[Tarantool-patches] [PATCH luajit 0/2] Fix stack overflow in pcall/xpcall

[Tarantool-patches] [PATCH luajit 0/2] Fix stack overflow in pcall/xpcall

[Sergey Bronnikov via Tarantool-patches]

The proposed patches fixes stack overflow in pcall/xpcall.

Related issues:
- https://github.com/LuaJIT/LuaJIT/issues/1048
- https://github.com/tarantool/tarantool/issues/11691
Git branch: https://github.com/tarantool/luajit/tree/ligurio/gh-xxxx-fix-stack-checks-in-vararg-calls

Mike Pall (2):
  LJ_FR2: Fix stack checks in vararg calls.
  Add stack check to pcall/xpcall.

 src/lj_def.h                                  |  2 +-
 src/lj_dispatch.c                             |  2 +-
 src/vm_arm.dasc                               |  7 ++
 src/vm_arm64.dasc                             |  9 ++
 src/vm_mips.dasc                              | 10 ++-
 src/vm_mips64.dasc                            | 13 ++-
 src/vm_ppc.dasc                               |  9 ++
 src/vm_x64.dasc                               |  6 ++
 src/vm_x86.dasc                               |  6 ++
 ...048-fix-stack-checks-vararg-calls.test.lua | 85 +++++++++++++++++++
 10 files changed, 144 insertions(+), 5 deletions(-)
 create mode 100644 test/tarantool-tests/lj-1048-fix-stack-checks-vararg-calls.test.lua

-- 
2.43.0

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

[Sergey Bronnikov via Tarantool-patches]

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

[Tarantool-patches] [PATCH luajit 2/2] Add stack check to pcall/xpcall.

[Sergey Bronnikov via Tarantool-patches]

Analyzed by Peter Cawley.

(cherry picked from commit a4c1640432a9d8a60624cdc8065b15078c228e36)

In the commit "LJ_FR2: Fix stack checks in vararg calls."
stack overflow in `pcall()`/`xpcall()` was fixed partially and
there are still cases where stack overflow happens, see comments
in the test. The patch add stack check to `pcall()` and `xpcall()`.

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

Part of tarantool/tarantool#11691
---
 src/vm_arm.dasc                               |  7 +++++
 src/vm_arm64.dasc                             |  8 +++++
 src/vm_mips.dasc                              | 10 +++++-
 src/vm_mips64.dasc                            | 12 +++++--
 src/vm_ppc.dasc                               |  9 ++++++
 src/vm_x64.dasc                               |  6 ++++
 src/vm_x86.dasc                               |  6 ++++
 ...048-fix-stack-checks-vararg-calls.test.lua | 31 ++++++++++++++++++-
 8 files changed, 85 insertions(+), 4 deletions(-)

