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From: Robin Murphy <robin.murphy@arm.com>
To: Nick Desaulniers <ndesaulniers@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>
Cc: Naohiro Aota <naohiro.aota@wdc.com>,
	Stephen Boyd <swboyd@google.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	linux-kernel@vger.kernel.org, Manoj Gupta <manojgupta@google.com>,
	Luis Lozano <llozano@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm
Date: Wed, 27 May 2020 14:45:32 +0100	[thread overview]
Message-ID: <2f58c2a4-0f37-d507-7767-00161c6b5d98@arm.com> (raw)
In-Reply-To: <20200526173117.155339-1-ndesaulniers@google.com>

On 2020-05-26 18:31, Nick Desaulniers wrote:
> Custom toolchains that modify the default target to -mthumb cannot
> compile the arm64 compat vdso32, as
> arch/arm64/include/asm/vdso/compat_gettimeofday.h
> contains assembly that's invalid in -mthumb.  Force the use of -marm,
> always.

FWIW, this seems suspicious - the only assembly instructions I see there 
are SWI(SVC), MRRC, and a MOV, all of which exist in Thumb for the 
-march=armv7a baseline that we set.

On a hunch, I've just bodged "VDSO_CFLAGS += -mthumb" into my tree and 
built a Thumb VDSO quite happily with Ubuntu 19.04's 
gcc-arm-linux-gnueabihf. What was the actual failure you saw?

Robin.

> Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084372
> Cc: Stephen Boyd <swboyd@google.com>
> Reported-by: Luis Lozano <llozano@google.com>
> Tested-by: Manoj Gupta <manojgupta@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Surgeon General's Warning: changing the compiler defaults is not
> recommended and can lead to spooky bugs that are hard to reproduce
> upstream.
> 
>   arch/arm64/kernel/vdso32/Makefile | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> index 3964738ebbde..c449a293d81e 100644
> --- a/arch/arm64/kernel/vdso32/Makefile
> +++ b/arch/arm64/kernel/vdso32/Makefile
> @@ -104,6 +104,8 @@ VDSO_CFLAGS += -D__uint128_t='void*'
>   # (on GCC 4.8 or older, there is unfortunately no way to silence this warning)
>   VDSO_CFLAGS += $(call cc32-disable-warning,shift-count-overflow)
>   VDSO_CFLAGS += -Wno-int-to-pointer-cast
> +# Force vdso to be compiled in ARM mode, not THUMB.
> +VDSO_CFLAGS += -marm
>   
>   VDSO_AFLAGS := $(VDSO_CAFLAGS)
>   VDSO_AFLAGS += -D__ASSEMBLY__
> 

WARNING: multiple messages have this Message-ID (diff)
From: Robin Murphy <robin.murphy@arm.com>
To: Nick Desaulniers <ndesaulniers@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>
Cc: Naohiro Aota <naohiro.aota@wdc.com>,
	Stephen Boyd <swboyd@google.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	linux-kernel@vger.kernel.org, Manoj Gupta <manojgupta@google.com>,
	Luis Lozano <llozano@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm
Date: Wed, 27 May 2020 14:45:32 +0100	[thread overview]
Message-ID: <2f58c2a4-0f37-d507-7767-00161c6b5d98@arm.com> (raw)
In-Reply-To: <20200526173117.155339-1-ndesaulniers@google.com>

On 2020-05-26 18:31, Nick Desaulniers wrote:
> Custom toolchains that modify the default target to -mthumb cannot
> compile the arm64 compat vdso32, as
> arch/arm64/include/asm/vdso/compat_gettimeofday.h
> contains assembly that's invalid in -mthumb.  Force the use of -marm,
> always.

FWIW, this seems suspicious - the only assembly instructions I see there 
are SWI(SVC), MRRC, and a MOV, all of which exist in Thumb for the 
-march=armv7a baseline that we set.

On a hunch, I've just bodged "VDSO_CFLAGS += -mthumb" into my tree and 
built a Thumb VDSO quite happily with Ubuntu 19.04's 
gcc-arm-linux-gnueabihf. What was the actual failure you saw?

Robin.

> Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084372
> Cc: Stephen Boyd <swboyd@google.com>
> Reported-by: Luis Lozano <llozano@google.com>
> Tested-by: Manoj Gupta <manojgupta@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Surgeon General's Warning: changing the compiler defaults is not
> recommended and can lead to spooky bugs that are hard to reproduce
> upstream.
> 
>   arch/arm64/kernel/vdso32/Makefile | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> index 3964738ebbde..c449a293d81e 100644
> --- a/arch/arm64/kernel/vdso32/Makefile
> +++ b/arch/arm64/kernel/vdso32/Makefile
> @@ -104,6 +104,8 @@ VDSO_CFLAGS += -D__uint128_t='void*'
>   # (on GCC 4.8 or older, there is unfortunately no way to silence this warning)
>   VDSO_CFLAGS += $(call cc32-disable-warning,shift-count-overflow)
>   VDSO_CFLAGS += -Wno-int-to-pointer-cast
> +# Force vdso to be compiled in ARM mode, not THUMB.
> +VDSO_CFLAGS += -marm
>   
>   VDSO_AFLAGS := $(VDSO_CAFLAGS)
>   VDSO_AFLAGS += -D__ASSEMBLY__
> 

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  parent reply	other threads:[~2020-05-27 13:45 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-26 17:31 [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm Nick Desaulniers
2020-05-26 17:31 ` Nick Desaulniers
2020-05-26 18:55 ` Stephen Boyd
2020-05-26 18:55   ` Stephen Boyd
2020-05-26 20:45 ` Will Deacon
2020-05-26 20:45   ` Will Deacon
2020-05-27 13:53   ` Dave Martin
2020-05-27 13:53     ` Dave Martin
2020-05-27 17:58     ` Nick Desaulniers
2020-05-27 17:58       ` Nick Desaulniers
2020-05-27 13:45 ` Robin Murphy [this message]
2020-05-27 13:45   ` Robin Murphy
2020-05-27 17:55   ` Nick Desaulniers
2020-05-27 17:55     ` Nick Desaulniers
2020-05-27 18:08     ` Will Deacon
2020-05-27 18:08       ` Will Deacon
2020-05-27 18:17       ` Nick Desaulniers
2020-05-27 18:17         ` Nick Desaulniers
2020-05-28  7:20         ` Will Deacon
2020-05-28  7:20           ` Will Deacon
2020-06-08 20:57           ` [PATCH v2] arm64: vdso32: add CONFIG_THUMB2_COMPAT_VDSO Nick Desaulniers
2020-06-08 20:57             ` Nick Desaulniers
2020-06-09 20:35             ` Catalin Marinas
2020-06-09 20:35               ` Catalin Marinas
2020-06-09 23:55               ` Nick Desaulniers
2020-06-09 23:55                 ` Nick Desaulniers
2020-06-10  8:47                 ` Will Deacon
2020-06-10  8:47                   ` Will Deacon
2020-06-10 10:29                   ` Catalin Marinas
2020-06-10 10:29                     ` Catalin Marinas
2020-06-10 10:32                     ` Will Deacon
2020-06-10 10:32                       ` Will Deacon
2020-06-10 11:21             ` Will Deacon
2020-06-10 11:21               ` Will Deacon
2020-05-27 19:28     ` [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm Robin Murphy
2020-05-27 19:28       ` Robin Murphy
2020-05-27 20:02       ` Robin Murphy
2020-05-27 20:02         ` Robin Murphy
2020-05-27 20:14       ` Nick Desaulniers
2020-05-27 20:14         ` Nick Desaulniers
2020-05-27 20:31         ` Nick Desaulniers
2020-05-27 20:31           ` Nick Desaulniers
2020-05-27 21:47           ` Nick Desaulniers
2020-05-27 21:47             ` Nick Desaulniers
2020-05-28  8:05           ` Peter Smith
2020-05-28  8:05             ` Peter Smith
2020-05-28  9:41             ` Catalin Marinas
2020-05-28  9:41               ` Catalin Marinas
2020-05-28 19:06               ` Nick Desaulniers
2020-05-28 19:06                 ` Nick Desaulniers

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