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Date:   Sat, 13 Oct 2018 04:14:16 +0200
From:   Eugene Syromiatnikov <esyr@redhat.com>
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Yury Norov <ynorov@caviumnetworks.com>,
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Subject: Re: [PATCH v9 00/24] ILP32 for ARM64
Message-ID: <20181013021416.GE21972@asgard.redhat.com>
References: <20180516081910.10067-1-ynorov@caviumnetworks.com>
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 <20181010141017.GA2881@asgard.redhat.com>
 <20181010153655.GA212880@arrakis.emea.arm.com>
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On Wed, Oct 10, 2018 at 04:36:56PM +0100, Catalin Marinas wrote:
> On Wed, Oct 10, 2018 at 04:10:21PM +0200, Eugene Syromiatnikov wrote:
> > I have some questions regarding AArch64 ILP32 implementation for which I
> > failed to find an answer myself:
> >  * How ptrace() tracer is supposed to distinguish between ILP32 and LP64
> >    tracees? For MIPS N32 and x32 this is possible based on syscall
> >    number, but for AArch64 ILP32 I do not see such a sign. There's also
> >    ARM_ip is employed for signalling entering/exiting, I wonder whether
> >    it's possible to employ it also for signalling tracee's personality.
> 
> With the current implementation, I don't think you can distinguish. From
> the kernel perspective, the register set is the same. What is the
> use-case for this?

Err, a ptrace()-based tracer trying to trace a process, for example?

> We could add a new regset to expose the ILP32 state (NT_ARM_..., I can't
> think of a name now but probably not PER* as this implies PER_LINUX_...
> which is independent from TIF_32BIT_*).

So that would require an additional ptrace() call on each syscall stop,
is that correct?

> >  * What's the reasoning behind capping syscall arguments to 32 bit? x32
> >    and MIPS N32 do not have such a restriction (and do not need special
> >    wrappers for syscalls that pass 64-bit values as a result, except
> >    when they do,  as it is the case for preadv2 on x32); moreover, that
> >    would lead to insurmountable difficulties for AArch64 ILP32 tracers
> >    that try to trace LP64 tracees, as it would be impossible to pass
> >    64-bit addresses to process_vm_{read,write} or ptrace PEEK/POKE.
> 
> We've attempted in earlier versions to allow a mix of 32 and 64-bit
> register values from ILP32 but it got pretty complicated. The entry code
> would need to know which registers need zeroing of the top 32-bit

If kernel specifies 64-bit wide registers for syscalls, then it's the
caller's (libc's) responsibility to properly sign-extend arguments when
needed, and glibc, for example, already has proper type definitions that
aimed to handle this.

> and the generic unistd.h wrapper hacks were not very nice.

They are already implemented in glibc during x32 introduction period.

> Some past discussions:
> 
> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1211716.html
> 
> -- 
> Catalin