diff --git a/src/vm_arm.dasc b/src/vm_arm.dasc
index 7095e660..efe9dcb2 100644
--- a/src/vm_arm.dasc
+++ b/src/vm_arm.dasc
@@ -1201,8 +1201,11 @@ static void build_subroutines(BuildCtx *ctx)
   |//-- Base library: catch errors ----------------------------------------
   |
   |.ffunc pcall
+  |   ldr RB, L->maxstack
+  |   add INS, BASE, NARGS8:RC
   |  ldrb RA, [DISPATCH, #DISPATCH_GL(hookmask)]
   |   cmp NARGS8:RC, #8
+  |   cmphs RB, INS
   |   blo ->fff_fallback
   |  tst RA, #HOOK_ACTIVE		// Remember active hook before pcall.
   |   mov RB, BASE
@@ -1213,7 +1216,11 @@ static void build_subroutines(BuildCtx *ctx)
   |  b ->vm_call_dispatch
   |
   |.ffunc_2 xpcall
+  |   ldr RB, L->maxstack
+  |   add INS, BASE, NARGS8:RC
   |  ldrb RA, [DISPATCH, #DISPATCH_GL(hookmask)]
+  |   cmp RB, INS
+  |   blo ->fff_fallback
   |  checkfunc CARG4, ->fff_fallback	// Traceback must be a function.
   |   mov RB, BASE
   |  strd CARG12, [BASE, #8]		// Swap function and traceback.
diff --git a/src/vm_arm64.dasc b/src/vm_arm64.dasc
index cf8e575a..53ff7162 100644
--- a/src/vm_arm64.dasc
+++ b/src/vm_arm64.dasc
@@ -1166,6 +1166,10 @@ static void build_subroutines(BuildCtx *ctx)
   |//-- Base library: catch errors ----------------------------------------
   |
   |.ffunc pcall
+  |  ldr TMP1, L->maxstack
+  |  add TMP2, BASE, NARGS8:RC
+  |  cmp TMP1, TMP2
+  |  blo ->fff_fallback
   |   cmp NARGS8:RC, #8
   |  ldrb TMP0w, GL->hookmask
   |   blo ->fff_fallback
@@ -1185,6 +1189,10 @@ static void build_subroutines(BuildCtx *ctx)
   |  b ->vm_call_dispatch
   |
   |.ffunc xpcall
+  |  ldr TMP1, L->maxstack
+  |  add TMP2, BASE, NARGS8:RC
+  |  cmp TMP1, TMP2
+  |  blo ->fff_fallback
   |     ldp CARG1, CARG2, [BASE]
   |  ldrb TMP0w, GL->hookmask
   |   subs NARGS8:TMP1, NARGS8:RC, #16
diff --git a/src/vm_mips.dasc b/src/vm_mips.dasc
index 32caabf7..69d09d52 100644
--- a/src/vm_mips.dasc
+++ b/src/vm_mips.dasc
@@ -1382,9 +1382,13 @@ static void build_subroutines(BuildCtx *ctx)
   |//-- Base library: catch errors ----------------------------------------
   |
   |.ffunc pcall
+  |   lw TMP1, L->maxstack
+  |   addu TMP2, BASE, NARGS8:RC
   |  lbu TMP3, DISPATCH_GL(hookmask)(DISPATCH)
   |  beqz NARGS8:RC, ->fff_fallback
-  |   move TMP2, BASE
+  |.  sltu AT, TMP1, TMP2
+  |   bnez AT, ->fff_fallback
+  |.   move TMP2, BASE
   |   addiu BASE, BASE, 8
   |  // Remember active hook before pcall.
   |  srl TMP3, TMP3, HOOK_ACTIVE_SHIFT
@@ -1394,8 +1398,12 @@ static void build_subroutines(BuildCtx *ctx)
   |.  addiu NARGS8:RC, NARGS8:RC, -8
   |
   |.ffunc xpcall
+  |   lw TMP1, L->maxstack
+  |   addu TMP2, BASE, NARGS8:RC
   |    sltiu AT, NARGS8:RC, 16
   |  lw CARG4, 8+HI(BASE)
+  |   sltu TMP1, TMP1, TMP2
+  |    or AT, AT, TMP1
   |    bnez AT, ->fff_fallback
   |.  lw CARG3, 8+LO(BASE)
   |   lw CARG1, LO(BASE)
diff --git a/src/vm_mips64.dasc b/src/vm_mips64.dasc
index 7f49df5b..06b143a2 100644
--- a/src/vm_mips64.dasc
+++ b/src/vm_mips64.dasc
@@ -1418,8 +1418,12 @@ static void build_subroutines(BuildCtx *ctx)
   |//-- Base library: catch errors ----------------------------------------
   |
   |.ffunc pcall
+  |  ld TMP1, L->maxstack
+  |  daddu TMP2, BASE, NARGS8:RC
+  |  sltu AT, TMP1, TMP2
+  |  bnez AT, ->fff_fallback
+  |.  lbu TMP3, DISPATCH_GL(hookmask)(DISPATCH)
   |  daddiu NARGS8:RC, NARGS8:RC, -8
-  |  lbu TMP3, DISPATCH_GL(hookmask)(DISPATCH)
   |  bltz NARGS8:RC, ->fff_fallback
   |.   move TMP2, BASE
   |   daddiu BASE, BASE, 16
