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Date:   Sat, 13 Oct 2018 10:34:11 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Eugene Syromiatnikov <esyr@redhat.com>
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Subject: Re: [PATCH v9 00/24] ILP32 for ARM64
Message-ID: <20181013093411.o3id6yzkspsxr5jt@mbp>
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 <20181010141017.GA2881@asgard.redhat.com>
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 <20181013021416.GE21972@asgard.redhat.com>
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On Sat, Oct 13, 2018 at 04:14:16AM +0200, Eugene Syromiatnikov wrote:
> 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?

I first thought it wouldn't matter for ptrace-based tracers since the
syscall numbers are (mostly) the same. But the arguments layout in
register is indeed different, so I see your point now about having to
distinguish.

> > 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?

The ILP32 state does not change at run-time, so it could only do a
ptrace() call once and save the information. No need to re-read it on
each syscall stop.

We could set a high bit in the syscall number reported to the ptrace
caller (though not changing the syscall ABI) but I haven't thought of
other consequences. For example, can the ptrace caller change the
syscall number?

> > >  * 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.

We tried, see my other reply.

-- 
Catalin