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[ Upstream commit7a078d2d18] 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 and guarding calls to hash_bits for empty maps in hashmap__for_each_key_entry and hashmap__for_each_entry_safe. Fixes:e3b9242240("libbpf: add resizable non-thread safe internal hashmap") Suggested-by: Andrii Nakryiko <andriin@fb.com>, Signed-off-by: Ian Rogers <irogers@google.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: Andrii Nakryiko <andrii@kernel.org> Acked-by: Song Liu <songliubraving@fb.com> Link: https://lore.kernel.org/bpf/20201029223707.494059-1-irogers@google.com Signed-off-by: Sasha Levin <sashal@kernel.org>