Re: [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation

Vincenzo Frascino
2019-06-26T19:01:16Z

Hi Dave,

thank you for the quick turn around.

On 6/26/19 5:14 PM, Dave Martin wrote:
> On Wed, Jun 26, 2019 at 02:27:59PM +0100, Vincenzo Frascino wrote:
>> Hi Dave,
>>
>> On 25/06/2019 16:33, Dave Martin wrote:
>>> On Fri, Jun 21, 2019 at 10:52:31AM +0100, Vincenzo Frascino wrote:
>>>> To take advantage of the commonly defined vdso interface for
>>>> gettimeofday the architectural code requires an adaptation.
>>>>
>>>> Re-implement the gettimeofday vdso in C in order to use lib/vdso.
>>>>
>>>> With the new implementation arm64 gains support for CLOCK_BOOTTIME
>>>> and CLOCK_TAI.
>>>>
>>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>>> Cc: Will Deacon <will.deacon@arm.com>
>>>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
>>>> Tested-by: Shijith Thotton <sthotton@marvell.com>
>>>> Tested-by: Andre Przywara <andre.przywara@arm.com>
>>>
>>> [...]
>>>
>>>> diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
>>>> new file mode 100644
>>>> index 000000000000..bc3cb6738051
>>>> --- /dev/null
>>>> +++ b/arch/arm64/include/asm/vdso/gettimeofday.h
>>>> @@ -0,0 +1,86 @@
>>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>>> +/*
>>>> + * Copyright (C) 2018 ARM Limited
>>>> + */
>>>> +#ifndef __ASM_VDSO_GETTIMEOFDAY_H
>>>> +#define __ASM_VDSO_GETTIMEOFDAY_H
>>>> +
>>>> +#ifndef __ASSEMBLY__
>>>> +
>>>> +#include <asm/unistd.h>
>>>> +#include <uapi/linux/time.h>
>>>> +
>>>> +#define VDSO_HAS_CLOCK_GETRES 1
>>>> +
>>>> +static __always_inline int gettimeofday_fallback(
>>>> + struct __kernel_old_timeval *_tv,
>>>> + struct timezone *_tz)
>>>
>>> Out of interest, does this need to be __always_inline?
>>>
>>
>> It is a design choice. Philosophically, I prefer to control and reduce the scope
>> of the decisions the compiler has to make in order to not have surprises.
>>
>>>> +{
>>>> + register struct timezone *tz asm("x1") = _tz;
>>>> + register struct __kernel_old_timeval *tv asm("x0") = _tv;
>>>> + register long ret asm ("x0");
>>>> + register long nr asm("x8") = __NR_gettimeofday;
>>>> +
>>>> + asm volatile(
>>>> + " svc #0\n"
>>>
>>> Can inlining of this function result in non-trivial expressions being
>>> substituted for _tz or _tv?
>>>
>>> A function call can clobber register asm vars that are assigned to the
>>> caller-save registers or that the PCS uses for function arguments, and
>>> the situations where this can happen are poorly defined AFAICT. There's
>>> also no reliable way to detect at build time whether the compiler has
>>> done this, and no robust way to stop if happening.
>>>
>>> (IMHO the compiler is wrong to do this, but it's been that way for ever,
>>> and I think I saw GCC 9 show this behaviour recently when I was
>>> investigating something related.)
>>>
>>>
>>> To be safe, it's better to put this out of line, or remove the reg asm()
>>> specifiers, mark x0-x18 and lr as clobbered here (so that the compiler
>>> doesn't map arguments to them), and put movs in the asm to move things
>>> into the right registers. The syscall number can be passed with an "i"
>>> constraint. (And yes, this sucks.)
>>>
>>> If the code this is inlined in is simple enough though, we can be fairly
>>> confident of getting away with it.
>>>
>>
>> I took very seriously what you are mentioning here because I think
>> that robustness of the code comes before than everything especially
>> in the kernel and I carried on some experiments to try to verify if
>> in this case is safe to assume that the compiler is doing the right
>> thing.
>>
>> Based on my investigation and on previous observations of the
>> generation of the vDSO library, I can conclude that the approach
>> seems safe due to the fact that the usage of this code is very
>> limited, the code itself is simple enough and that gcc would inline
>> this code anyway based on the current compilation options.
>
> I'd caution about "seems safe". A lot of subtly wrong code not only
> seems safe, but _is_ safe in its original context, in practice. Add
> some code to the vdso over time though, or tweak the compilation options
> at some point in the future, or use a different compiler, and things
> could still go wrong.
>
> (Further comments below.)
>

