Re: [PATCH v2] Cygwin: spawn: Fix segfalt when too many command line args are specified.

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

On Aug 28 18:46, Takashi Yano wrote:
> 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, the total length of the arguments and the size of
> argv[] is restricted to 1/4 of total stack size for the process, and
> spawnve() returns E2BIG if the size exceeds the limit.
> [...]
> +static size_t
> +get_stack_size (const WCHAR *filename)
> +{
> +  HANDLE h;
> +  h = CreateFileW (filename, GENERIC_READ, FILE_SHARE_READ,
> +		   NULL, OPEN_EXISTING, 0, NULL);
> +  char buf[1024];
> +  DWORD n;
> +  ReadFile (h, buf, sizeof (buf), &n, 0);
> +  CloseHandle (h);
> +  IMAGE_NT_HEADERS32 *p = (IMAGE_NT_HEADERS32 *) memmem (buf, n, "PE\0\0", 4);
> +  if (!p)
> +    return 0;
> +  if ((char *) &p->OptionalHeader.SizeOfStackCommit > buf + n)
> +    return 0; /* buf[] is not enough */
> +  if (p->OptionalHeader.Magic == IMAGE_NT_OPTIONAL_HDR32_MAGIC)
> +    return p->OptionalHeader.SizeOfStackReserve;
> +  IMAGE_NT_HEADERS64 *p64 = (IMAGE_NT_HEADERS64 *) p;
> +  if ((char *) &p64->OptionalHeader.SizeOfStackCommit > buf + n)
> +    return 0; /* buf[] is not enough */
> +  return p64->OptionalHeader.SizeOfStackReserve;
> +}

Sorry, but this proposal is too complicated, IMHO.

Checking the stacksize in the file header for each single execve
is quite a bit time consuming, isn't it?

The question is rather, why storing argv on the stack at all?  I guess
the original idea was that argv is always a rather overseeable number.
But it doesn't have to stay on the stack.

I tried this simple patch:

diff --git a/winsup/cygwin/dcrt0.cc b/winsup/cygwin/dcrt0.cc
index 49b7a44aeb15..961dea4ab993 100644
--- a/winsup/cygwin/dcrt0.cc
+++ b/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 **nav = newargv;
-      char **oav = __argv;
-      while ((*nav++ = *oav++) != NULL)
-	continue;
+      char **newargv = (char **) malloc ((__argc + 1) * sizeof (char **));
+      memcpy (newargv, __argv, (__argc + 1) * sizeof (char **));
       /* Handle any signals which may have arrived */
       sig_dispatch_pending (false);
       _my_tls.call_signal_handler ();

and the testcase `LC_ALL=C sed 's/x/y/' $(seq 1000000)' simply ran
as desired.  Combined with a bit of error checking...

> diff --git a/winsup/cygwin/sysconf.cc b/winsup/cygwin/sysconf.cc
> index 2db92e4de..6cb2aecd0 100644
> --- a/winsup/cygwin/sysconf.cc
> +++ b/winsup/cygwin/sysconf.cc
> @@ -21,6 +21,13 @@ details. */
>  #include "cpuid.h"
>  #include "clock.h"
>  
> +#define DEFAULT_STACKGUARD (wincap.def_guard_page_size() + wincap.page_size ())
> +static long
> +get_arg_max (int in)
> +{
> +  return (long) (get_rlimit_stack () + DEFAULT_STACKGUARD) / 4;
> +}
> +
>  static long
>  get_page_size (int in)
>  {
> @@ -485,7 +492,7 @@ static struct
>      };
>  } sca[] =
>  {
> -  {cons, {c:ARG_MAX}},			/*   0, _SC_ARG_MAX */
> +  {func, {f:get_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 */
> -- 
> 2.39.0

Along these lines, there's no reason to couple SC_ARG_MAX to the
size of the stack.  I'd propose to return the value 2097152.  It's
the default value returned by getconf ARG_MAX on LInx as well.

Corinna