On Sat, Oct 13, 2018 at 4:07 AM Eugene Syromiatnikov [off-list ref] wrote:

On Wed, Oct 10, 2018 at 03:39:07PM +0100, Szabolcs Nagy wrote:

quoted

On 10/10/18 15:10, Eugene Syromiatnikov wrote:

quoted

 * 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.

quoted

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.

quoted

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.

I have no idea why you think this is a problem specific to aarch64-ilp32:
If we want to be able to debug 64-bit tasks from a 32-bit task on any
architecture that has compat mode, we should solve it once and
extend the ptrace interface to allow it on *all* of them. We certainly
don't need /more/ special cases for the x32 hack, there should really
be fewer of them.

      Arnd