Re: [RFC PATCH bpf-next 3/6] selftests/test: test gen_prologue and gen_epilogue

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2024-08-15 at 16:41 -0700, Martin KaFai Lau wrote:

[...]

> >      SEC("struct_ops/test_epilogue")
> >      __naked int test_epilogue(void)
> >      {
> >      	asm volatile (
> >      	"r0 = 0;"
> 
> I also want to test a struct_ops prog making kfunc call, e.g. the 
> BPF_PROG(test_epilogue_kfunc) in this patch. I have never tried this in asm, so 
> a n00b question. Do you know if there is an example how to call kfunc?

Here is an example:
progs/verifier_ref_tracking.c, specifically take a look at
acquire_release_user_key_reference(). The main trick is to have
__kfunc_btf_root() with dummy calls, so that there are BTF signatures
for kfuncs included in the object file.

> >      	"exit;"
> >      	::: __clobber_all);
> >      }
> >      
> >      SEC(".struct_ops.link")
> >      struct bpf_testmod_st_ops st_ops = {
> >      	.test_epilogue = (void *)test_epilogue,
> >      };
> > 
> > (Complete example is in the attachment).
> > test_loader based tests can also trigger program execution via __retval() macro.
> > The only (minor) shortcoming that I see, is that test_loader would
> > load/unload st_ops map multiple times because of the following
> > interaction:
> > - test_loader assumes that each bpf program defines a test;
> > - test_loader re-creates all maps before each test;
> > - libbpf struct_ops autocreate logic marks all programs referenced
> >    from struct_ops map as autoloaded.
> 
> If I understand correctly, there are redundant works but still work?

Yes.

> Potentially the test_loader can check all the loaded struct_ops progs of a 
> st_ops map at once which is an optimization.

Yes, I should look into this.

> Re: __retval(), the struct_ops progs is triggered by a SEC("syscall") prog. 
> Before calling this syscall prog, the st_ops map needs to be attached first. I 
> think the attach part is missing also? or there is a way?

I think libbpf handles the attachment automatically, I'll double check and reply.