@@ -1440,8 +1444,12 @@ static void build_subroutines(BuildCtx *ctx)
   |.  nop
   |
   |.ffunc xpcall
+  |  ld TMP1, L->maxstack
+  |  daddu TMP2, BASE, NARGS8:RC
+  |  sltu AT, TMP1, TMP2
+  |  bnez AT, ->fff_fallback
+  |.  ld CARG1, 0(BASE)
   |  daddiu NARGS8:RC, NARGS8:RC, -16
-  |  ld CARG1, 0(BASE)
   |   ld CARG2, 8(BASE)
   |    bltz NARGS8:RC, ->fff_fallback
   |.    lbu TMP1, DISPATCH_GL(hookmask)(DISPATCH)
diff --git a/src/vm_ppc.dasc b/src/vm_ppc.dasc
index 980ad897..f2ea933b 100644
--- a/src/vm_ppc.dasc
+++ b/src/vm_ppc.dasc
@@ -1755,8 +1755,12 @@ static void build_subroutines(BuildCtx *ctx)
   |//-- Base library: catch errors ----------------------------------------
   |
   |.ffunc pcall
+  |    lwz TMP1, L->maxstack
+  |    add TMP2, BASE, NARGS8:RC
   |  cmplwi NARGS8:RC, 8
   |   lbz TMP3, DISPATCH_GL(hookmask)(DISPATCH)
+  |    cmplw cr1, TMP1, TMP2
+  |  cror 4*cr0+lt, 4*cr0+lt, 4*cr1+lt
   |  blt ->fff_fallback
   |   mr TMP2, BASE
   |   la BASE, 8(BASE)
@@ -1767,14 +1771,19 @@ static void build_subroutines(BuildCtx *ctx)
   |  b ->vm_call_dispatch
   |
   |.ffunc xpcall
+  |     lwz TMP1, L->maxstack
+  |     add TMP2, BASE, NARGS8:RC
   |  cmplwi NARGS8:RC, 16
   |   lwz CARG3, 8(BASE)
+  |     cmplw cr1, TMP1, TMP2
   |.if FPU
   |    lfd FARG2, 8(BASE)
+  |  cror 4*cr0+lt, 4*cr0+lt, 4*cr1+lt
   |    lfd FARG1, 0(BASE)
   |.else
   |    lwz CARG1, 0(BASE)
   |    lwz CARG2, 4(BASE)
+  |  cror 4*cr0+lt, 4*cr0+lt, 4*cr1+lt
   |    lwz CARG4, 12(BASE)
   |.endif
   |  blt ->fff_fallback
diff --git a/src/vm_x64.dasc b/src/vm_x64.dasc
index d5296759..141f5f82 100644
--- a/src/vm_x64.dasc
+++ b/src/vm_x64.dasc
@@ -1545,6 +1545,9 @@ static void build_subroutines(BuildCtx *ctx)
   |//-- Base library: catch errors ----------------------------------------
   |
   |.ffunc_1 pcall
+  |  mov L:RB, SAVE_L
+  |  lea RA, [BASE+NARGS:RD*8]
+  |  cmp RA, L:RB->maxstack; ja ->fff_fallback
   |  lea RA, [BASE+16]
   |  sub NARGS:RDd, 1
   |  mov PCd, 16+FRAME_PCALL
@@ -1563,6 +1566,9 @@ static void build_subroutines(BuildCtx *ctx)
   |  jmp ->vm_call_dispatch
   |
   |.ffunc_2 xpcall
+  |  mov L:RB, SAVE_L
+  |  lea RA, [BASE+NARGS:RD*8]
+  |  cmp RA, L:RB->maxstack; ja ->fff_fallback
   |  mov LFUNC:RA, [BASE+8]
   |  checktp_nc LFUNC:RA, LJ_TFUNC, ->fff_fallback
   |  mov LFUNC:RB, [BASE]		// Swap function and traceback.
diff --git a/src/vm_x86.dasc b/src/vm_x86.dasc
index b043b830..1ba5abce 100644
--- a/src/vm_x86.dasc
+++ b/src/vm_x86.dasc
@@ -1914,6 +1914,9 @@ static void build_subroutines(BuildCtx *ctx)
   |//-- Base library: catch errors ----------------------------------------
   |
   |.ffunc_1 pcall
+  |  mov L:RB, SAVE_L
+  |  lea RA, [BASE+NARGS:RD*8]
+  |  cmp RA, L:RB->maxstack; ja ->fff_fallback
   |  lea RA, [BASE+8]
   |  sub NARGS:RD, 1
   |  mov PC, 8+FRAME_PCALL
@@ -1925,6 +1928,9 @@ static void build_subroutines(BuildCtx *ctx)
   |  jmp ->vm_call_dispatch
   |
   |.ffunc_2 xpcall
+  |  mov L:RB, SAVE_L
+  |  lea RA, [BASE+NARGS:RD*8]
+  |  cmp RA, L:RB->maxstack; ja ->fff_fallback
   |  cmp dword [BASE+12], LJ_TFUNC;  jne ->fff_fallback
   |  mov RB, [BASE+4]			// Swap function and traceback.
   |  mov [BASE+12], RB
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
index e300d5c1..367aecb6 100644
--- 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
@@ -7,7 +7,7 @@ local test = tap.test('lj-1048-fix-stack-checks-vararg-calls'):skipcond({
   ['Test requires JIT enabled'] = not jit.status(),
 })
 
