[Tarantool-patches] [PATCH luajit 1/2] LJ_FR2: Fix stack checks in vararg calls.

[Sergey Bronnikov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Thanks to Peter Cawley.

(cherry picked from commit d1a2fef8a8f53b0055ee041f7f63d83a27444ffa)

The builtin `pcall()` has two separate ways by which it can
grow the stack by one slot:

1. Resolving the `__call` metamethod of its first argument.
2. Growing the stack by one slot in LJ_FR2 mode.

The first case leads to a stack smash if `pcall()` is used as
`__call`. Setting a metatable with this metamethod will cause
an infinite loop which fills up the stack with `pcall`-frames
and then keeps going beyond the end of the stack until it segfaults.
Either of these points can cause an issue if `pcall()` is used as
`__newindex`. The patch partially fixes aforementioned issues.

Sergey Bronnikov:
* added the description and the test for the problem

Part of tarantool/tarantool#11691
---
 src/lj_def.h                                  |  2 +-
 src/lj_dispatch.c                             |  2 +-
 src/vm_arm64.dasc                             |  1 +
 src/vm_mips64.dasc                            |  1 +
 ...048-fix-stack-checks-vararg-calls.test.lua | 56 +++++++++++++++++++
 5 files changed, 60 insertions(+), 2 deletions(-)
 create mode 100644 test/tarantool-tests/lj-1048-fix-stack-checks-vararg-calls.test.lua

diff --git a/src/lj_def.h b/src/lj_def.h
index a5bca6b0..7e4f251e 100644
--- a/src/lj_def.h
+++ b/src/lj_def.h
@@ -69,7 +69,7 @@ typedef unsigned int uintptr_t;
 #define LJ_MAX_UPVAL	60		/* Max. # of upvalues. */
 
 #define LJ_MAX_IDXCHAIN	100		/* __index/__newindex chain limit. */
-#define LJ_STACK_EXTRA	(5+2*LJ_FR2)	/* Extra stack space (metamethods). */
+#define LJ_STACK_EXTRA	(5+3*LJ_FR2)	/* Extra stack space (metamethods). */
 
 #define LJ_NUM_CBPAGE	1		/* Number of FFI callback pages. */
 
diff --git a/src/lj_dispatch.c b/src/lj_dispatch.c
index a44a5adf..431cb3c2 100644
--- a/src/lj_dispatch.c
+++ b/src/lj_dispatch.c
@@ -453,7 +453,7 @@ static int call_init(lua_State *L, GCfunc *fn)
     int numparams = pt->numparams;
     int gotparams = (int)(L->top - L->base);
     int need = pt->framesize;
-    if ((pt->flags & PROTO_VARARG)) need += 1+gotparams;
+    if ((pt->flags & PROTO_VARARG)) need += 1+LJ_FR2+gotparams;
     lj_state_checkstack(L, (MSize)need);
     numparams -= gotparams;
     return numparams >= 0 ? numparams : 0;
diff --git a/src/vm_arm64.dasc b/src/vm_arm64.dasc
index c5f0a7a7..cf8e575a 100644
--- a/src/vm_arm64.dasc
+++ b/src/vm_arm64.dasc
@@ -3779,6 +3779,7 @@ static void build_ins(BuildCtx *ctx, BCOp op, int defop)
     |   add TMP2, BASE, RC
     |   add LFUNC:CARG3, CARG3, TMP0, lsl #47
     |  add RA, RA, RC
+    |  sub CARG1, CARG1, #8
     |   add TMP0, RC, #16+FRAME_VARG
     |   str LFUNC:CARG3, [TMP2], #8	// Store (tagged) copy of LFUNC.
     |    ldr KBASE, [PC, #-4+PC2PROTO(k)]
diff --git a/src/vm_mips64.dasc b/src/vm_mips64.dasc
index 44fba36c..7f49df5b 100644
--- a/src/vm_mips64.dasc
+++ b/src/vm_mips64.dasc
@@ -5267,6 +5267,7 @@ static void build_ins(BuildCtx *ctx, BCOp op, int defop)
     |   settp LFUNC:RB, TMP0
     |  daddu TMP0, RA, RC
     |   sd LFUNC:RB, 0(TMP1)		// Store (tagged) copy of LFUNC.
+    |  daddiu TMP2, TMP2, -8
     |   daddiu TMP3, RC, 16+FRAME_VARG
     |  sltu AT, TMP0, TMP2
     |    ld KBASE, -4+PC2PROTO(k)(PC)
diff --git a/test/tarantool-tests/lj-1048-fix-stack-checks-vararg-calls.test.lua b/test/tarantool-tests/lj-1048-fix-stack-checks-vararg-calls.test.lua
new file mode 100644
index 00000000..e300d5c1
--- /dev/null
+++ b/test/tarantool-tests/lj-1048-fix-stack-checks-vararg-calls.test.lua
@@ -0,0 +1,56 @@
+local tap = require('tap')
+
+-- A test file to demonstrate a stack overflow in `pcall()` in
+-- some cases, see below testcase descriptions.
+-- See also https://github.com/LuaJIT/LuaJIT/issues/1048.
+local test = tap.test('lj-1048-fix-stack-checks-vararg-calls'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+test:plan(2)
+
+-- The first testcase demonstrate a stack overflow in `pcall()`
+-- by recursive calling `pcall()`. The functions are vararg
+-- because stack check in BC_IFUNCV is off by one without the
+-- patch.
+local function prober_1(...) -- luacheck: no unused
+  pcall(pcall, pcall, pcall, pcall, pcall, pcall, pcall, pcall, pairs, {})
+end
+
+local function looper_1(n, ...)
+  prober_1(...)
+  prober_1(nil, ...)
+  return looper_1(n + 1, n, ...)
+end
+
+pcall(coroutine.wrap(looper_1), 0)
+
+test:ok(true, 'no stack overflow with recursive pcall')
+
+-- The second testcase demonstrate a stack overflow in `pcall()`
+-- with using metamethods. A stack overflow is triggered when
+-- `pcall()` is used as `__call` metamethod, setting metatable
+-- will cause an infinite loop which fills up the stack with
+-- `pcall`-frames and then keeps going beyond the end of the
+-- stack until it segfaults. Also, a stack overflow can be
+-- triggered when `pcall()` is used as `__newindex` metamethod.
+-- The functions are vararg because stack check in BC_IFUNCV is
+-- off by one without the patch.
+
+local mt = setmetatable({}, { __newindex = pcall, __call = pairs })
+
+local function prober_2(...) -- luacheck: no unused
+  mt[mt] = mt
+end
+
+local function looper_2(n, ...)
+  prober_2(...)
+  prober_2(nil, ...)
+  return looper_2(n + 1, n, ...)
+end
+
+pcall(coroutine.wrap(looper_2), 0)
+
+test:ok(true, 'no stack overflow with using metamethod')
+
+test:done(true)
-- 
2.43.0