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

[kevin.brodsky@arm.com (Kevin Brodsky)]

On 28/10/16 04:09, Jisheng Zhang wrote:
> 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 <kevin.brodsky@arm.com>
>> ---
>>   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 <kevin.brodsky@arm.com>
>> +# 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

That's a fair point. I guess -march=armv8-a is not strictly necessary and the 
produced vDSO should be fine if arch/arm/vdso also compiles fine. However we would 
still need to pass -march=armv7-a. I'm not sure what to do between:
* Checking that the compiler supports -march=armv8-a when inspecting 
CROSS_COMPILE_ARM32, and if it doesn't vdso32 will not be built.
* Checking whether -march=armv8-a is available here, and if it is not fall back to 
-march=armv7-a.

Thanks!
Kevin