2024年9月28日 06:18，Jonas Oberhauser <[email protected]> wrote：
>
>
>
> Am 9/27/2024 um 10:10 PM schrieb Mathieu Desnoyers:
>> On 2024-09-27 21:23, Jonas Oberhauser wrote:
>> [...]
>>> That idea seems to be confirmed by this (atrocious, not to be copied!)
>>> example:
>>>
>>> int fct_escape_address_of_b(void)
>>> {
>>> int *a, *b;
>>>
>>> do {
>>> a = READ_ONCE(p);
>>> asm volatile ("" : : : "memory");
>>> b = READ_ONCE(p);
>>> } while (a != b);
>>>
>>> // really really hide b
>>> int **p = &b;
>>> OPTIMIZER_HIDE_VAR(p);
>>>
>>> asm volatile ("" : : : "memory");
>>> return *b;
>>> }
>>>
>>> This also does not generate any additional instructions, unlike just using
>>> OPTIMIZER_HIDE_VAR(b).
>>>
>>> What is the advantage of defining OPTIMIZE_HIDE_VAR the way it currently
>>> works instead of like above?
>> Did you try it on godbolt.org ? Does it have the intended effect ?
>
> I certainly did try and certainly read it as having the intended effect,
> otherwise I wouldn't have written that it seems confirmed.
>
> However, just because my eyes read it doesn't mean that's what happened, and
> even if it happened doesn't mean that it is guaranteed to happen.
>
>> By the looks of it, you're just creating another version of @b called
>> "p", which is then never used and would be discarded by further
>> optimization. >
>> I'm unsure what you are trying to achieve here.
>
> Simply put I'm trying to let the compiler think that I leaked the address of
> b. After that, the memory barrier should let it think that the b after the
> memory barrier might not be the same as the one before it (which was equal to
> a), forcing it to read from b.
>
> However, I suppose on second thought that that might not be enough, because
> the compiler could still simply do b = a right after exiting the while loop.
>
> And that is true no matter what we put behind the while loop or before the
> condition, as long as the condition compares a and b, right after it the
> compiler can do b = a. Just took me a while to see :))
>
> I'm not sure why gcc does the b=a with the normal OPTIMIZER_HIDE_VAR but (as
> far as I read the code) doesn't do it with the above. Maybe just a weird
> corner case...
Let the p to be a static variable out of the function will make a difference.
Or the following:
int **p = &b;
barrier_data(p);
also works.
BTW, barrier_data(&b) generates more instructions than godbolt when build the
kernel.
>
> Have fun,
> jonas
>
>
