netdev - [PATCH 05/15] bpf: avoid gcc-10 stringop-overflow warning

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Date:   Thu, 30 Apr 2020 23:30:47 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Brad Spengler <spender@...ecurity.net>,
        Daniel Borkmann <dborkman@...hat.com>,
        Alexei Starovoitov <ast@...mgrid.com>,
        Kees Cook <keescook@...omium.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiri Olsa <jolsa@...nel.org>,
        Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@...el.com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: [PATCH 05/15] bpf: avoid gcc-10 stringop-overflow warning

gcc-10 warns about accesses to zero-length arrays:

kernel/bpf/core.c: In function 'bpf_patch_insn_single':
cc1: warning: writing 8 bytes into a region of size 0 [-Wstringop-overflow=]
In file included from kernel/bpf/core.c:21:
include/linux/filter.h:550:20: note: at offset 0 to object 'insnsi' with size 0 declared here
  550 |   struct bpf_insn  insnsi[0];
      |                    ^~~~~~

In this case, we really want to have two flexible-array members,
but that is not possible. Removing the union to make insnsi a
flexible-array member while leaving insns as a zero-length array
fixes the warning, as nothing writes to the other one in that way.

This trick only works on linux-3.18 or higher, as older versions
had additional members in the union.

Fixes: 60a3b2253c41 ("net: bpf: make eBPF interpreter images read-only")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 include/linux/filter.h | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index af37318bb1c5..73d06a39e2d6 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -545,10 +545,8 @@ struct bpf_prog {
 	unsigned int		(*bpf_func)(const void *ctx,
 					    const struct bpf_insn *insn);
 	/* Instructions for interpreter */
-	union {
-		struct sock_filter	insns[0];
-		struct bpf_insn		insnsi[0];
-	};
+	struct sock_filter	insns[0];
+	struct bpf_insn		insnsi[];
 };
 
 struct sk_filter {
-- 
2.26.0