Re: [Qemu-devel] [PATCH v2] bitops.h: Add field32() and field64() functions to extract bitfields

[Eric Blake]

On 06/27/2012 04:29 AM, Peter Maydell wrote:
> Add field32() and field64() functions which extract a particular
> bit field from a word and return it. Based on an idea by Jia Liu.
> 

> +static inline uint64_t field64(uint64_t value, int start, int length)
> +{
> +    assert(start >= 0 && start <= 63 && length > 0 && start + length <= 64);

You're failing to account for wraparound:

field64(value, 63, MAX_INT)

gives undefined behavior in the addition, and even on (most) platforms
where it silently wraps around to a negative number, you have then
missed triggering the assert and proceed to do more unefined behavior
with a negative shift.  You can solve that, and use one less conjunct,
by using:

assert(start >= 0 && length > 0 && (unsigned) start + length <= 64);

Same comment for field32().

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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