Re: os/2 c-stack [was: snapshot in preparation for m4 1.4.12]

[Eric Blake]

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According to Elbert Pol on 8/13/2008 12:17 PM:
> Hello Eric,

[please don't top-post]

> > However, I'm still interested in seeing what this does when libsigsegv is
> > not present (you can use './configure --without-libsigsegv-prefix' to
> > rebuild m4 without it, rather than having to uninstall the library).

> 
> I build now with --without-libsigsegv-prefix
> 
> I attach resulst

Hmm.  According to that most recent log, using an explicit
 ./configure --without-libsigsegv-prefix
did not do what I expected; configure still detected the library located
in --prefix, and still linked with it.  I'm not sure why libsigsegv.m4 is
not honoring an explicit --without.  Bruno, might this be a bug in
lib-prefix.m4?

At any rate, I'm installing this patch to gnulib, since it appears (from
your first log), that even though your platform supports SA_ONSTACK, it
does not handle stack overflow as a SIGSEGV that can be handled by the
alternate stack.  It basically splits stack overflow detection into two
tests, so that c-stack doesn't even try installing a handler if it appears
that stack overflow SIGSEGV can't really be handled.

I'm planning on building a new m4 snapshot with all the recent gnulib
fixes later today.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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From e3ea810706b94a625eadc27dda079d74cdd68440 Mon Sep 17 00:00:00 2001
From: Eric Blake <address@hidden>
Date: Mon, 25 Aug 2008 06:26:56 -0600
Subject: [PATCH] c-stack: test that stack overflow can be caught

* m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): Also check
that platform allows handling stack overflow; at least OS/2 EMX
has sigaltstack, but crashes before transferring control to
handler on stack overflow.
* lib/c-stack.c [HAVE_SIGALTSTACK]: Adjust conditions to also
check for HAVE_STACK_OVERFLOW_HANDLING.
Reported by Elbert Pol.

Signed-off-by: Eric Blake <address@hidden>
---
 ChangeLog     |   11 +++++++
 lib/c-stack.c |    8 +++--
 m4/c-stack.m4 |   92 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 106 insertions(+), 5 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 708cefd..92dd185 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+2008-08-25  Eric Blake  <address@hidden>
+
+       c-stack: test that stack overflow can be caught
+       * m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): Also check
+       that platform allows handling stack overflow; at least OS/2 EMX
+       has sigaltstack, but crashes before transferring control to
+       handler on stack overflow.
+       * lib/c-stack.c [HAVE_SIGALTSTACK]: Adjust conditions to also
+       check for HAVE_STACK_OVERFLOW_HANDLING.
+       Reported by Elbert Pol.
+
 2008-08-25  Bruno Haible  <address@hidden>
 
        * doc/posix-functions/strftime.texi: Fix description of strftime
diff --git a/lib/c-stack.c b/lib/c-stack.c
index c46c231..6802665 100644
--- a/lib/c-stack.c
+++ b/lib/c-stack.c
@@ -117,7 +117,8 @@ die (int signo)
   abort ();
 }
 
-#if (HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK) || HAVE_LIBSIGSEGV
+#if (HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK \
+     && HAVE_STACK_OVERFLOW_HANDLING) || HAVE_LIBSIGSEGV
 
 /* Storage for the alternate signal stack.  */
 static union
@@ -210,7 +211,7 @@ c_stack_action (void (*action) (int))
   return 0;
 }
 
-#elif HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK
+#elif HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK && HAVE_STACK_OVERFLOW_HANDLING
 
 /* Direction of the C runtime stack.  This function is
    async-signal-safe.  */
@@ -316,7 +317,8 @@ c_stack_action (void (*action) (int))
   return sigaction (SIGSEGV, &act, NULL);
 }
 
-#else /* ! ((HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK) || HAVE_LIBSIGSEGV) */
+#else /* ! ((HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK
+            && HAVE_STACK_OVERFLOW_HANDLING) || HAVE_LIBSIGSEGV) */
 
 int
 c_stack_action (void (*action) (int)  __attribute__ ((unused)))
diff --git a/m4/c-stack.m4 b/m4/c-stack.m4
index 1db2a50..885b662 100644
--- a/m4/c-stack.m4
+++ b/m4/c-stack.m4
@@ -7,7 +7,7 @@
 
