Re: [PATCH] powerpc/vdso: Don't use r30 to avoid breaking Go lang

[Paul Menzel <pmenzel@molgen.mpg.de>]

Dear Michael,


Am 29.07.21 um 15:12 schrieb Michael Ellerman:
> The Go runtime uses r30 for some special value called 'g'. It assumes
> that value will remain unchanged even when calling VDSO functions.
> Although r30 is non-volatile across function calls, the callee is free
> to use it, as long as the callee saves the value and restores it before
> returning.
> 
> It used to be true by accident that the VDSO didn't use r30, because the
> VDSO was hand-written asm. When we switched to building the VDSO from C
> the compiler started using r30, at least in some builds, leading to
> crashes in Go. eg:
> 
>    ~/go/src$ ./all.bash
>    Building Go cmd/dist using /usr/lib/go-1.16. (go1.16.2 linux/ppc64le)
>    Building Go toolchain1 using /usr/lib/go-1.16.
>    go build os/exec: /usr/lib/go-1.16/pkg/tool/linux_ppc64le/compile: signal: segmentation fault
>    go build reflect: /usr/lib/go-1.16/pkg/tool/linux_ppc64le/compile: signal: segmentation fault
>    go tool dist: FAILED: /usr/lib/go-1.16/bin/go install -gcflags=-l -tags=math_big_pure_go compiler_bootstrap bootstrap/cmd/...: exit status 1
> 
> There are patches in flight to fix Go[1], but until they are released
> and widely deployed we can workaround it in the VDSO by avoiding use of

Nit: work around is spelled with a space.

> r30.
> 
> Note this only works with GCC, clang does not support -ffixed-rN.

Maybe the clang/LLVM build support folks (in CC) have an idea.

> 1: https://go-review.googlesource.com/c/go/+/328110
> 
> Fixes: ab037dd87a2f ("powerpc/vdso: Switch VDSO to generic C implementation.")
> Cc: stable@vger.kernel.org # v5.11+
> Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
> Tested-by: Paul Menzel <pmenzel@molgen.mpg.de>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>   arch/powerpc/kernel/vdso64/Makefile | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile
> index 2813e3f98db6..3c5baaa6f1e7 100644
> --- a/arch/powerpc/kernel/vdso64/Makefile
> +++ b/arch/powerpc/kernel/vdso64/Makefile
> @@ -27,6 +27,13 @@ KASAN_SANITIZE := n
>   
>   ccflags-y := -shared -fno-common -fno-builtin -nostdlib \
>   	-Wl,-soname=linux-vdso64.so.1 -Wl,--hash-style=both
> +
> +# Go prior to 1.16.x assumes r30 is not clobbered by any VDSO code. That used to be true
> +# by accident when the VDSO was hand-written asm code, but may not be now that the VDSO is
> +# compiler generated. To avoid breaking Go tell GCC not to use r30. Impact on code
> +# generation is minimal, it will just use r29 instead.
> +ccflags-y += $(call cc-option, -ffixed-r30)
> +
>   asflags-y := -D__VDSO64__ -s
>   
>   targets += vdso64.lds
> 

The rest looks good.


Kind regards,

Paul