On 12/08/2017 02:14 PM, David Laight wrote: > 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. >
Ok, *could* be a bug. > ... >> 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 :-) The gcc developers must have good self-preservation instinct, that's why they usually provide switch off for optimizations like this.
