netdev - [PATCH bpf] libbpf: sanitize VAR to conservative 1-byte INT

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Date:   Fri, 19 Jul 2019 12:46:03 -0700
From:   Andrii Nakryiko <andriin@...com>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...com>,
        <daniel@...earbox.net>, <rdna@...com>
CC:     <andrii.nakryiko@...il.com>, <kernel-team@...com>,
        Andrii Nakryiko <andriin@...com>
Subject: [PATCH bpf] libbpf: sanitize VAR to conservative 1-byte INT

If VAR in non-sanitized BTF was size less than 4, converting such VAR
into an INT with size=4 will cause BTF validation failure due to
violationg of STRUCT (into which DATASEC was converted) member size.
Fix by conservatively using size=1.

Signed-off-by: Andrii Nakryiko <andriin@...com>
---
 tools/lib/bpf/libbpf.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 87168f21ef43..d8833ff6c4a1 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -1377,8 +1377,13 @@ static void bpf_object__sanitize_btf(struct bpf_object *obj)
 		if (!has_datasec && kind == BTF_KIND_VAR) {
 			/* replace VAR with INT */
 			t->info = BTF_INFO_ENC(BTF_KIND_INT, 0, 0);
-			t->size = sizeof(int);
-			*(int *)(t+1) = BTF_INT_ENC(0, 0, 32);
+			/*
+			 * using size = 1 is the safest choice, 4 will be too
+			 * big and cause kernel BTF validation failure if
+			 * original variable took less than 4 bytes
+			 */
+			t->size = 1;
+			*(int *)(t+1) = BTF_INT_ENC(0, 0, 8);
 		} else if (!has_datasec && kind == BTF_KIND_DATASEC) {
 			/* replace DATASEC with STRUCT */
 			struct btf_var_secinfo *v = (void *)(t + 1);
-- 
2.17.1