Re: [RFC PATCH v2 13/13] objtool: arm64: Enable stack validation for arm64

[Julien Thierry <jthierry@redhat.com>]

On 3/7/21 11:25 AM, Ard Biesheuvel wrote:
> On Wed, 3 Mar 2021 at 18:10, Julien Thierry <jthierry@redhat.com> wrote:
>>
>> From: Raphael Gault <raphael.gault@arm.com>
>>
>> Add build option to run stack validation at compile time.
>>
>> When requiring stack validation, jump tables are disabled as it
>> simplifies objtool analysis (without having to introduce unreliable
>> artifacs). In local testing, this does not appear to significaly
>> affect final binary size nor system performance.
>>
>> Signed-off-by: Raphael Gault <raphael.gault@arm.com>
>> Signed-off-by: Julien Thierry <jthierry@redhat.com>
>> ---
>>   arch/arm64/Kconfig  | 1 +
>>   arch/arm64/Makefile | 4 ++++
>>   2 files changed, 5 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 1f212b47a48a..928323c03318 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -187,6 +187,7 @@ config ARM64
>>          select MMU_GATHER_RCU_TABLE_FREE
>>          select HAVE_RSEQ
>>          select HAVE_STACKPROTECTOR
>> +       select HAVE_STACK_VALIDATION
>>          select HAVE_SYSCALL_TRACEPOINTS
>>          select HAVE_KPROBES
>>          select HAVE_KRETPROBES
>> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
>> index 5b84aec31ed3..b819fb2e8eda 100644
>> --- a/arch/arm64/Makefile
>> +++ b/arch/arm64/Makefile
>> @@ -136,6 +136,10 @@ ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_REGS),y)
>>     CC_FLAGS_FTRACE := -fpatchable-function-entry=2
>>   endif
>>
>> +ifeq ($(CONFIG_STACK_VALIDATION),y)
>> +KBUILD_CFLAGS  += -fno-jump-tables
>> +endif
>> +
> 
> This is a bit misleading: the Kconfig option in question is selected
> automatically in all cases, so jump tables are always disabled.
> 

So, at the moment, the arch Kconfig only advertises that arm64 has stack 
validation with objtool, but currently stack validation itself is not 
enabled by default.

> However, I think disabling jump tables make sense anyway, at least
> when building the relocatable kernel for KASLR: we currently don't use
> -fpic/fpie in that case when building the vmlinux objects (because we
> don't want/need GOT tables), and so jump tables are emitted using
> absolute addresses, which induce some space overhead in the image. (24
> bytes of RELA data per absolute address)
> 
> ... unless I am missing something, and jump tables can/will be emitted
> as relative, even when not compiling in PIC mode?
> 

Personally I don't have enough context to assess whether it's the way to 
go. But if nobody opposes I'm fine having -fno-jump-tables in the 
default arm64 CFLAGS.

Thanks,

-- 
Julien Thierry