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

Dave Martin
2019-06-27T10:02:02Z

On Wed, Jun 26, 2019 at 08:01:58PM +0100, Vincenzo Frascino wrote:

[...]

> 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.

That turned to be a spurious hypothesis on my part -- LTO isn't the
smoking gun. (See below.)

> > 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.

So, after a bit of further experimentation, I found that I could trigger
it with implicit function calls on an older compiler. I couldn't show
it with explicit function calls (as in your example).

With the following code, inlining if an expression that causes an
implicit call to a libgcc helper can trigger this issue, but I had to
try an older compiler:

int foo(int x, int y)
{
register int res asm("r0");
register const int x_ asm("r0") = x;
register const int y_ asm("r1") = y;

asm volatile (
"svc #0"
: "=r" (res)
: "r" (x_), "r" (y_)
: "memory"
);

return res;
}

int bar(int x, int y)
{
return foo(x, x / y);
}

-> (arm-linux-gnueabihf-gcc 9.1 -O2)

00000000 <foo>:
0: df00 svc 0
2: 4770 bx lr

00000004 <bar>:
4: b510 push {r4, lr}
6: 4604 mov r4, r0
8: f7ff fffe bl 0 <__aeabi_idiv>
c: 4601 mov r1, r0
e: 4620 mov r0, r4
10: df00 svc 0
12: bd10 pop {r4, pc}

-> (arm-linux-gnueabihf-gcc 5.1 -O2)

00000000 <foo>:
0: df00 svc 0
2: 4770 bx lr

00000004 <bar>:
4: b508 push {r3, lr}
6: f7ff fffe bl 0 <__aeabi_idiv>
a: 4601 mov r1, r0
c: df00 svc 0
e: bd08 pop {r3, pc}

I was struggling to find a way to emit an implicit function call for
AArch64, except for 128-bit divide, which would complicate things since
uint128_t doesn't fit in a single register anyway.

Maybe this was considered a bug and fixed sometime after GCC 5, but I
think the GCC documentation is still quite unclear on the semantics of
register asm vars that alias call-clobbered registers in the PCS.

If we can get a promise out of the GCC folks that this will not happen
with any future compiler, then maybe we could just require a new enough
compiler to be used.

Then of course there is clang.

Cheers
---Dave