musl - Re: thoughts on reallocarray, explicit

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Message-ID: <CAPLrYERvsFF=0UN-gRchd7zY0ViKNf6QA5bt7Oj5mGcxpLzbdA@mail.gmail.com>
Date: Mon, 19 May 2014 18:45:08 +0200
From: Daniel Cegiełka <daniel.cegielka@...il.com>
To: musl@...ts.openwall.com
Subject: Re: thoughts on reallocarray, explicit_bzero?

2014-05-19 18:25 GMT+02:00 Szabolcs Nagy <nsz@...t70.net>:

> i don't see how the openbsd explicit_bzero stops the
> compiler to do optimizations..
>
> (i guess they rely on that their gcc does not do lto
> or that libc is dynamic linked and the compiler has no
> 'explicit_bzero' builtin, neither of which is a great
> solution..)
>
> the usual approach to this is volatile function pointer:
>
> static void *(*volatile force_memset)(void,int,size_t) = memset;
>
> in general in c one cannot be sure that the secret bits
> are not leaked somewhere since the languge spec cannot
> give such guarantees
>
> that said either the volatile funcptr or actually reusing
> the memory such that it cannot be optimized away works in
> practice

first version:

void explicit_bzero(void * const b, const size_t l)
{
    volatile unsigned char *p = (volatile unsigned char *) b;
    size_t i = (size_t) 0U;

    while (i < l) {
        p[i++] = 0U;
    }
}

Of course, if someone has better ideas... I'm very curious :)

Daniel

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