linux-kernel - [RFC PATCH bpf-next 2/6] bpf: btf->resolved_[ids,sizes] should not be used for vmlinux BTF

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Date:   Fri, 17 Apr 2020 11:42:36 +0100
From:   Alan Maguire <alan.maguire@...cle.com>
To:     ast@...nel.org, daniel@...earbox.net, yhs@...com
Cc:     kafai@...com, songliubraving@...com, andriin@...com,
        john.fastabend@...il.com, kpsingh@...omium.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Alan Maguire <alan.maguire@...cle.com>
Subject: [RFC PATCH bpf-next 2/6] bpf: btf->resolved_[ids,sizes] should not be used for vmlinux BTF

When testing support for print data structures using patches
later in this series, I hit NULL pointer dereference bugs when
printing "struct sk_buff". The problem seems to revolve around
that structure's use of a zero-length array in the middle of
the data structure - headers_start[0].

We see in btf_type_id_size() we consult btf->resolved_ids and
btf->resolved_sizes; both of which are not used in kernel
vmlinux BTF so should not be used when handling vmlinux
BTF data.

Signed-off-by: Alan Maguire <alan.maguire@...cle.com>
---
 kernel/bpf/btf.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 50080ad..a474839 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -1170,7 +1170,7 @@ const struct btf_type *btf_type_id_size(const struct btf *btf,
 
 	if (btf_type_has_size(size_type)) {
 		size = size_type->size;
-	} else if (btf_type_is_array(size_type)) {
+	} else if (btf_type_is_array(size_type) && btf->resolved_sizes) {
 		size = btf->resolved_sizes[size_type_id];
 	} else if (btf_type_is_ptr(size_type)) {
 		size = sizeof(void *);
@@ -1179,6 +1179,9 @@ const struct btf_type *btf_type_id_size(const struct btf *btf,
 				 !btf_type_is_var(size_type)))
 			return NULL;
 
+		if (!btf->resolved_ids)
+			return NULL;
+
 		size_type_id = btf->resolved_ids[size_type_id];
 		size_type = btf_type_by_id(btf, size_type_id);
 		if (btf_type_nosize_or_null(size_type))
-- 
1.8.3.1