-test:plan(2)
+test:plan(4)
 
 -- The first testcase demonstrate a stack overflow in `pcall()`
 -- by recursive calling `pcall()`. The functions are vararg
@@ -53,4 +53,33 @@ pcall(coroutine.wrap(looper_2), 0)
 
 test:ok(true, 'no stack overflow with using metamethod')
 
+-- The third testcase demonstrate a stack overflow in
+-- `pcall()`/xpcall()` similar to the first testcase, but it is
+-- triggered using hand-crafted Lua chunk with a lot `pcall()`
+-- builtins.
+
+for i = 1, 100 do
+  local code = 'return pcall(' .. string.rep('pcall, ', i) .. 'pairs, {})'
+  local f = load(code)
+  coroutine.wrap(f)()
+end
+
+test:ok(true, 'no stack overflow with pcalls in load()')
+
+-- The fourth testcase demonstrate a stack overflow in
+-- `pcall()`/`xpcall()` similar to the first testcase, but it is
+-- triggered using `unpack()`.
+
+local t = {}
+local function f()
+  return pcall(unpack(t))
+end
+
+for i = 1, 100 do
+  t[i], t[i + 1], t[i + 2] = pcall, pairs, {}
+  coroutine.wrap(f)()
+end
+
+test:ok(true, 'no stack overflow with unpacked pcalls')
+
 test:done(true)
-- 
2.43.0

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

[Sergey Kaplun via Tarantool-patches]

Hi, Sergey!
Thanks for the patch!
Please consider my comments below.

On 27.08.25, Sergey Bronnikov wrote:
> 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.

This is unrelated to this patch, so it can be omitted.

> 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.

This issue is not related to this patch.

> Either of these points can cause an issue if `pcall()` is used as
> `__newindex`.

Looks like the metamethods are not required for issue reproducing.

>               The patch partially fixes aforementioned issues.

By how?

> 
> 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

<snipped>

> 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;

I can't see the test related to this change. Not `prober_1()` nor
`prober_2()` lead to the assertion failure for x86_64 or aarch64 without
it.

>      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

Please mention in the commit message why the original stack check was
incorrect (for aarch64 and mips64).

Also, mention why the x64 isn't affected:

x64:
| RA == BASE + (RD=NARGS+1)*8 + framesize * 8 +8 > maxstack
The last summand here is the `LJ_FR2` adjustment.

arm64|mips64 -- incorrect check:
| RA == BASE + (RD=NARGS)*8 + framesize * 8 >= maxstack


>      |   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

<snipped>

> 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

Minor: by one for the arm64, mips64 architectures.

> +-- patch.
> +local function prober_1(...) -- luacheck: no unused
> +  pcall(pcall, pcall, pcall, pcall, pcall, pcall, pcall, pcall, pairs, {})
> +end

Why do we want to use probber_1 here? Why is this different from the
second example? Only because of the metamethods?

If we want to keep it, please describe why we need at least 9 pcall-s.

Also, there is no need for `pairs()` here. Let's use another simpler fast
function (like `type()`). Also, please add a comment about fast function
usage, see the example below.

