selftests/bpf: add precision propagation tests in the presence of subprogs

Add a bunch of tests validating verifier's precision backpropagation
logic in the presence of subprog calls and/or callback-calling
helpers/kfuncs.

We validate the following conditions:
  - subprog_result_precise: static subprog r0 result precision handling;
  - global_subprog_result_precise: global subprog r0 precision
    shortcutting, similar to BPF helper handling;
  - callback_result_precise: similarly r0 marking precise for
    callback-calling helpers;
  - parent_callee_saved_reg_precise, parent_callee_saved_reg_precise_global:
    propagation of precision for callee-saved registers bypassing
    static/global subprogs;
  - parent_callee_saved_reg_precise_with_callback: same as above, but in
    the presence of callback-calling helper;
  - parent_stack_slot_precise, parent_stack_slot_precise_global:
    similar to above, but instead propagating precision of stack slot
    (spilled SCALAR reg);
  - parent_stack_slot_precise_with_callback: same as above, but in the
    presence of callback-calling helper;
  - subprog_arg_precise: propagation of precision of static subprog's
    input argument back to caller;
  - subprog_spill_into_parent_stack_slot_precise: negative test
    validating that verifier currently can't support backtracking of stack
    access with non-r10 register, we validate that we fallback to
    forcing precision for all SCALARs.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230505043317.3629845-10-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/verifier.c

@@ -55,6 +55,7 @@
 #include "verifier_spill_fill.skel.h"
 #include "verifier_spin_lock.skel.h"
 #include "verifier_stack_ptr.skel.h"
+#include "verifier_subprog_precision.skel.h"
 #include "verifier_subreg.skel.h"
 #include "verifier_uninit.skel.h"
 #include "verifier_unpriv.skel.h"
@@ -154,6 +155,7 @@ void test_verifier_sock(void)                 { RUN(verifier_sock); }
 void test_verifier_spill_fill(void)           { RUN(verifier_spill_fill); }
 void test_verifier_spin_lock(void)            { RUN(verifier_spin_lock); }
 void test_verifier_stack_ptr(void)            { RUN(verifier_stack_ptr); }
+void test_verifier_subprog_precision(void)    { RUN(verifier_subprog_precision); }
 void test_verifier_subreg(void)               { RUN(verifier_subreg); }
 void test_verifier_uninit(void)               { RUN(verifier_uninit); }
 void test_verifier_unpriv(void)               { RUN(verifier_unpriv); }

tools/testing/selftests/bpf/progs/bpf_misc.h

@@ -86,6 +86,10 @@
 #define POINTER_VALUE	0xcafe4all
 #define TEST_DATA_LEN	64
 
+#ifndef __used
+#define __used __attribute__((used))
+#endif
+
 #if defined(__TARGET_ARCH_x86)
 #define SYSCALL_WRAPPER 1
 #define SYS_PREFIX "__x64_"