 # Written by Paul Eggert.
 
-# serial 4
+# serial 5
 
 AC_DEFUN([AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC],
   [# for STACK_DIRECTION
@@ -16,6 +16,90 @@ AC_DEFUN([AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC],
    AC_CHECK_HEADERS_ONCE([ucontext.h])
 
    AC_CACHE_CHECK([for working C stack overflow detection],
+     [ac_cv_sys_stack_overflow_works],
+     [AC_TRY_RUN(
+       [
+        #include <unistd.h>
+        #include <signal.h>
+        #if HAVE_SETRLIMIT
+        # include <sys/types.h>
+        # include <sys/time.h>
+        # include <sys/resource.h>
+        #endif
+         #ifndef SIGSTKSZ
+         # define SIGSTKSZ 16384
+         #endif
+
+        static union
+        {
+          char buffer[SIGSTKSZ];
+          long double ld;
+          long u;
+          void *p;
+        } alternate_signal_stack;
+
+        static void
+        segv_handler (int signo)
+        {
+          _exit (0);
+        }
+
+        static int
+        c_stack_action ()
+        {
+          stack_t st;
+          struct sigaction act;
+          int r;
+
+          st.ss_flags = 0;
+          st.ss_sp = alternate_signal_stack.buffer;
+          st.ss_size = sizeof alternate_signal_stack.buffer;
+          r = sigaltstack (&st, 0);
+          if (r != 0)
+            return r;
+
+          sigemptyset (&act.sa_mask);
+          act.sa_flags = SA_NODEFER | SA_ONSTACK | SA_RESETHAND;
+          act.sa_handler = segv_handler;
+          return sigaction (SIGSEGV, &act, 0);
+        }
+
+        static int
+        recurse (char *p)
+        {
+          char array[500];
+          array[0] = 1;
+          return *p + recurse (array);
+        }
+
+        int
+        main ()
+        {
+          #if HAVE_SETRLIMIT && defined RLIMIT_STACK
+          /* Before starting the endless recursion, try to be friendly
+             to the user's machine.  On some Linux 2.2.x systems, there
+             is no stack limit for user processes at all.  We don't want
+             to kill such systems.  */
+          struct rlimit rl;
+          rl.rlim_cur = rl.rlim_max = 0x100000; /* 1 MB */
+          setrlimit (RLIMIT_STACK, &rl);
+          #endif
+
+          c_stack_action ();
+          return recurse ("\1");
+        }
+       ],
+       [ac_cv_sys_stack_overflow_works=yes],
+       [ac_cv_sys_stack_overflow_works=no],
+       [ac_cv_sys_stack_overflow_works=cross-compiling])])
+
+  if test $ac_cv_sys_stack_overflow_works = yes; then
+   AC_DEFINE([HAVE_STACK_OVERFLOW_HANDLING], [1],
+     [Define to 1 if extending the stack slightly past the limit causes
+      a SIGSEGV which can be handled on an alternate stack established
+      with sigaltstack.])
+
+   AC_CACHE_CHECK([for precise C stack overflow detection],
      ac_cv_sys_xsi_stack_overflow_heuristic,
      [AC_TRY_RUN(
        [
@@ -29,6 +113,9 @@ AC_DEFUN([AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC],
         # include <sys/time.h>
         # include <sys/resource.h>
         #endif
+         #ifndef SIGSTKSZ
+         # define SIGSTKSZ 16384
+         #endif
 
         static union
         {
@@ -131,7 +218,8 @@ AC_DEFUN([AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC],
        with the X/Open System Interface (XSI) option
        and is a standardized way to implement a SEGV-based stack
        overflow detection heuristic.])
-   fi])
+   fi
+  fi])
 
 
 AC_DEFUN([gl_PREREQ_C_STACK],
-- 
1.6.0