bpf: Let bpf_iter_task_new accept null task ptr

When using task_iter to iterate all threads of a specific task, we enforce
that the user must pass a valid task pointer to ensure safety. However,
when iterating all threads/process in the system, BPF verifier still
require a valid ptr instead of "nullable" pointer, even though it's
pointless, which is a kind of surprising from usability standpoint. It
would be nice if we could let that kfunc accept a explicit null pointer
when we are using BPF_TASK_ITER_ALL_{PROCS, THREADS} and a valid pointer
when using BPF_TASK_ITER_THREAD.

Given a trival kfunc:
	__bpf_kfunc void FN(struct TYPE_A *obj);

BPF Prog would reject a nullptr for obj. The error info is:
"arg#x pointer type xx xx must point to scalar, or struct with scalar"
reported by get_kfunc_ptr_arg_type(). The reg->type is SCALAR_VALUE and
the btf type of ref_t is not scalar or scalar_struct which leads to the
rejection of get_kfunc_ptr_arg_type.

This patch add "__nullable" annotation:
	__bpf_kfunc void FN(struct TYPE_A *obj__nullable);
Here __nullable indicates obj can be optional, user can pass a explicit
nullptr or a normal TYPE_A pointer. In get_kfunc_ptr_arg_type(), we will
detect whether the current arg is optional and register is null, If so,
return a new kfunc_ptr_arg_type KF_ARG_PTR_TO_NULL and skip to the next
arg in check_kfunc_args().

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231018061746.111364-7-zhouchuyi@bytedance.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index faa1712c1df5..59e747938bdb 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -976,7 +976,7 @@ __diag_ignore_all("-Wmissing-prototypes",
 		  "Global functions as their definitions will be in vmlinux BTF");
 
 __bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it,
-		struct task_struct *task, unsigned int flags)
+		struct task_struct *task__nullable, unsigned int flags)
 {
 	struct bpf_iter_task_kern *kit = (void *)it;
 
@@ -988,14 +988,17 @@ __bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it,
 	switch (flags) {
 	case BPF_TASK_ITER_ALL_THREADS:
 	case BPF_TASK_ITER_ALL_PROCS:
+		break;
 	case BPF_TASK_ITER_PROC_THREADS:
+		if (!task__nullable)
+			return -EINVAL;
 		break;
 	default:
 		return -EINVAL;
 	}
 
 	if (flags == BPF_TASK_ITER_PROC_THREADS)
-		kit->task = task;
+		kit->task = task__nullable;
 	else
 		kit->task = &init_task;
 	kit->pos = kit->task;