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 <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Allan Jude <[email protected]>
Reviewed-by: Serapheim Dimitropoulos <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #10611 
Closes #10613 

1 files changed, 1 insertions, 0 deletions

diff --git a/module/lua/lstate.h b/module/lua/lstate.h
index 22e575e9a..b636396a6 100644
--- a/module/lua/lstate.h
+++ b/module/lua/lstate.h
@@ -166,6 +166,7 @@ struct lua_State {
   unsigned short nCcalls;  /* number of nested C calls */
   lu_byte hookmask;
   lu_byte allowhook;
+  lu_byte runerror; /* handling a runtime error */
   int basehookcount;
   int hookcount;
   lua_Hook hook;