code review 141080043: syscall: in linux/arm Syscall, zero R3, R4, R5

syscall: in linux/arm Syscall, zero R3, R4, R5

The general kernel system call interface
takes 6 arguments: R0, R1, R2, R3, R4, R5.

Syscall is for calls that only need 3.
The amd64 and 386 versions zero the extra arg registers,
but the arm version does not.

func utimensat calls Syscall with 3 arguments.
The kernel expects a 4th argument.
That turns out to be whatever is in R3 at the time of the call.
CL 137160043 changed various pieces of code and apparently
changed the value left in R3 at the time of utimensat's Syscall.
This causes the kernel to return EINVAL.

Change linux/arm Syscall to zero R3, R4, R5, so that calls will
behave deterministically, even if they pass too few arguments.

Arguably, utimensat could be fixed too, but the predictable
zeroing is certainly worth doing, and once done utimensat's
use of Syscall is fine.

Fixes arm build.

[rsc, 2014-09-05 03:12:08]

Hello bradfitz (cc: golang-codereviews@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go/

[rsc, 2014-09-05 03:12:15]

*** Submitted as https://code.google.com/p/go/source/detail?r=4fb2eea9ceb0 ***

syscall: in linux/arm Syscall, zero R3, R4, R5

The general kernel system call interface
takes 6 arguments: R0, R1, R2, R3, R4, R5.

Syscall is for calls that only need 3.
The amd64 and 386 versions zero the extra arg registers,
but the arm version does not.

func utimensat calls Syscall with 3 arguments.
The kernel expects a 4th argument.
That turns out to be whatever is in R3 at the time of the call.
CL 137160043 changed various pieces of code and apparently
changed the value left in R3 at the time of utimensat's Syscall.
This causes the kernel to return EINVAL.

Change linux/arm Syscall to zero R3, R4, R5, so that calls will
behave deterministically, even if they pass too few arguments.

Arguably, utimensat could be fixed too, but the predictable
zeroing is certainly worth doing, and once done utimensat's
use of Syscall is fine.

Fixes arm build.

TBR=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/141080043

[gobot, 2014-09-05 03:15:00]

This CL appears to have broken the windows-386 builder.
See http://build.golang.org/log/422b875114a0c33eac74796944721e61e24f5c63

[minux, 2014-09-05 03:31:49]

LGTM.

[dave_cheney.net, 2014-09-05 03:41:46]

LGTM.
On 5 Sep 2014 13:12, <rsc@golang.org> wrote:

> *** Submitted as
> https://code.google.com/p/go/source/detail?r=4fb2eea9ceb0 ***
>
> syscall: in linux/arm Syscall, zero R3, R4, R5
>
> The general kernel system call interface
> takes 6 arguments: R0, R1, R2, R3, R4, R5.
>
> Syscall is for calls that only need 3.
> The amd64 and 386 versions zero the extra arg registers,
> but the arm version does not.
>
> func utimensat calls Syscall with 3 arguments.
> The kernel expects a 4th argument.
> That turns out to be whatever is in R3 at the time of the call.
> CL 137160043 changed various pieces of code and apparently
> changed the value left in R3 at the time of utimensat's Syscall.
> This causes the kernel to return EINVAL.
>
> Change linux/arm Syscall to zero R3, R4, R5, so that calls will
> behave deterministically, even if they pass too few arguments.
>
> Arguably, utimensat could be fixed too, but the predictable
> zeroing is certainly worth doing, and once done utimensat's
> use of Syscall is fine.
>
> Fixes arm build.
>
> TBR=bradfitz
> CC=golang-codereviews
> https://codereview.appspot.com/141080043
>
>
> https://codereview.appspot.com/141080043/
>
> --
> You received this message because you are subscribed to the Google Groups
> "golang-codereviews" group.
> To unsubscribe from this group and stop receiving emails from it, send an
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>

[bradfitz, 2014-09-05 04:23:50]

LGTM
 On Sep 4, 2014 8:12 PM, <rsc@golang.org> wrote:

> Reviewers: bradfitz,
>
> Message:
> Hello bradfitz (cc: golang-codereviews@googlegroups.com),
>
> I'd like you to review this change to
> https://code.google.com/p/go/
>
>
> Description:
> syscall: in linux/arm Syscall, zero R3, R4, R5
>
> The general kernel system call interface
> takes 6 arguments: R0, R1, R2, R3, R4, R5.
>
> Syscall is for calls that only need 3.
> The amd64 and 386 versions zero the extra arg registers,
> but the arm version does not.
>
> func utimensat calls Syscall with 3 arguments.
> The kernel expects a 4th argument.
> That turns out to be whatever is in R3 at the time of the call.
> CL 137160043 changed various pieces of code and apparently
> changed the value left in R3 at the time of utimensat's Syscall.
> This causes the kernel to return EINVAL.
>
> Change linux/arm Syscall to zero R3, R4, R5, so that calls will
> behave deterministically, even if they pass too few arguments.
>
> Arguably, utimensat could be fixed too, but the predictable
> zeroing is certainly worth doing, and once done utimensat's
> use of Syscall is fine.
>
> Fixes arm build.
>
> Please review this at https://codereview.appspot.com/141080043/
>
> Affected files (+3, -0 lines):
>   M src/pkg/syscall/asm_linux_arm.s
>
>
> Index: src/pkg/syscall/asm_linux_arm.s
> ===================================================================
> --- a/src/pkg/syscall/asm_linux_arm.s
> +++ b/src/pkg/syscall/asm_linux_arm.s
> @@ -18,6 +18,9 @@
>         MOVW    8(SP), R0
>         MOVW    12(SP), R1
>         MOVW    16(SP), R2
> +       MOVW    $0, R3
> +       MOVW    $0, R4
> +       MOVW    $0, R5
>         SWI             $0
>         MOVW    $0xfffff001, R1
>         CMP             R1, R0
>
>
>