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

[Dmitry Safonov <dsafonov@virtuozzo.com>]

On 04/08/2016 11:44 PM, Andy Lutomirski wrote:
> Feel free to ask for help on some of these details.  user_64bit_mode
> will be helpful too.
Hello again,

here are some questions on  TIF_IA32 removal:
- in function intel_pmu_pebs_fixup_ip: there is need to
know if process was it native/compat mode for instruction
interpreter for IP + one instruction fixup. There are
registers, but they are from PEBS, which does not contain
segment descriptors (even for PEBSv3). Other values
are from interrupt regs (look at setup_pebs_sample_data).
So, I guess, we may use user_64bit_mode on interrupt
register set, which will be racy with changing task's mode,
but quite ok?
- the same with LBR branching: I may got cs value for
user_64bit_mode or all registers set from intel_pmu_handle_irq
and pass it through intel_pmu_lbr_read => intel_pmu_lbr_filter
to branch_type for instruction decoder, which may
missinterpret opcode for the same racy-mode-switching app.
Is it also fine?
- for coredumping/ptracing, I will change test_thread_flag(TIF_IA32)
by user_64bit_mode(task_pt_regs()) - that looks/should be simple.
It's also valid as at the moment of coredump or of
PTRACE_GETREGSET task isn't running.
- I do not know what to do with uprobes - as you noted,
the way it cheks ia32_compat is buggy even now: task that
switches CS to __USER32_CS or back to __USER_CS will have
lousy inserted uprobe in mm.
So, how do we know on insert-time, with which descriptor
will be program on uprobed code?
- for MPX, I guess, tracking which syscall called
mpx_enable_management will work, at least it may be
documented, that before switching, one need to disable mpx.
- perf_reg_abi everywhere is used with current, so it's
also simple-switching to user_64bit_mode(task_pt_regs(current)).

For the conclusion:
I will send those patches, but I do not know what to do with
uprobes tracing. Could you give an advice what to do with
that?
It seems like, if I do those things, I will only need a way to
change vdso blob, without swapping some compatible flags,
as 64-bit tasks will differ from 32-bit only by the way they
execute syscalls.