Allow me to provide a clarification on "seems safe" vs "is safe": my approach
"seems safe" because I am providing empirical evidence to support my thesis, but
I guess we both know that there is no simple way to prove in one way or another
that the problem has a complete solution.
The proposed problem involves suppositions on potential future code additions
and changes of behavior of the compiler that I can't either control or prevent.
In other words, I can comment and propose solutions only based on the current
status of the things, and it is what my analysis targets, not on what will
happen in future.

I will reply point by point below.

>> The experiment that I did was to define some self-contained code that
>> tries to mimic what you are describing and compile it with 3
>> different versions of gcc (6.4, 8.1 and 8.3) and in all the tree
>> cases the behavior seems correct.
>>
>> Code:
>> =====
>>
>> typedef int ssize_t;
>> typedef int size_t;
>>
>> static int my_strlen(const char *s)
>> {
>> int i = 0;
>>
>> while (s[i] == '\0')
>> i++;
>>
>> return i;
>> }
>>
>> static inline ssize_t my_syscall(int fd, const void *buf, size_t count)
>> {
>> register ssize_t arg1 asm ("x0") = fd;
>> register const void *arg2 asm ("x1") = buf;
>> register size_t arg3 asm ("x2") = count;
>>
>> __asm__ volatile (
>> "mov x8, #64\n"
>> "svc #0\n"
>> : "=&r" (arg1)
>> : "r" (arg2), "r" (arg3)
>> : "x8"
>> );
>>
>> return arg1;
>> }
>>
>> void sys_caller(const char *s)
>> {
>> my_syscall(1, s, my_strlen(s));
>> }
>>
>>
>> GCC 8.3.0:
>> ==========
>>
>> main.8.3.0.o: file format elf64-littleaarch64
>>
>>
>> Disassembly of section .text:
>>
>> 0000000000000000 <sys_caller>:
>> 0: 39400001 ldrb w1, [x0]
>> 4: 35000161 cbnz w1, 30 <sys_caller+0x30>
>> 8: d2800023 mov x3, #0x1 // #1
>> c: d1000404 sub x4, x0, #0x1
>> 10: 2a0303e2 mov w2, w3
>> 14: 91000463 add x3, x3, #0x1
>> 18: 38636881 ldrb w1, [x4, x3]
>> 1c: 34ffffa1 cbz w1, 10 <sys_caller+0x10>
>> 20: aa0003e1 mov x1, x0
>> 24: d2800808 mov x8, #0x40 // #64
>> 28: d4000001 svc #0x0
>> 2c: d65f03c0 ret
>> 30: 52800002 mov w2, #0x0 // #0
>> 34: 17fffffb b 20 <sys_caller+0x20>
>>
>>
>> GCC 8.1.0:
>> ==========
>>
>> main.8.1.0.o: file format elf64-littleaarch64
>>
>>
>> Disassembly of section .text:
>>
>> 0000000000000000 <sys_caller>:
>> 0: 39400001 ldrb w1, [x0]
>> 4: 35000161 cbnz w1, 30 <sys_caller+0x30>
>> 8: d2800023 mov x3, #0x1 // #1
>> c: d1000404 sub x4, x0, #0x1
>> 10: 2a0303e2 mov w2, w3
>> 14: 91000463 add x3, x3, #0x1
>> 18: 38636881 ldrb w1, [x4, x3]
>> 1c: 34ffffa1 cbz w1, 10 <sys_caller+0x10>
>> 20: aa0003e1 mov x1, x0
>> 24: d2800808 mov x8, #0x40 // #64
>> 28: d4000001 svc #0x0
>> 2c: d65f03c0 ret
>> 30: 52800002 mov w2, #0x0 // #0
>> 34: 17fffffb b 20 <sys_caller+0x20>
>>
>>
>>
>> GCC 6.4.0:
>> ==========
>>
>> main.6.4.0.o: file format elf64-littleaarch64
>>
>>
>> Disassembly of section .text:
>>
>> 0000000000000000 <sys_caller>:
>> 0: 39400001 ldrb w1, [x0]
>> 4: 35000161 cbnz w1, 30 <sys_caller+0x30>
>> 8: d2800023 mov x3, #0x1 // #1
>> c: d1000404 sub x4, x0, #0x1
>> 10: 2a0303e2 mov w2, w3
>> 14: 91000463 add x3, x3, #0x1
>> 18: 38636881 ldrb w1, [x4, x3]
>> 1c: 34ffffa1 cbz w1, 10 <sys_caller+0x10>
>> 20: aa0003e1 mov x1, x0
>> 24: d2800808 mov x8, #0x40 // #64
>> 28: d4000001 svc #0x0
>> 2c: d65f03c0 ret
>> 30: 52800002 mov w2, #0x0 // #0
>> 34: 17fffffb b 20 <sys_caller+0x20>
>
> Thanks for having a go at this. If the compiler can show the
> problematic behaviour, it looks like your could could probably trigger
> it, and as you observe, it doesn't trigger.
>
> I am sure I have seen it in the past, but today I am struggling
> to tickle the compiler in the right way. My original reproducer may
> have involved LTO, but either way I don't still have it :(
>

