Cygwin: Fix segfalt when too many command line args are specified.

Previously, the number of command line args was not checked for
cygwin process. Due to this, segmentation fault was caused if too
many command line args are specified.
https://cygwin.com/pipermail/cygwin/2023-August/254333.html

Since char *argv[argc + 1] is placed on the stack in dll_crt0_1(),
STATUS_STACK_OVERFLOW occurs if the stack does not have enough
space.

With this patch, char *argv[] is placed in heap instead of stack
and ARG_MAX is increased from 32000 to 2097152 which is default
value of Linux. The argument length is also compared with ARG_MAX
and spawnve() returns E2BIG if it is too long.

Reported-by: Ed Morton
Reviewed-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>

winsup/cygwin/dcrt0.cc

@@ -978,11 +978,8 @@ dll_crt0_1 (void *)
 	 a change to an element of argv[] it does not affect Cygwin's argv.
 	 Changing the the contents of what argv[n] points to will still
 	 affect Cygwin.  This is similar (but not exactly like) Linux. */
-      char *newargv[__argc + 1];
+      char **newargv = (char **) malloc ((__argc + 1) * sizeof (char *));
-      char **nav = newargv;
+      memcpy (newargv, __argv, (__argc + 1) * sizeof (char *));
-      char **oav = __argv;
-      while ((*nav++ = *oav++) != NULL)
-	continue;
       /* Handle any signals which may have arrived */
       sig_dispatch_pending (false);
       _my_tls.call_signal_handler ();

winsup/cygwin/local_includes/winsup.h

@@ -73,6 +73,10 @@ uint32_t cygwin_inet_addr (const char *cp);
    application provided path strings we handle. */
 #define NT_MAX_PATH 32768
 
+/* CYG_ARG_MAX is the maximum total length of command line args.
+   The value 2097152 is the default ARG_MAX value in Linux. */
+#define CYG_ARG_MAX 2097152
+
 /* This definition allows to define wide char strings using macros as
    parameters.  See the definition of __CONCAT in newlib's sys/cdefs.h
    and accompanying comment. */

winsup/cygwin/release/3.4.9

@@ -8,3 +8,6 @@ Bug Fixes
 - For the time being, disable creating special files using mknod/mkfifo
   on NFS.
   Addresses: https://cygwin.com/pipermail/cygwin/2023-August/254266.html
+
+- Fix segfault when too many command line args are specified.
+  Addresses: https://cygwin.com/pipermail/cygwin/2023-August/254333.html

winsup/cygwin/spawn.cc

@@ -351,8 +351,9 @@ child_info_spawn::worker (const char *prog_arg, const char *const *argv,
 	 We need to quote any argument that has whitespace or embedded "'s.  */
 
       int ac;
+      size_t arg_len = 0;
       for (ac = 0; argv[ac]; ac++)
-	/* nothing */;
+	arg_len += strlen (argv[ac]) + 1;
 
       int err;
       const char *ext;
@@ -521,6 +522,12 @@ child_info_spawn::worker (const char *prog_arg, const char *const *argv,
 	  __leave;
 	}
       set (chtype, real_path.iscygexec ());
+      if (iscygwin () && arg_len > (size_t) sysconf (_SC_ARG_MAX))
+	{
+	  set_errno (E2BIG);
+	  res = -1;
+	  __leave;
+	}
       __stdin = in__stdin;
       __stdout = in__stdout;
       record_children ();

winsup/cygwin/sysconf.cc

@@ -485,7 +485,7 @@ static struct
     };
 } sca[] =
 {
-  {cons, {c:ARG_MAX}},			/*   0, _SC_ARG_MAX */
+  {cons, {c:CYG_ARG_MAX}},		/*   0, _SC_ARG_MAX */
   {cons, {c:CHILD_MAX}},		/*   1, _SC_CHILD_MAX */
   {cons, {c:CLOCKS_PER_SEC}},		/*   2, _SC_CLK_TCK */
   {cons, {c:NGROUPS_MAX}},		/*   3, _SC_NGROUPS_MAX */