conform. - Rather than making the system implementation conform to some unspecified behavior, I think it might be a wise idea to fix emacs instead. Looking at the code fragment you posted below(*), I’m not entirely convinced that the code would operate as intended on modern Linux or Unix. Given that, it may be better to make an implementation which does something like the current behavior was intended to do or better yet just remove it as a likely latent bug. (*) Emacs' handle_sigsegv function. Of course, for testing purposes this is still nice to have, so thank you for this test, I really appreciate it. As for getrlimit(RLIMIT_STACK), I changed that as outlined in my former mail in git. On second thought, I also changed the values of MINSIGSTKSZ and SIGSTKSZ. Instead of 2K and 8K, they are now defined as 32K and 64K. The reason is that we then have enough space on the alternate stack to install a _cygtls area, should the need arise. I created new developer snapshots on https://cygwin.com/snapshots/ Please give them a try. Remember to tweak STACK_DANGER_ZONE. You'll have to rebuild emacs anyway due to the change to [MIN]SIGSTKSZ. Thanks, Corinna -- Corinna Vinschen Please, send mails regarding Cygwin to Cygwin Maintainer cygwin AT cygwin DOT com Red Hat
#include <alloca.h>
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <setjmp.h>
#include <sys/time.h>
#include <sys/resource.h>
int stack_direction;
char *stack_bottom;
sigjmp_buf return_to_command_loop;
/* Attempt to recover from SIGSEGV caused by C stack overflow. */
static void
handle_sigsegv (int sig, siginfo_t *siginfo, void *arg)
{
struct rlimit rlim;
if (!getrlimit (RLIMIT_STACK, &rlim))
{
enum { STACK_DANGER_ZONE = 32 * 1024 };
char *beg, *end, *addr;
beg = stack_bottom;
end = stack_bottom + stack_direction * rlim.rlim_cur;
if (beg > end)
addr = beg, beg = end, end = addr;
addr = (char *) siginfo->si_addr;
/* If we're somewhere on stack and too close to
one of its boundaries, most likely this is it. */
if (beg < addr && addr < end
&& (addr - beg < STACK_DANGER_ZONE
|| end - addr < STACK_DANGER_ZONE))
siglongjmp (return_to_command_loop, 1);
}
/* Otherwise we can't do anything with this. */
abort ();
}
static int
init_sigsegv (void)
{
struct sigaction sa;
stack_t ss;
stack_direction = ((char *) &ss < stack_bottom) ? -1 : 1;
ss.ss_sp = malloc (SIGSTKSZ);
ss.ss_size = SIGSTKSZ;
ss.ss_flags = 0;
if (sigaltstack (&ss, NULL) < 0)
return 0;
sigfillset (&sa.sa_mask);
sa.sa_sigaction = handle_sigsegv;
sa.sa_flags = SA_SIGINFO | SA_ONSTACK;
return sigaction (SIGSEGV, &sa, NULL) < 0 ? 0 : 1;
}
void foo ()
{
int buf[512];
foo ();
}
int
main ()
{
char stack_bottom_variable;
/* Record (approximately) where the stack begins. */
stack_bottom = &stack_bottom_variable;
init_sigsegv ();
if (!sigsetjmp (return_to_command_loop, 1))
{
printf ("command loop before crash\n");
foo ();
}
else
printf ("command loop after crash\n");
return 0;
}
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