From: Nick Desaulniers <ndesaulniers@google.com> To: Arnd Bergmann <arnd@kernel.org> Cc: Russell King <linux@armlinux.org.uk>, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>, Ard Biesheuvel <ardb@kernel.org>, Linus Walleij <linus.walleij@linaro.org>, Nathan Chancellor <nathan@kernel.org>, llvm@lists.linux.dev, David Spickett <david.spickett@linaro.org> Subject: Re: [PATCH 06/14] ARM: disallow CONFIG_THUMB with ARMv4 Date: Wed, 29 Sep 2021 11:52:11 -0700 [thread overview] Message-ID: <CAKwvOdngJj-b=MJ2-Bik6dQGoambMZxGdZyVeO0O5KkaMhhU3Q@mail.gmail.com> (raw) In-Reply-To: <20210928154143.2106903-7-arnd@kernel.org> On Tue, Sep 28, 2021 at 8:42 AM Arnd Bergmann <arnd@kernel.org> wrote: > > From: Arnd Bergmann <arnd@arndb.de> > > We can currently build a multi-cpu enabled kernel that allows both ARMv4 > and ARMv5 CPUs, and also supports THUMB mode in user space. > > However, returning to user space in this configuration with the usr_ret > macro requires the use of the 'bx' instruction, which is refused by > the assembler: > > arch/arm/kernel/entry-armv.S: Assembler messages: > arch/arm/kernel/entry-armv.S:937: Error: selected processor does not support `bx lr' in ARM mode > arch/arm/kernel/entry-armv.S:960: Error: selected processor does not support `bx lr' in ARM mode > arch/arm/kernel/entry-armv.S:1003: Error: selected processor does not support `bx lr' in ARM mode > <instantiation>:2:2: note: instruction requires: armv4t > bx lr > > While it would be possible to handle this correctly in principle, doing so > seems to not be worth it, if we can simply avoid the problem by enforcing does `mov pc, lr` work here, with a preprocessor guard on CPU_32v4? Or better yet... If `ret` is just an assembler macro (arch/arm/include/asm/assembler.h#L449), then perhaps always just using `ret` would be preferable here? In that case, it looks like `usr_ret` could be outright replaced with just expansions of `ret`? Just spitballing ideas; like you said, maybe not worth fixing. > that a kernel supporting both ARMv4 and a later CPU architecture cannot > run THUMB binaries. > > This turned up while build-testing with clang; for some reason, > gcc never triggered the problem. I suspect this is a Clang's integrated assembler vs GAS difference (compiler irrelevant); clang's assembler is more strict that `bx lr` requires armvt (CPU_32v4T). Though I can reproduce that exact message in local testing with GAS: $ cat foo.s bx lr $ arm-linux-gnueabi-as -march=armv4 -c foo.s foo.s: Assembler messages: foo.s:1: Error: selected processor does not support `bx lr' in ARM mode $ arm-linux-gnueabi-as -march=armv4t -c foo.s $ arm-linux-gnueabi-objdump -dr a.out ... 00000000 <.text>: 0: e12fff1e bx lr 0: R_ARM_V4BX *ABS* The `<instantiation>:2:2` makes me think of inline asm. > > Signed-off-by: Arnd Bergmann <arnd@arndb.de> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> > --- > arch/arm/mm/Kconfig | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig > index 82aa990c4180..58afba346729 100644 > --- a/arch/arm/mm/Kconfig > +++ b/arch/arm/mm/Kconfig > @@ -675,7 +675,7 @@ config ARM_PV_FIXUP > > config ARM_THUMB > bool "Support Thumb user binaries" if !CPU_THUMBONLY && EXPERT > - depends on CPU_THUMB_CAPABLE > + depends on CPU_THUMB_CAPABLE && !CPU_32v4 > default y > help > Say Y if you want to include kernel support for running user space > -- > 2.29.2 > -- Thanks, ~Nick Desaulniers
WARNING: multiple messages have this Message-ID (diff)
From: Nick Desaulniers <ndesaulniers@google.com> To: Arnd Bergmann <arnd@kernel.org> Cc: Russell King <linux@armlinux.org.uk>, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>, Ard Biesheuvel <ardb@kernel.org>, Linus Walleij <linus.walleij@linaro.org>, Nathan Chancellor <nathan@kernel.org>, llvm@lists.linux.dev, David Spickett <david.spickett@linaro.org> Subject: Re: [PATCH 06/14] ARM: disallow CONFIG_THUMB with ARMv4 Date: Wed, 29 Sep 2021 11:52:11 -0700 [thread overview] Message-ID: <CAKwvOdngJj-b=MJ2-Bik6dQGoambMZxGdZyVeO0O5KkaMhhU3Q@mail.gmail.com> (raw) In-Reply-To: <20210928154143.2106903-7-arnd@kernel.org> On Tue, Sep 28, 2021 at 8:42 AM Arnd Bergmann <arnd@kernel.org> wrote: > > From: Arnd Bergmann <arnd@arndb.de> > > We can currently build a multi-cpu enabled kernel that allows both ARMv4 > and ARMv5 CPUs, and also supports THUMB mode in user space. > > However, returning to user space in this configuration with the usr_ret > macro requires the use of the 'bx' instruction, which is refused by > the assembler: > > arch/arm/kernel/entry-armv.S: Assembler messages: > arch/arm/kernel/entry-armv.S:937: Error: selected processor does not support `bx lr' in ARM mode > arch/arm/kernel/entry-armv.S:960: Error: selected processor does not support `bx lr' in ARM mode > arch/arm/kernel/entry-armv.S:1003: Error: selected processor does not support `bx lr' in ARM mode > <instantiation>:2:2: note: instruction requires: armv4t > bx lr > > While it would be possible to handle this correctly in principle, doing so > seems to not be worth it, if we can simply avoid the problem by