> +
> +local function looper_1(n, ...)
> +  prober_1(...)
> +  prober_1(nil, ...)

Why do we need `nil` here? I suppose this line is excess, see the
comment with the example below.

> +  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.

This comment is unrelated to this test.

>                                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)

This can be simplified to the following:
| src/luajit -e '
| -- Do not use a Lua function as metamethod -- since it will check
| -- the stack on each invocation. Use simple `type()` built-in
| -- instead.
| local t = setmetatable({}, {__newindex = pcall, __call = type})
| local function prober(...)
|     -- Invokes `pcall(t, t, t)`.
|     t[t] = t
| end
| local function looper(n, ...)
|     prober(...)
|     return looper(n+1, n, ...)
| end
| pcall(coroutine.wrap(looper), 0)
| '

> +
> +test:ok(true, 'no stack overflow with using metamethod')
> +
> +test:done(true)
> -- 
> 2.43.0
> 

-- 
Best regards,
Sergey Kaplun

Re: [Tarantool-patches] [PATCH luajit 2/2] Add stack check to pcall/xpcall.

[Sergey Kaplun via Tarantool-patches]

Hi, Sergey!
Thanks for the patch!
Please consider my comments below.

On 27.08.25, Sergey Bronnikov wrote:
> Analyzed by Peter Cawley.
> 
> (cherry picked from commit a4c1640432a9d8a60624cdc8065b15078c228e36)
> 
> In the commit "LJ_FR2: Fix stack checks in vararg calls."

Minor: In the previous commit ("...")

> stack overflow in `pcall()`/`xpcall()` was fixed partially and

This handles stack overflow for vararg functions and metamethod
invocations not `xpcall()/pcall()` (directly).

> there are still cases where stack overflow happens, see comments
> in the test. The patch add stack check to `pcall()` and `xpcall()`.

Please, mention in the commit message that the issue was fixed by adding
the stack check to these fast functions.

