Re: [Tarantool-patches] [PATCH luajit 4/4] ARM64: Fix xpcall() error case (really).

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

Hi!

Thanks for the patch!

Just some updates to the message, LGTM.

Sergos

> On 24 May 2021, at 16:27, Sergey Kaplun <skaplun@tarantool.org> wrote:
> 
> From: Mike Pall <mike>
> 
> Thanks to François Perrad and Stefan Pejic.
> 
> (cherry picked from commit d417ded17945b4211608d497d50b509e0274f5e0)
> 
> Premature decrementing VM's RC register before switch to fff_fallback
	    decrement of

> handler during processing `xpcall()` fast function leads to incorrect
> stack layout (not enough arguments on stack), when `xpcall()` calls
> without a second argument or if it is not a function (see <301-basic.t>
> test in lua-Harness test suite). While further error processing it leads
> to incorrect error message, due to stack inconsistency.

Mention this test verifies the patch behavior.

> 
> This patch stores intermediate result into TMP1 register (it does not
> determine fallback's behaviour and there is no way to return from
> fallback back to xpcall processing with spoiled TMP1) and moves RC
> setting after possible switching to fallback handler.
		the switch            the

> 
> Sergey Kaplun:
> * added the description for the problem
> 
> Resolves tarantool/tarantool#6093
> Part of tarantool/tarantool#5629
> ---
> src/vm_arm64.dasc | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/src/vm_arm64.dasc b/src/vm_arm64.dasc
> index e16a77ab..6e298255 100644
> --- a/src/vm_arm64.dasc
> +++ b/src/vm_arm64.dasc
> @@ -1183,7 +1183,7 @@ static void build_subroutines(BuildCtx *ctx)
>   |.ffunc xpcall
>   |     ldp CARG1, CARG2, [BASE]
>   |  ldrb TMP0w, GL->hookmask
> -  |   subs NARGS8:RC, NARGS8:RC, #16
> +  |   subs NARGS8:TMP1, NARGS8:RC, #16
>   |   blo ->fff_fallback
>   |    mov RB, BASE
>   |     asr ITYPE, CARG2, #47
> @@ -1191,6 +1191,7 @@ static void build_subroutines(BuildCtx *ctx)
>   |     cmn ITYPE, #-LJ_TFUNC
>   |  add PC, TMP0, #24+FRAME_PCALL
>   |     bne ->fff_fallback		// Traceback must be a function.
> +  |   mov NARGS8:RC, NARGS8:TMP1
>   |    add BASE, BASE, #24
>   |     stp CARG2, CARG1, [RB]		// Swap function and traceback.
>   |   cbz NARGS8:RC, ->vm_call_dispatch
> -- 
> 2.31.0
>