Subject: [RFC PATCH v2 4/8] arm64: compat: Add a 32-bit vDSO

From: [email-masked] (Jisheng Zhang)
Date: Fri, 28 Oct 2016 11:09:26 +0800

Dear Kevin,

On Thu, 27 Oct 2016 17:30:54 +0100 Kevin Brodsky wrote:

> Provide the files necessary for building a compat (AArch32) vDSO in
> kernel/vdso32.
> 
> This is mostly an adaptation of the arm vDSO. The most significant
> change in vgettimeofday.c is the use of the arm64 vdso_data struct,
> allowing the vDSO data page to be shared between the 32 and 64-bit
> vDSOs.
> 
> In addition to the time functions, sigreturn trampolines are also
> provided, aiming at replacing those in the vector page. To improve
> debugging, CFI and unwinding directives are used, based on glibc's
> implementation. Symbol offsets are made available to the kernel using
> the same method as the 64-bit vDSO.
> 
> There is unfortunately an important caveat to all this: we cannot get
> away with hand-coding 32-bit instructions like in kernel/kuser32.S,
> this time we really need a 32-bit compiler. The compat vDSO Makefile
> relies on CROSS_COMPILE_ARM32 to provide a 32-bit compiler,
> appropriate logic will be added to the arm64 Makefile later on to
> ensure that an attempt to build the compat vDSO is made only if this
> variable has been set properly.
> 
> Signed-off-by: Kevin Brodsky <[email-masked]>
> ---
>  arch/arm64/kernel/vdso32/Makefile        | 121 +++++++++++++
>  arch/arm64/kernel/vdso32/sigreturn.S     |  86 +++++++++
>  arch/arm64/kernel/vdso32/vdso.S          |  32 ++++
>  arch/arm64/kernel/vdso32/vdso.lds.S      |  98 +++++++++++
>  arch/arm64/kernel/vdso32/vgettimeofday.c | 294 +++++++++++++++++++++++++++++++
>  5 files changed, 631 insertions(+)
>  create mode 100644 arch/arm64/kernel/vdso32/Makefile
>  create mode 100644 arch/arm64/kernel/vdso32/sigreturn.S
>  create mode 100644 arch/arm64/kernel/vdso32/vdso.S
>  create mode 100644 arch/arm64/kernel/vdso32/vdso.lds.S
>  create mode 100644 arch/arm64/kernel/vdso32/vgettimeofday.c
> 
> diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> new file mode 100644
> index 000000000000..38facc870f6e
> --- /dev/null
> +++ b/arch/arm64/kernel/vdso32/Makefile
> @@ -0,0 +1,121 @@
> +#
> +# Building a vDSO image for AArch32.
> +#
> +# Author: Kevin Brodsky <[email-masked]>
> +# A mix between the arm64 and arm vDSO Makefiles.
> +
> +CC_ARM32 := $(CROSS_COMPILE_ARM32)gcc
> +
> +# Same as cc-ldoption, but using CC_ARM32 instead of CC
> +cc32-ldoption = $(call try-run,\
> +        $(CC_ARM32) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
> +
> +# Borrow vdsomunge.c from the arm vDSO
> +munge := arch/arm/vdso/vdsomunge
> +hostprogs-y := $(srctree)/$(munge)
> +
> +c-obj-vdso := vgettimeofday.o
> +asm-obj-vdso := sigreturn.o
> +
> +# Build rules
> +targets := $(c-obj-vdso) $(asm-obj-vdso) vdso.so vdso.so.dbg vdso.so.raw
> +c-obj-vdso := $(addprefix $(obj)/, $(c-obj-vdso))
> +asm-obj-vdso := $(addprefix $(obj)/, $(asm-obj-vdso))
> +obj-vdso := $(c-obj-vdso) $(asm-obj-vdso)
> +
> +ccflags-y := -fPIC -fno-common -fno-builtin -fno-stack-protector
> +ccflags-y += -DDISABLE_BRANCH_PROFILING
> +
> +# Force -O2 to avoid libgcc dependencies
> +VDSO_CFLAGS := -march=armv8-a -O2

For completeness, bringing 32bit compiler need to check whether the 32bit
toolchain support some options. IIRC, armv8-a support isn't enabled until
gcc 4.8, so old toolchains such gcc-4.7 will complain:
 error: unrecognized argument in option ?-march=armv8-a?

Thanks,
Jisheng