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

[Jiri Olsa <olsajiri@gmail.com>]

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

jirka