enforcing does `mov pc, lr` work here, with a preprocessor guard on CPU_32v4? Or better yet... If `ret` is just an assembler macro (arch/arm/include/asm/assembler.h#L449), then perhaps always just using `ret` would be preferable here? In that case, it looks like `usr_ret` could be outright replaced with just expansions of `ret`? Just spitballing ideas; like you said, maybe not worth fixing. > that a kernel supporting both ARMv4 and a later CPU architecture cannot > run THUMB binaries. > > This turned up while build-testing with clang; for some reason, > gcc never triggered the problem. I suspect this is a Clang's integrated assembler vs GAS difference (compiler irrelevant); clang's assembler is more strict that `bx lr` requires armvt (CPU_32v4T). Though I can reproduce that exact message in local testing with GAS: $ cat foo.s bx lr $ arm-linux-gnueabi-as -march=armv4 -c foo.s foo.s: Assembler messages: foo.s:1: Error: selected processor does not support `bx lr' in ARM mode $ arm-linux-gnueabi-as -march=armv4t -c foo.s $ arm-linux-gnueabi-objdump -dr a.out ... 00000000 <.text>: 0: e12fff1e bx lr 0: R_ARM_V4BX *ABS* The `<instantiation>:2:2` makes me think of inline asm. > > Signed-off-by: Arnd Bergmann <arnd@arndb.de> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> > --- > arch/arm/mm/Kconfig | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig > index 82aa990c4180..58afba346729 100644 > --- a/arch/arm/mm/Kconfig > +++ b/arch/arm/mm/Kconfig > @@ -675,7 +675,7 @@ config ARM_PV_FIXUP > > config ARM_THUMB > bool "Support Thumb user binaries" if !CPU_THUMBONLY && EXPERT > - depends on CPU_THUMB_CAPABLE > + depends on CPU_THUMB_CAPABLE && !CPU_32v4 > default y > help > Say Y if you want to include kernel support for running user space > -- > 2.29.2 > -- Thanks, ~Nick Desaulniers _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2021-09-29 18:52 UTC|newest] Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-09-28 15:41 [PATCH 00/14] ARM: randconfig build fixes Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 15:41 ` [PATCH 01/14] ARM: RiscPC needs older gcc version Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 16:03 ` Arnd Bergmann 2021-09-28 16:03 ` Arnd Bergmann 2021-09-28 16:03 ` Arnd Bergmann 2021-09-29 8:45 ` Arnd Bergmann 2021-09-29 8:45 ` Arnd Bergmann 2021-09-29 8:45 ` Arnd Bergmann 2021-09-28 15:41 ` [PATCH 02/14] ARM: patch: fix BE32 compilation Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 21:33 ` Linus Walleij 2021-09-28 21:33 ` Linus Walleij 2021-09-28 21:33 ` Linus Walleij 2021-09-28 15:41 ` [PATCH 03/14] ARM: remove duplicate memcpy() definition Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 21:39 ` Linus Walleij 2021-09-28 21:39 ` Linus Walleij 2021-09-28 21:39 ` Linus Walleij 2021-09-28 15:41 ` [PATCH 04/14] ARM: kprobes: address gcc -Wempty-body warning Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 15:41 ` [PATCH 05/14] ARM: ARMv7-M uses BE-8, not BE-32 Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 15:41 ` [PATCH 06/14] ARM: disallow CONFIG_THUMB with ARMv4 Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-29 18:52 ` Nick Desaulniers [this message] 2021-09-29 18:52 ` Nick Desaulniers 2021-09-29 18:52 ` Nick Desaulniers 2021-10-06 16:01 ` Ard Biesheuvel 2021-10-06 16:01 ` Ard Biesheuvel 2021-09-28 15:41 ` [PATCH 07/14] ARM: fix link warning with XIP + frame-pointer Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 15:41 ` [PATCH 08/14] ARM: kprobes: fix arch_init_kprobes() prototype Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 15:41 ` [PATCH 09/14] ARM: allow compile-testing without machine record Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 15:41 ` [PATCH 10/14] ARM: only warn about XIP address when not compile testing Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 15:41 ` [PATCH 11/14] ARM: kasan: work around LPAE build warning Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 15:41 ` [PATCH 12/14] ARM: add CONFIG_PHYS_OFFSET default values Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 16:02 ` Nicolas Pitre 2021-09-28 16:02 ` Nicolas Pitre 2021-09-28 16:02 ` Nicolas Pitre 2021-09-28 21:44 ` Linus Walleij 2021-09-28 21:44 ` Linus Walleij 2021-09-28 21:44 ` Linus Walleij 2021-09-28 15:41 ` [PATCH 13/14] ARM: use .arch directives instead of assembler command line flags Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 17:10 ` Nick Desaulniers 2021-09-28 17:10 ` Nick Desaulniers 2021-09-28 17:10 ` Nick Desaulniers 2021-09-28 18:32 ` Arnd Bergmann 2021-09-28 18:32 ` Arnd Bergmann 2021-09-28 18:32 ` Arnd Bergmann 2021-09-29 18:11 ` Nick Desaulniers 2021-09-29 18:11 ` Nick Desaulniers 2021-09-29 18:11 ` Nick Desaulniers 2022-01-21 22:17 ` Nathan Chancellor 2022-01-21 22:17 ` Nathan Chancellor 2021-09-28 15:41 ` [PATCH 14/14] [RFC] ARM: forbid ftrace with clang and thumb2_kernel Arnd Bergmann 2021-09-28 15:41 ` Arnd Bergmann 2021-09-28 17:20 ` Nick Desaulniers 2021-09-28 17:20 ` Nick Desaulniers 2021-09-28 17:20 ` Nick Desaulniers
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