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Date:   Sat, 13 Oct 2018 04:07:31 +0200
From:   Eugene Syromiatnikov <esyr@redhat.com>
To:     Szabolcs Nagy <szabolcs.nagy@arm.com>
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Subject: Re: [PATCH v9 00/24] ILP32 for ARM64
Message-ID: <20181013020731.GD21972@asgard.redhat.com>
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On Wed, Oct 10, 2018 at 03:39:07PM +0100, Szabolcs Nagy wrote:
> On 10/10/18 15:10, Eugene Syromiatnikov wrote:
> >  * 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.
> 
> but that's necessarily the case for all ilp32 abis:
> the userspace syscall function receives 32bit
> arguments so even if the kernel abi takes 64bit
> args you cannot use that from c code. (the libc
> does not even know which args should be sign or
> zero extended.)

glibc's syscall() prototype has kernel_ulong_t as its arguments (more
specifically, to __syscall_ulong_t, which is 64-bit wide on x32; it
should also have kernel_long_t as its return type instead of long,
but that's another story), so it works perfectly fine in case of x32.

> process_vm_readv/writev is limited by the ilp32
> iovec struct, not by the syscall arguments.

Right, on x32/N32 this issue is worked around by the usage of the respective
x86_64/N64 call, and it looks like another thing that is impossible
with AArch64 ilp32.

> ptrace is specified to take void* addr argument,
> and void* is 32bit on all ilp32 targets.
> so again on the c language level there is no
> way around the 32bit limitation.

Which is an issue.