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Re: [PATCH 1/2] explicit_bzero-tests: pacify GCC

From: Bruno Haible
Subject: Re: [PATCH 1/2] explicit_bzero-tests: pacify GCC
Date: Sun, 18 Jul 2021 11:12:53 +0200
User-agent: KMail/5.1.3 (Linux/4.4.0-210-generic; KDE/5.18.0; x86_64; ; )

Hi Paul,

> diff --git a/tests/test-explicit_bzero.c b/tests/test-explicit_bzero.c
> index cdb839245..c42aba93f 100644
> --- a/tests/test-explicit_bzero.c
> +++ b/tests/test-explicit_bzero.c
> @@ -126,12 +126,12 @@ test_heap (void)
>  /* There are two passes:
>       1. Put a secret in memory and invoke explicit_bzero on it.
>       2. Verify that the memory has been erased.
> -   Implement them in the same function, so that they access the same memory
> -   range on the stack.  */
> +   Access the memory via a volatile pointer, so the compiler
> +   does not assume the pointer's value and optimize away accesses.  */
> +static char *volatile stackbuf;
>  do_secret_stuff (volatile int pass)
>  {
> -  char stackbuf[SECRET_SIZE];
>    if (pass == 1)
>      {
>        memcpy (stackbuf, SECRET, SECRET_SIZE);

I disagree with this change, as it significantly reduces the strength of the

The purpose of the test is to verify that the compiler does not eliminate
a call to explicit_bzero, even if data flow analysis reveals that the stack
area is "dead" at the end of the function.

With this patch, it was turned into a weaker assertion: namely, that the
compiler does not eliminate a call to explicit_bzero if it cannot make
inferences about the pointer argument.

I would suggest to revert this patch, and instead use a #pragma, like you
did in the test-memrchr.c patch.


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