Re: [PATCH] libbpf hashmap: Fix undefined behavior in hash_bits

[Song Liu]

> On Oct 29, 2020, at 9:09 AM, Ian Rogers <irogers@xxxxxxxxxx> wrote:
> 
> 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 when running with
> address sanitizer.
> 
> A variant of this patch was posted previously as:
> https://lore.kernel.org/lkml/20200508063954.256593-1-irogers@xxxxxxxxxx/
> 
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
> tools/lib/bpf/hashmap.h | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
> 
> diff --git a/tools/lib/bpf/hashmap.h b/tools/lib/bpf/hashmap.h
> index d9b385fe808c..27d0556527d3 100644
> --- a/tools/lib/bpf/hashmap.h
> +++ b/tools/lib/bpf/hashmap.h
> @@ -12,9 +12,23 @@
> #include <stddef.h>
> #include <limits.h>
> 
> +#ifdef __has_feature
> +#define HAVE_FEATURE(f) __has_feature(f)
> +#else
> +#define HAVE_FEATURE(f) 0
> +#endif
> +
> static inline size_t hash_bits(size_t h, int bits)
> {
> 	/* shuffle bits and return requested number of upper bits */
> +#if defined(ADDRESS_SANITIZER) || HAVE_FEATURE(address_sanitizer)

I am not very familiar with these features. Is address sanitizer same
as undefined behavior sanitizer (mentioned in previous version)?

> +	/*
> +	 * If the requested bits == 0 avoid undefined behavior from a
> +	 * greater-than bit width shift right (aka invalid-shift-exponent).
> +	 */
> +	if (bits == 0)
> +		return -1;

Shall we return 0 or -1 (0xffffffff) here? 

Also, we have HASHMAP_MIN_CAP_BITS == 2. Shall we just make sure we
never feed bits == 0 into hash_bits()?

Thanks,
Song


> +#endif
> #if (__SIZEOF_SIZE_T__ == __SIZEOF_LONG_LONG__)
> 	/* LP64 case */
> 	return (h * 11400714819323198485llu) >> (__SIZEOF_LONG_LONG__ * 8 - bits);
> -- 
> 2.29.1.341.ge80a0c044ae-goog
>