Re: [PATCH 2/2 v2] ntdll: Zero debug registers after thread creation

[Michael Müller]

Am 07.04.2017 um 17:52 schrieb Christian Inci:
>  DWORD tid = 0;
>  int request_pipe[2];
> -
>
> CONTEXT ctx;
> NTSTATUS status;
>
> [...]
> -
>
> ctx.ContextFlags = CONTEXT_DEBUG_REGISTERS;
> -
>
> NtSetContextThread(handle, &ctx);
> -

You didn't initialize the register values and therefore set the debug
registers to random stack values. You should either initialize them
manually or memset the whole struct.

[PATCH 2/2 v2] ntdll: Zero debug registers after thread creation

[Christian Inci]

Thanks, I weren't sure about that one. I thought that every recent compiler will zero-initialize local variables/structures.
Are there compilers or some special cases which wouldn't let that happen?
Are they often enough not explicit set to zero (by using non-previously-used memory for the first time), so it'll be assumed to be zero?

On 04/07/2017 06:43 PM, Michael Müller wrote:
> Am 07.04.2017 um 17:52 schrieb Christian Inci:
> >  DWORD tid = 0;
> >  int request_pipe[2];
> > -
> >
> > CONTEXT ctx;
> > NTSTATUS status;
> >
> > [...]
> > -
> >
> > ctx.ContextFlags = CONTEXT_DEBUG_REGISTERS;
> > -
> >
> > NtSetContextThread(handle, &ctx);
> > -
>
> You didn't initialize the register values and therefore set the debug
> registers to random stack values. You should either initialize them
> manually or memset the whole struct.

[PATCH 2/2 v2] ntdll: Zero debug registers after thread creation

[Matteo Bruni]

2017-04-07 19:22 GMT+02:00 Christian Inci :
> Thanks, I weren't sure about that one. I thought that every recent compiler will zero-initialize local variables/structures.
> Are there compilers or some special cases which wouldn't let that happen?
> Are they often enough not explicit set to zero (by using non-previously-used memory for the first time), so it'll be assumed to be zero?

Huh? No, automatic variables aren't implicitly initialized in C and I
wouldn't want a compiler that goes out of its way to initialize them,
slowing down the code for everyone.

[PATCH 2/2 v2] ntdll: Zero debug registers after thread creation

[Christian Inci]

Thanks. I guess that I should quit coding altogether.

On 04/09/2017 07:53 PM, Matteo Bruni wrote:
> 2017-04-07 19:22 GMT+02:00 Christian Inci :
> > Thanks, I weren't sure about that one. I thought that every recent compiler will zero-initialize local variables/structures.
> > Are there compilers or some special cases which wouldn't let that happen?
> > Are they often enough not explicit set to zero (by using non-previously-used memory for the first time), so it'll be assumed to be zero?
>
> Huh? No, automatic variables aren't implicitly initialized in C and I
> wouldn't want a compiler that goes out of its way to initialize them,
> slowing down the code for everyone.

[PATCH 2/2 v2] ntdll: Zero debug registers after thread creation

[Matteo Bruni]

2017-04-09 20:15 GMT+02:00 Christian Inci :
> Thanks. I guess that I should quit coding altogether.

That's a bit of an overreaction...

BTW, thank you for debugging and finding a fix for the bug!