> 
> Sergey Bronnikov:
> * added the description and the test for the problem
> 
> Part of tarantool/tarantool#11691
> ---
>  src/vm_arm.dasc                               |  7 +++++
>  src/vm_arm64.dasc                             |  8 +++++
>  src/vm_mips.dasc                              | 10 +++++-
>  src/vm_mips64.dasc                            | 12 +++++--
>  src/vm_ppc.dasc                               |  9 ++++++
>  src/vm_x64.dasc                               |  6 ++++
>  src/vm_x86.dasc                               |  6 ++++
>  ...048-fix-stack-checks-vararg-calls.test.lua | 31 ++++++++++++++++++-
>  8 files changed, 85 insertions(+), 4 deletions(-)
> 
> diff --git a/src/vm_arm.dasc b/src/vm_arm.dasc
> index 7095e660..efe9dcb2 100644
> --- a/src/vm_arm.dasc
> +++ b/src/vm_arm.dasc
> @@ -1201,8 +1201,11 @@ static void build_subroutines(BuildCtx *ctx)
>    |//-- Base library: catch errors ----------------------------------------
>    |
>    |.ffunc pcall
> +  |   ldr RB, L->maxstack
> +  |   add INS, BASE, NARGS8:RC
>    |  ldrb RA, [DISPATCH, #DISPATCH_GL(hookmask)]
>    |   cmp NARGS8:RC, #8
> +  |   cmphs RB, INS
>    |   blo ->fff_fallback
>    |  tst RA, #HOOK_ACTIVE		// Remember active hook before pcall.
>    |   mov RB, BASE
> @@ -1213,7 +1216,11 @@ static void build_subroutines(BuildCtx *ctx)
>    |  b ->vm_call_dispatch
>    |
>    |.ffunc_2 xpcall
> +  |   ldr RB, L->maxstack
> +  |   add INS, BASE, NARGS8:RC
>    |  ldrb RA, [DISPATCH, #DISPATCH_GL(hookmask)]
> +  |   cmp RB, INS
> +  |   blo ->fff_fallback
>    |  checkfunc CARG4, ->fff_fallback	// Traceback must be a function.
>    |   mov RB, BASE
>    |  strd CARG12, [BASE, #8]		// Swap function and traceback.
> diff --git a/src/vm_arm64.dasc b/src/vm_arm64.dasc
> index cf8e575a..53ff7162 100644
> --- a/src/vm_arm64.dasc
> +++ b/src/vm_arm64.dasc
> @@ -1166,6 +1166,10 @@ static void build_subroutines(BuildCtx *ctx)
>    |//-- Base library: catch errors ----------------------------------------
>    |
>    |.ffunc pcall
> +  |  ldr TMP1, L->maxstack
> +  |  add TMP2, BASE, NARGS8:RC
> +  |  cmp TMP1, TMP2
> +  |  blo ->fff_fallback
>    |   cmp NARGS8:RC, #8
>    |  ldrb TMP0w, GL->hookmask
>    |   blo ->fff_fallback
> @@ -1185,6 +1189,10 @@ static void build_subroutines(BuildCtx *ctx)
>    |  b ->vm_call_dispatch
>    |
>    |.ffunc xpcall
> +  |  ldr TMP1, L->maxstack
> +  |  add TMP2, BASE, NARGS8:RC
> +  |  cmp TMP1, TMP2
> +  |  blo ->fff_fallback
>    |     ldp CARG1, CARG2, [BASE]
>    |  ldrb TMP0w, GL->hookmask
>    |   subs NARGS8:TMP1, NARGS8:RC, #16
> diff --git a/src/vm_mips.dasc b/src/vm_mips.dasc
> index 32caabf7..69d09d52 100644
> --- a/src/vm_mips.dasc
> +++ b/src/vm_mips.dasc
> @@ -1382,9 +1382,13 @@ static void build_subroutines(BuildCtx *ctx)
>    |//-- Base library: catch errors ----------------------------------------
>    |
>    |.ffunc pcall
> +  |   lw TMP1, L->maxstack
> +  |   addu TMP2, BASE, NARGS8:RC
>    |  lbu TMP3, DISPATCH_GL(hookmask)(DISPATCH)
>    |  beqz NARGS8:RC, ->fff_fallback
> -  |   move TMP2, BASE
> +  |.  sltu AT, TMP1, TMP2
> +  |   bnez AT, ->fff_fallback
> +  |.   move TMP2, BASE
>    |   addiu BASE, BASE, 8
>    |  // Remember active hook before pcall.
>    |  srl TMP3, TMP3, HOOK_ACTIVE_SHIFT
> @@ -1394,8 +1398,12 @@ static void build_subroutines(BuildCtx *ctx)
>    |.  addiu NARGS8:RC, NARGS8:RC, -8
>    |
>    |.ffunc xpcall
> +  |   lw TMP1, L->maxstack
> +  |   addu TMP2, BASE, NARGS8:RC
>    |    sltiu AT, NARGS8:RC, 16
>    |  lw CARG4, 8+HI(BASE)
> +  |   sltu TMP1, TMP1, TMP2
> +  |    or AT, AT, TMP1
>    |    bnez AT, ->fff_fallback
>    |.  lw CARG3, 8+LO(BASE)
>    |   lw CARG1, LO(BASE)
> diff --git a/src/vm_mips64.dasc b/src/vm_mips64.dasc
> index 7f49df5b..06b143a2 100644
> --- a/src/vm_mips64.dasc
> +++ b/src/vm_mips64.dasc
> @@ -1418,8 +1418,12 @@ static void build_subroutines(BuildCtx *ctx)
>    |//-- Base library: catch errors ----------------------------------------
>    |
>    |.ffunc pcall
> +  |  ld TMP1, L->maxstack
> +  |  daddu TMP2, BASE, NARGS8:RC
> +  |  sltu AT, TMP1, TMP2
> +  |  bnez AT, ->fff_fallback
> +  |.  lbu TMP3, DISPATCH_GL(hookmask)(DISPATCH)
>    |  daddiu NARGS8:RC, NARGS8:RC, -8
> -  |  lbu TMP3, DISPATCH_GL(hookmask)(DISPATCH)
>    |  bltz NARGS8:RC, ->fff_fallback
>    |.   move TMP2, BASE
>    |   daddiu BASE, BASE, 16

I see that the original patch in the upstream has another diff. Please
backport the commit ea7071d3 ("MIPS64: Fix xpcall() error case") first
(as the first commit in the patch series) to avoid conflicts in the
future.

