From: Andrey Ryabinin
> Sent: 08 December 2017 10:49
...
> CONFIG_UBSAN_ALIGNMENT is already disabled by default for
> HAVE_EFFICIENT_UNALIGNED_ACCESS=y because it's noisy,
> but we still allow users to enable it if they want to.
>
> I don't think we should completely forbid enabling it for
> HAVE_EFFICIENT_UNALIGNED_ACCESS=y.
> Unaligned access is still a bug in non-arch code and outside of sections like
> #ifdef HAVE_EFFICIENT_UNALIGNED_ACCESS .. #endif .
Don't think so.
Code that knows that unaligned accesses don't fault can set up pointers
that non-arch code dereferences.
Happens all the time in the networking stack.
...
> And one day, GCC might start doing optimizations based on this, e.g.:
>
> u64 *ptr;
> ...
> x = *ptr;
> ...
> if (ptr & 7) // Compiler can assume that this statement is always
> false, because 'ptr' was deferenced, so it must be aligned
> do_something();
Ugg - shoot the gcc developers :-)
David
