netdev - [PATCH v3 bpf-next 2/5] bpf: Mark cgroups and dfl

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Date:   Mon, 27 Feb 2023 20:01:18 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     davem@...emloft.net
Cc:     daniel@...earbox.net, andrii@...nel.org, martin.lau@...nel.org,
        void@...ifault.com, davemarchevsky@...a.com, tj@...nel.org,
        memxor@...il.com, netdev@...r.kernel.org, bpf@...r.kernel.org,
        kernel-team@...com
Subject: [PATCH v3 bpf-next 2/5] bpf: Mark cgroups and dfl_cgrp fields as trusted.

From: Alexei Starovoitov <ast@...nel.org>

bpf programs sometimes do:
bpf_cgrp_storage_get(&map, task->cgroups->dfl_cgrp, ...);
It is safe to do, because cgroups->dfl_cgrp pointer is set diring init and
never changes. The task->cgroups is also never NULL. It is also set during init
and will change when task switches cgroups. For any trusted task pointer
dereference of cgroups and dfl_cgrp should yield trusted pointers. The verifier
wasn't aware of this. Hence in gcc compiled kernels task->cgroups dereference
was producing PTR_TO_BTF_ID without modifiers while in clang compiled kernels
the verifier recognizes __rcu tag in cgroups field and produces
PTR_TO_BTF_ID | MEM_RCU | MAYBE_NULL.
Tag cgroups and dfl_cgrp as trusted to equalize clang and gcc behavior.
When GCC supports btf_type_tag such tagging will done directly in the type.

Signed-off-by: Alexei Starovoitov <ast@...nel.org>
---
 kernel/bpf/verifier.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5cb8b623f639..e4234266e76d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5011,6 +5011,10 @@ static int bpf_map_direct_read(struct bpf_map *map, int off, int size, u64 *val)

 BTF_TYPE_SAFE_NESTED(struct task_struct) {
 	const cpumask_t *cpus_ptr;
+	struct css_set *cgroups;
+};
+BTF_TYPE_SAFE_NESTED(struct css_set) {
+	struct cgroup *dfl_cgrp;
 };

 static bool nested_ptr_is_trusted(struct bpf_verifier_env *env,
@@ -5022,6 +5026,7 @@ static bool nested_ptr_is_trusted(struct bpf_verifier_env *env,
 		return false;

 	BTF_TYPE_EMIT(BTF_TYPE_SAFE_NESTED(struct task_struct));
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_NESTED(struct css_set));

 	return btf_nested_type_is_trusted(&env->log, reg, off);
 }
-- 
2.30.2