Fix lua stack overflow on recursive call to gsub()

The `zfs program` subcommand invokes a LUA interpreter to run ZFS
"channel programs".  This interpreter runs in a constrained environment,
with defined memory limits.  The LUA stack (used for LUA functions that
call each other) is allocated in the kernel's heap, and is limited by
the `-m MEMORY-LIMIT` flag and the `zfs_lua_max_memlimit` module
parameter.  The C stack is used by certain LUA features that are
implemented in C.  The C stack is limited by `LUAI_MAXCCALLS=20`, which
limits call depth.

Some LUA C calls use more stack space than others, and `gsub()` uses an
unusually large amount.  With a programming trick, it can be invoked
recursively using the C stack (rather than the LUA stack).  This
overflows the 16KB Linux kernel stack after about 11 iterations, less
than the limit of 20.

One solution would be to decrease `LUAI_MAXCCALLS`.  This could be made
to work, but it has a few drawbacks:

1. The existing test suite does not pass with `LUAI_MAXCCALLS=10`.

2. There may be other LUA functions that use a lot of stack space, and
the stack space may change depending on compiler version and options.

This commit addresses the problem by adding a new limit on the amount of
free space (in bytes) remaining on the C stack while running the LUA
interpreter: `LUAI_MINCSTACK=4096`.  If there is less than this amount
of stack space remaining, a LUA runtime error is generated.

Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Allan Jude <allanjude@freebsd.org>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10611 
Closes #10613

module/lua/ldo.c

@@ -29,6 +29,24 @@
 
 
 
+/* Return the number of bytes available on the stack. */
+#if defined (_KERNEL) && defined(__linux__)
+#include <asm/current.h>
+static intptr_t stack_remaining(void) {
+  char local;
+  return (intptr_t)(&local - (char *)current->stack);
+}
+#elif defined (_KERNEL) && defined(__FreeBSD__)
+#include <sys/pcpu.h>
+static intptr_t stack_remaining(void) {
+  char local;
+  return (intptr_t)(&local - (char *)curthread->td_kstack);
+}
+#else
+static intptr_t stack_remaining(void) {
+  return INTPTR_MAX;
+}
+#endif
 
 /*
 ** {======================================================
@@ -445,8 +463,13 @@ void luaD_call (lua_State *L, StkId func, int nResults, int allowyield) {
     if (L->nCcalls == LUAI_MAXCCALLS)
       luaG_runerror(L, "C stack overflow");
     else if (L->nCcalls >= (LUAI_MAXCCALLS + (LUAI_MAXCCALLS>>3)))
-      luaD_throw(L, LUA_ERRERR);  /* error while handing stack error */
+      luaD_throw(L, LUA_ERRERR);  /* error while handling stack error */
   }
+  intptr_t remaining = stack_remaining();
+  if (L->runerror == 0 && remaining < LUAI_MINCSTACK)
+    luaG_runerror(L, "C stack overflow");
+  if (L->runerror != 0 && remaining < LUAI_MINCSTACK / 2)
+    luaD_throw(L, LUA_ERRERR);  /* error while handling stack error */
   if (!allowyield) L->nny++;
   if (!luaD_precall(L, func, nResults))  /* is a Lua function? */
     luaV_execute(L);  /* call it */