Spectre V1 defense in GCC

Spectre V1 defense in GCC

The docs part of the patches provide a better description of the actual behavior. It always returns the given value when executing non-speculatively. When executing speculatively, it either blocks until all speculation is resolved, or if the architecture supports it, returns the fallback value if there is outstanding speculation that could turn out to be incorrect. This basically means that in the speculative scenarios that could cause issues, you will just work with the dummy value, and the speculation will be useless but safe.

+(speculation_safe_value,
+"This target hook can be used to generate a target-specific code\n\
+ sequence that implements the @code{__builtin_speculation_safe_value}\n\
+ built-in function.  The function must always return @var{val} in\n\
+ @var{result} in mode @var{mode} when the cpu is not executing\n\
+ speculatively, but must never return that when speculating until it\n\
+ is known that the speculation will not be unwound.  The hook supports\n\
+ two primary mechanisms for implementing the requirements.  The first\n\
+ is to emit a speculation barrier which forces the processor to wait\n\
+ until all prior speculative operations have been resolved; the second\n\
+ is to use a target-specific mechanism that can track the speculation\n\
+ state and to return @var{failval} if it can determine that\n\
+ speculation must be unwound at a later time.\n\
+ \n\
+ The default implementation simply copies @var{val} to @var{result} and\n\
+ emits a @code{speculation_barrier} instruction if that is defined.  If\n\
+ @code{speculation_barrier} is not defined for the target a warning will\n\
+ be generated.",

There are some examples later in the thread that shows how this can be used, such as the following. This is written with the assumption that mem[0] is a safe, if potentially incorrect, value to return:

void *mem;

void* f(unsigned untrusted)
{
  if (untrusted < 100)
    return mem[__builtin_speculation_safe_value (untrusted)];
  return NULL;
}