[PATCH bpf-next v2 4/7] selftests/bpf: Use _exit() in uprobe_multi_test clone-ed child

[KaFai Wan <kafai.wan@linux.dev>]

child_func() terminates with exit(), but spawn_child_flag() also starts
it via clone(CLONE_VM), which shares the parent's address space. That
exit() then runs atexit handlers and static destructors against globals
the parent is still using.

LLVM commit `3c3ed59c6a2a ("[llvm] Add format check for
MCSubtargetFeatures")` [1], included in the LLVM 23.1 releases, added
Target::isValidFeatureListFormat() with a function-local static holding
the compiled regex:

	static const llvm::Regex pattern("^([+-][^,]+)(,[+-][^,]+)*,?$");
	return pattern.match(Features);

The child exit destroys that static while its guard variable still marks
it as initialized, so the regex is never rebuilt in the parent. From that
point on, get_jited_program_text() gets NULL from
LLVMCreateDisasmCPUFeatures(), so every jited program text check in the
tests running after filter_clone_vm fails, e.g. "verifier_jit_convergence",
"verifier_private_stack", "verifier_tailcall_jit" and "verifier_jit_inline".

Use _exit(), which runs neither atexit handlers nor static destructors, as
uprobe_syscall.c already does for its own CLONE_VM child.

[1] https://github.com/llvm/llvm-project/pull/180943
Fixes: d2520bdb1932f ("selftests/bpf: Add uprobe multi pid filter test for clone-ed processes")
Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
---
 tools/testing/selftests/bpf/prog_tests/uprobe_multi_test.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/uprobe_multi_test.c b/tools/testing/selftests/bpf/prog_tests/uprobe_multi_test.c
index f0baf5738b751..7fbcfddc3cc36 100644
--- a/tools/testing/selftests/bpf/prog_tests/uprobe_multi_test.c
+++ b/tools/testing/selftests/bpf/prog_tests/uprobe_multi_test.c
@@ -94,14 +94,14 @@ static int child_func(void *arg)
 	/* wait for parent's kick */
 	err = read(child->go[0], &c, 1);
 	if (err != 1)
-		exit(err);
+		_exit(err);
 
 	uprobe_multi_func_1();
 	uprobe_multi_func_2();
 	uprobe_multi_func_3();
 	usdt_trigger();
 
-	exit(errno);
+	_exit(errno);
 }
 
 static int spawn_child_flag(struct child *child, bool clone_vm)
-- 
2.43.0