Re: [PATCH bpf-next v6 8/9] selftests/bpf: Add tracing prog private stack tests

[Yonghong Song <yonghong.song@linux.dev>]

On 10/21/24 3:40 AM, Jiri Olsa wrote:
> On Sun, Oct 20, 2024 at 09:32:38PM -0700, Yonghong Song wrote:
>> On 10/20/24 2:59 PM, Jiri Olsa wrote:
>>> On Sun, Oct 20, 2024 at 12:14:31PM -0700, Yonghong Song wrote:
>>>
>>> SNIP
>>>
>>>> +__naked __noinline __used
>>>> +static unsigned long loop_callback(void)
>>>> +{
>>>> +	asm volatile (
>>>> +	"call %[bpf_get_prandom_u32];"
>>>> +	"r1 = 42;"
>>>> +	"*(u64 *)(r10 - 512) = r1;"
>>>> +	"call cumulative_stack_depth_subprog;"
>>>> +	"r0 = 0;"
>>>> +	"exit;"
>>>> +	:
>>>> +	: __imm(bpf_get_prandom_u32)
>>>> +	: __clobber_common);
>>>> +}
>>>> +
>>>> +SEC("raw_tp")
>>>> +__description("Private stack, callback")
>>>> +__success
>>>> +__arch_x86_64
>>>> +/* for func loop_callback */
>>>> +__jited("func #1")
>>>> +__jited("	endbr64")
>>> this should fail if CONFIG_X86_KERNEL_IBT is not enabled, right?
>>>
>>> hm, but I can see that also in other tests, so I guess it's fine,
>>> should we add it to config.x86_64 ?
>> The CI has CONFIG_X86_KERNEL_IBT as well.
>>
>> I checked x86 kconfig, I see
>>
>> config CC_HAS_IBT
>>          # GCC >= 9 and binutils >= 2.29
>>          # Retpoline check to work around https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93654
>>          # Clang/LLVM >= 14
>>          # https://github.com/llvm/llvm-project/commit/e0b89df2e0f0130881bf6c39bf31d7f6aac00e0f
>>          # https://github.com/llvm/llvm-project/commit/dfcf69770bc522b9e411c66454934a37c1f35332
>>          def_bool ((CC_IS_GCC && $(cc-option, -fcf-protection=branch -mindirect-branch-register)) || \
>>                    (CC_IS_CLANG && CLANG_VERSION >= 140000)) && \
>>                    $(as-instr,endbr64)
>>
>> config X86_KERNEL_IBT
>>          prompt "Indirect Branch Tracking"
>>          def_bool y
>>          depends on X86_64 && CC_HAS_IBT && HAVE_OBJTOOL
>>          # https://github.com/llvm/llvm-project/commit/9d7001eba9c4cb311e03cd8cdc231f9e579f2d0f
>>          depends on !LD_IS_LLD || LLD_VERSION >= 140000
>>          select OBJTOOL
>>          select X86_CET
>>          help
>>            Build the kernel with support for Indirect Branch Tracking, a
>>            hardware support course-grain forward-edge Control Flow Integrity
>>            protection. It enforces that all indirect calls must land on
>>            an ENDBR instruction, as such, the compiler will instrument the
>>            code with them to make this happen.
>>            In addition to building the kernel with IBT, seal all functions that
>>            are not indirect call targets, avoiding them ever becoming one.
>>            This requires LTO like objtool runs and will slow down the build. It
>>            does significantly reduce the number of ENDBR instructions in the
>>            kernel image.
>>
>> So CONFIG_X86_KERNEL_IBT will be enabled if clang >= version_14 or newer gcc.
> IIUC it's just dependency, no? doesn't mean it'll get enabled automatically
>
>> In my system, the gcc version is 13.1. So there is no need to explicitly add
>> CONFIG_X86_KERNEL_IBT to the selftests/bpf/config.x86_64 file.
> I had to enable it manualy for gcc 13.3.1

IIUC, the ci config is generated based on config + config.x86_64 + config.vm
in tools/testing/selftests/bpf directory.

In my case .config is generated from config + config.x86_64 + config.vm
With my local gcc 11.5, I did
    make olddefconfig
and I see CONFIG_X86_KERNEL_IBT=y is set.

Maybe your base config is a little bit different from what ci used.
My local config is based on ci config + some more e.g. enabling KASAN etc.

Could you debug a little more on why CONFIG_X86_KERNEL_IBT not enabled
by default in your case? For

config X86_KERNEL_IBT
         prompt "Indirect Branch Tracking"
         def_bool y
         depends on X86_64 && CC_HAS_IBT && HAVE_OBJTOOL
         # https://github.com/llvm/llvm-project/commit/9d7001eba9c4cb311e03cd8cdc231f9e579f2d0f
         depends on !LD_IS_LLD || LLD_VERSION >= 140000
         select OBJTOOL
         select X86_CET

default is 'y' so if all dependencies are met, CONFIG_X86_KERNEL_IBT
is supposed to be on by default.

>
> jirka