vDSO library is a shared object not compiled with LTO as far as I can see, hence
if this involved LTO should not applicable in this case.

>
> The classic example of this (triggered directly and not due to inlining)
> would be something like:
>
> int bar(int, int);
>
> void foo(int x, int y)
> {
> register int x_ asm("r0") = x;
> register int y_ asm("r1") = bar(x, y);
>
> asm volatile (
> "svc #0"
> :: "r" (x_), "r" (y_)
> : "memory"
> );
> }
>
> ->
>
> 0000000000000000 <foo>:
> 0: a9bf7bfd stp x29, x30, [sp, #-16]!
> 4: 910003fd mov x29, sp
> 8: 94000000 bl 0 <bar>
> c: 2a0003e1 mov w1, w0
> 10: d4000001 svc #0x0
> 14: a8c17bfd ldp x29, x30, [sp], #16
> 18: d65f03c0 ret
>

Contextualized to what my vdso fallback functions do, this should not be a
concern because in no case a function result is directly set to a variable
declared as register.

Since the vdso fallback functions serve a very specific and limited purpose, I
do not expect that that code is going to change much in future.

The only thing that can happen is something similar to what I wrote in my
example, which as I empirically proved does not trigger the problematic behavior.

>
> The gcc documentation is vague and ambiguous about precisely whan this
> can happen and about how to avoid it.
>

On this I agree, it is not very clear, but this seems more something to raise
with the gcc folks in order to have a more "explicit" description that leaves no
room to the interpretation.

...

>
> However, the workaround is cheap, and to avoid the chance of subtle
> intermittent code gen bugs it may be worth it:
>
> void foo(int x, int y)
> {
> asm volatile (
> "mov x0, %0\n\t"
> "mov x1, %1\n\t"
> "svc #0"
> :: "r" (x), "r" (bar(x, y))
> : "r0", "r1", "memory"
> );
> }
>
> ->
>
> 0000000000000000 <foo>:
> 0: a9be7bfd stp x29, x30, [sp, #-32]!
> 4: 910003fd mov x29, sp
> 8: f9000bf3 str x19, [sp, #16]
> c: 2a0003f3 mov w19, w0
> 10: 94000000 bl 0 <bar>
> 14: 2a0003e2 mov w2, w0
> 18: aa1303e0 mov x0, x19
> 1c: aa0203e1 mov x1, x2
> 20: d4000001 svc #0x0
> 24: f9400bf3 ldr x19, [sp, #16]
> 28: a8c27bfd ldp x29, x30, [sp], #32
> 2c: d65f03c0 ret
>
>
> What do you think?
>

The solution seems ok, thanks for providing it, but IMHO I think we should find
a workaround for something that is broken, which, unless I am missing something
major, this seems not the case.

> Cheers
> ---Dave
>

--
Regards,
Vincenzo