selftests/bpf: Add tests for callx

Add verifier_callx tests:
- calls via r0, callee saved regs, pointer passed as argument, returned
  from subprog, spilled/filled, different callees on different paths.
  These are executed to test JIT.
- invalid operands: scalar, other pointers, modified and variable
  PTR_TO_FUNC, global functions, reserved fields, JMP32.
- callx under lock.
- bounded and unbounded recursion via callx.
- stack depth of call chains through callx, including address taken
  several frames above callx.
- tail calls in the callee and the caller of callx.
- stack liveness: caller slots read by callee stay alive.
- no const propagation across callx.
- precision backtracking through callx.
- address of a function that is never called, with and without callx.
- function pointers in C.

callx needs JIT. Add RUN_JITED() to skip the tests when it's off.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20260924031042.1690890-16-alexei.starovoitov@gmail.com

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

@@ -28,6 +28,7 @@
 #include "verifier_btf_ctx_access.skel.h"
 #include "verifier_btf_unreliable_prog.skel.h"
 #include "verifier_call_large_imm.skel.h"
+#include "verifier_callx.skel.h"
 #include "verifier_cfg.skel.h"
 #include "verifier_cgroup_inv_retcode.skel.h"
 #include "verifier_cgroup_skb.skel.h"
@@ -173,6 +174,14 @@ static void run_tests_aux(const char *skel_name,
 
 #define RUN(skel) run_tests_aux(#skel, skel##__elf_bytes, NULL)
 
+/* for tests of what the interpreter doesn't support */
+#define RUN_JITED(skel) do {		\
+	if (is_jit_enabled())		\
+		RUN(skel);		\
+	else				\
+		test__skip();		\
+} while (0)
+
 void test_arena_kfunc(void)                   { RUN_TESTS(arena_kfunc); }
 
 void test_arena_kfunc_jit(void)               { RUN_TESTS(arena_kfunc_jit); }
@@ -199,6 +208,7 @@ void test_verifier_bswap(void)                { RUN(verifier_bswap); }
 void test_verifier_btf_ctx_access(void)       { RUN(verifier_btf_ctx_access); }
 void test_verifier_btf_unreliable_prog(void)  { RUN(verifier_btf_unreliable_prog); }
 void test_verifier_call_large_imm(void)       { RUN(verifier_call_large_imm); }
+void test_verifier_callx(void)                { RUN_JITED(verifier_callx); }
 void test_verifier_cfg(void)                  { RUN(verifier_cfg); }
 void test_verifier_cgroup_inv_retcode(void)   { RUN(verifier_cgroup_inv_retcode); }
 void test_verifier_cgroup_skb(void)           { RUN(verifier_cgroup_skb); }