Re: [PATCH 1/2] x86/arch_prctl: add ARCH_SET_{COMPAT,NATIVE} to change compatible mode

[Dmitry Safonov]

On 04/08/2016 06:56 PM, Andy Lutomirski wrote:
> On Fri, Apr 8, 2016 at 6:50 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> wrote:
> > Hello again,
> > what do you think about attached patch?
> > I think it should fix landing problem for i386 vdso mremap.
> > It does not touch fast syscall path, so there should be no
> > speed regression.
> For this thing:
>
> +    /* Fixing userspace landing - look at do_fast_syscall_32 */
> +    if (current_thread_info()->status & TS_COMPAT)
> +        regs->ip = (unsigned long)current->mm->context.vdso +
> +            vdso_image_32.sym_int80_landing_pad;
>
> Either check that ip was where you expected it
And if it's not there - return error?
>   or simply remove this
> code -- user programs that are mremapping the vdso are already playing
> with fire and can just use int $0x80 to do it.
>
> Other than that, it looks generally sane.  The .mremap hook didn't
> exist last time I looked at this :)
>
> The main downside of your approach is that it doesn't allow switching
> between the 32-bit, 64-bit, and x32 images.  Also, it requires
> awareness of how vvar and vdso line up, whereas a dedicated API could
> do the whole thing.
Yes, I'm working on it. This patch will only allow moving vdso
image with general mremap - so I could use arch_prctl for
that API, as for native i386 one may move vdso with mremap
and cannot map any other vdso blobs.
Does it sound fine?

So, I have some difficulties with removing TIF_IA32 flag:
it's checked by perf for interpreting stack frames/instructions
and may be checked out of syscall executing (when tracing
page fault events, for example). I doubt, is it sane to remove
TS_COMPAT instead, leaving TIF_IA32, as for some cases
we need to know if task is compatible outside of syscall's path?
And the comment in asm/syscall.h says:
>  * TIF_IA32 tasks should always have TS_COMPAT set at
>  * system call time.
that means, that TS_COMPAT is always set on TIF_IA32, so
is meaningless.
What do you think?

Thanks,
Dmitry.