> @@ -1440,8 +1444,12 @@ static void build_subroutines(BuildCtx *ctx)
>    |.  nop
>    |
>    |.ffunc xpcall
> +  |  ld TMP1, L->maxstack
> +  |  daddu TMP2, BASE, NARGS8:RC
> +  |  sltu AT, TMP1, TMP2
> +  |  bnez AT, ->fff_fallback
> +  |.  ld CARG1, 0(BASE)
>    |  daddiu NARGS8:RC, NARGS8:RC, -16
> -  |  ld CARG1, 0(BASE)
>    |   ld CARG2, 8(BASE)
>    |    bltz NARGS8:RC, ->fff_fallback
>    |.    lbu TMP1, DISPATCH_GL(hookmask)(DISPATCH)
> diff --git a/src/vm_ppc.dasc b/src/vm_ppc.dasc
> index 980ad897..f2ea933b 100644
> --- a/src/vm_ppc.dasc
> +++ b/src/vm_ppc.dasc
> @@ -1755,8 +1755,12 @@ static void build_subroutines(BuildCtx *ctx)
>    |//-- Base library: catch errors ----------------------------------------
>    |
>    |.ffunc pcall
> +  |    lwz TMP1, L->maxstack
> +  |    add TMP2, BASE, NARGS8:RC
>    |  cmplwi NARGS8:RC, 8
>    |   lbz TMP3, DISPATCH_GL(hookmask)(DISPATCH)
> +  |    cmplw cr1, TMP1, TMP2
> +  |  cror 4*cr0+lt, 4*cr0+lt, 4*cr1+lt
>    |  blt ->fff_fallback
>    |   mr TMP2, BASE
>    |   la BASE, 8(BASE)
> @@ -1767,14 +1771,19 @@ static void build_subroutines(BuildCtx *ctx)
>    |  b ->vm_call_dispatch
>    |
>    |.ffunc xpcall
> +  |     lwz TMP1, L->maxstack
> +  |     add TMP2, BASE, NARGS8:RC
>    |  cmplwi NARGS8:RC, 16
>    |   lwz CARG3, 8(BASE)
> +  |     cmplw cr1, TMP1, TMP2
>    |.if FPU
>    |    lfd FARG2, 8(BASE)
> +  |  cror 4*cr0+lt, 4*cr0+lt, 4*cr1+lt
>    |    lfd FARG1, 0(BASE)
>    |.else
>    |    lwz CARG1, 0(BASE)
>    |    lwz CARG2, 4(BASE)
> +  |  cror 4*cr0+lt, 4*cr0+lt, 4*cr1+lt
>    |    lwz CARG4, 12(BASE)
>    |.endif
>    |  blt ->fff_fallback
> diff --git a/src/vm_x64.dasc b/src/vm_x64.dasc
> index d5296759..141f5f82 100644
> --- a/src/vm_x64.dasc
> +++ b/src/vm_x64.dasc
> @@ -1545,6 +1545,9 @@ static void build_subroutines(BuildCtx *ctx)
>    |//-- Base library: catch errors ----------------------------------------
>    |
>    |.ffunc_1 pcall
> +  |  mov L:RB, SAVE_L
> +  |  lea RA, [BASE+NARGS:RD*8]
> +  |  cmp RA, L:RB->maxstack; ja ->fff_fallback
>    |  lea RA, [BASE+16]
>    |  sub NARGS:RDd, 1
>    |  mov PCd, 16+FRAME_PCALL
> @@ -1563,6 +1566,9 @@ static void build_subroutines(BuildCtx *ctx)
>    |  jmp ->vm_call_dispatch
>    |
>    |.ffunc_2 xpcall
> +  |  mov L:RB, SAVE_L
> +  |  lea RA, [BASE+NARGS:RD*8]
> +  |  cmp RA, L:RB->maxstack; ja ->fff_fallback
>    |  mov LFUNC:RA, [BASE+8]
>    |  checktp_nc LFUNC:RA, LJ_TFUNC, ->fff_fallback
>    |  mov LFUNC:RB, [BASE]		// Swap function and traceback.
> diff --git a/src/vm_x86.dasc b/src/vm_x86.dasc
> index b043b830..1ba5abce 100644
> --- a/src/vm_x86.dasc
> +++ b/src/vm_x86.dasc
> @@ -1914,6 +1914,9 @@ static void build_subroutines(BuildCtx *ctx)
>    |//-- Base library: catch errors ----------------------------------------
>    |
>    |.ffunc_1 pcall
> +  |  mov L:RB, SAVE_L
> +  |  lea RA, [BASE+NARGS:RD*8]
> +  |  cmp RA, L:RB->maxstack; ja ->fff_fallback
>    |  lea RA, [BASE+8]
>    |  sub NARGS:RD, 1
>    |  mov PC, 8+FRAME_PCALL
> @@ -1925,6 +1928,9 @@ static void build_subroutines(BuildCtx *ctx)
>    |  jmp ->vm_call_dispatch
>    |
>    |.ffunc_2 xpcall
> +  |  mov L:RB, SAVE_L
> +  |  lea RA, [BASE+NARGS:RD*8]
> +  |  cmp RA, L:RB->maxstack; ja ->fff_fallback
>    |  cmp dword [BASE+12], LJ_TFUNC;  jne ->fff_fallback
>    |  mov RB, [BASE+4]			// Swap function and traceback.
>    |  mov [BASE+12], RB
> 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
> index e300d5c1..367aecb6 100644
> --- 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
> @@ -7,7 +7,7 @@ local test = tap.test('lj-1048-fix-stack-checks-vararg-calls'):skipcond({
>    ['Test requires JIT enabled'] = not jit.status(),
>  })
>  
> -test:plan(2)
> +test:plan(4)
>  
>  -- The first testcase demonstrate a stack overflow in `pcall()`
>  -- by recursive calling `pcall()`. The functions are vararg
> @@ -53,4 +53,33 @@ pcall(coroutine.wrap(looper_2), 0)
>  
>  test:ok(true, 'no stack overflow with using metamethod')
>  

