[PATCH] libbpf hashmap: fix undefined behavior in hash_bits

From: Ian Rogers <irogers@google.com>
To: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	netdev@vger.kernel.org, bpf@vger.kernel.org,
	linux-kernel@vger.kernel.org
Cc: Ian Rogers <irogers@google.com>
Subject: [PATCH] libbpf hashmap: fix undefined behavior in hash_bits
Date: Thu,  7 May 2020 23:39:54 -0700	[thread overview]
Message-ID: <20200508063954.256593-1-irogers@google.com> (raw)

If bits is 0, the case when the map is empty, then the >> is the size of
the register which is undefined behavior - on x86 it is the same as a
shift by 0. Fix by handling the 0 case explicitly.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/lib/bpf/hashmap.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/lib/bpf/hashmap.h b/tools/lib/bpf/hashmap.h
index d5ef212a55ba..781db653d16c 100644
--- a/tools/lib/bpf/hashmap.h
+++ b/tools/lib/bpf/hashmap.h
@@ -19,6 +19,8 @@
 static inline size_t hash_bits(size_t h, int bits)
 {
 	/* shuffle bits and return requested number of upper bits */
+	if (bits == 0)
+		return 0;
 	return (h * 11400714819323198485llu) >> (__WORDSIZE - bits);
 }
 
-- 
2.26.2.645.ge9eca65c58-goog

