__jit_debug_register_code was not invoked in debug build

[Mingwei Zhang]

Dear V8 developer,

I recently tried to use the debugging interface for gdb on JIT code. When I tried building a debug build of V8 and following the instructions in:

https://github.com/v8/v8/wiki/GDB-JIT-Interface

I launched my program using the following command:

gdb --args ./d8 --gdbjit helloworld.js

In the gdb prompt, I setup a breakpoint in __jit_debug_register_code before launch my program, but looks like the hook was never invoked.

I am using gdb 7.11 in Ubuntu 16.04.

d8 version 6.7.0 (candidate)

My helloworld.js is very simple like this:

function func() {

        console.log('hello');

}

func()

Thanks.

[Ben Noordhuis]

You don't have to set a breakpoint in __jit_debug_register_code, gdb
does that for you. It's a magic method that gdb recognizes, see
https://sourceware.org/gdb/onlinedocs/gdb/Declarations.html for
details.

That said, the gdbjit code is unmaintained - or at least not regularly
tested - and quite possibly broken. YMMV.

[Mingwei Zhang]

Hi Ben,

The problem is that I don't get any breakpoint hit notification in gdb. The program just execute and exited normally, regardless of whether i set the breakpoint or not in __jit_debug_register_code.

I double checked in instructions that when I am using gm.py with target x64.debug, it does use the option ENABLE_GDB_JIT_INTERFACE.

So, then I am very confused why I don't get breakpoint hit in gdb.

Appreciate your help.

[Ben Noordhuis]

On Tue, Jun 19, 2018 at 4:11 AM, Mingwei Zhang <mingwa...@gmail.com> wrote:
> Hi Ben,
>
> The problem is that I don't get any breakpoint hit notification in gdb. The
> program just execute and exited normally, regardless of whether i set the
> breakpoint or not in __jit_debug_register_code.
>
> I double checked in instructions that when I am using gm.py with target
> x64.debug, it does use the option ENABLE_GDB_JIT_INTERFACE.
>
> So, then I am very confused why I don't get breakpoint hit in gdb.
>
> Appreciate your help.

Do things go better when you also pass --gdbjit_full to d8? If not,
I'd say it's safe to assume the code has bitrotted (but fix-up CLs
undoubtedly welcome in that case.)

[Mingwei]

No. it does not. Neither --gdbjit or --gdbjit_full works on d8.

One thing I noticed is that in this version of d8, I am not seeing any
RWX code pages. Obviously, V8 enforces W^X policy on JIT code pages.
But I am not sure if this is the reason for that.

Let me know. Thanks.

best

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--
Mingwei Zhang