[PATCH v3] explicit_bzero yet again

[Szabolcs Nagy]

On 07/12/15 14:13, Zack Weinberg wrote:
> On 11/19/2015 10:55 AM, Rich Felker wrote:
>> On Thu, Nov 19, 2015 at 09:46:22AM -0500, Zack Weinberg wrote:
>>> [PATCH 1/3] New library function, explicit_bzero
>>>
>>> The new function explicit_bzero is functionally identical to bzero,
>>> but the compiler will not delete a call to it, even if the memory
>>> region it's applied to is dead after the call.  You should use this
>>> function, for instance, to erase cryptographic keys or similar when
>>> you are done with them.  Taken from OpenBSD.
>>
>> Perhaps glibc explicitly does not support LTO, but with LTO this is
>> trivially removable by the compiler. IMO from the beginning there
>> should be asm constraints to make it impossible to remove the code
>> even if it's inlined.
>
> Presently there *is* no such asm construct (all possibilities either
> don't work sufficiently universally, someone on this list has already
> objected to them, or both).  Therefore I do not think this is feasible.
>
> However, presently no compiler will LTO-optimize across shared library
> boundaries, and even if such a compiler existed I would argue that the
> optimization you contemplate is invalid for functions whose semantics
> are unknown to the compiler, because the library could be swapped out at
> runtime.
>

obviously the concern is about static linking.