[cfe-dev] -fsanitize=undefined and alignment on x86_64

[Richard Smith]

On Fri, Jan 18, 2013 at 11:14 AM, Martin Martin
<martin at silverliningsystems.com> wrote:
> This is essentially what I was doing.
>
> So that's potentially bad even on x86_64, because optimizations are free to
> assume the lower order bits of the pointer are zero?

Yes, exactly.

> On Fri, Jan 18, 2013 at 2:04 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>>
>> On Fri, Jan 18, 2013 at 4:21 AM, Martin Martin
>> <martin at silverliningsystems.com> wrote:
>> > Hi all,
>> >
>> > sanitize=undefined is giving me an error message about unaligned access
>> > to a
>> > size_t.  It's in a packed structure, and indeed is not 8 byte aligned
>> > (although it is 4 byte aligned.)  This is in Linux on an x86_64
>> > architecture.
>> >
>> > My question is: why is this undefined behavior?  Can't modern Intel and
>> > AMD
>> > processors accesses unaligned ints, although with a possible performance
>> > penalty?  So this would be a performance problem, but not correctness?
>> > Is
>> > the problem that the compiler could use SSE or some other instructions
>> > that
>> > require alignment and will seg fault or give wrong results with
>> > unaligned
>> > access?
>>
>> Please can you provide a code sample which triggers the issue? Packed
>> structs are supported by -fsanitize=alignment. However, note that
>> taking the address of a misaligned member of a packed struct will not
>> in general work:
>>
>> struct __attribute__((packed)) S {
>>   char c;
>>   long l;
>> } s;
>> int f() { return s.l; } // ok
>> int g() { return *&s.l; } // undefined behavior: long* access requires
>> 8 byte alignment
>
>