Why do you drop the original test case?

Expected behaviour:
| src/luajit -e 'local t = {setmetatable({},{__call=pcall})()} print(t[#t])'
| stack overflow
Actual behaviour -- dirty read detected by ASAN.

> +-- The third testcase demonstrate a stack overflow in
> +-- `pcall()`/xpcall()` similar to the first testcase, but it is
> +-- triggered using hand-crafted Lua chunk with a lot `pcall()`
> +-- builtins.
> +
> +for i = 1, 100 do
> +  local code = 'return pcall(' .. string.rep('pcall, ', i) .. 'pairs, {})'

Why do we need this test if it has the same semantics as the second
one?

> +  local f = load(code)
> +  coroutine.wrap(f)()
> +end
> +
> +test:ok(true, 'no stack overflow with pcalls in load()')
> +
> +-- The fourth testcase demonstrate a stack overflow in
> +-- `pcall()`/`xpcall()` similar to the first testcase, but it is
> +-- triggered using `unpack()`.
> +
> +local t = {}
> +local function f()
> +  return pcall(unpack(t))
> +end
> +
> +for i = 1, 100 do

This limit isn't enough for GC64 or non-GC64 mode.
| src/luajit -e '
| local t = {}
| local function f() return pcall(unpack(t)) end
| for i = 1, 100 do
|   t[i], t[i+1], t[i+2] = pcall, pairs, {}
|   coroutine.wrap(f)()
| end
| '

For the GC64 build it is necessary to set the limit as 180, (179 -- not
SegFault).

Please provide two different limits depending on the GC64 mode
configuration. Please, describe why the __exact__ limit is chosen for
the particular configuration.

> +  t[i], t[i + 1], t[i + 2] = pcall, pairs, {}
> +  coroutine.wrap(f)()
> +end
> +
> +test:ok(true, 'no stack overflow with unpacked pcalls')
> +
>  test:done(true)
> -- 
> 2.43.0
> 

-- 
Best regards,
Sergey Kaplun