Re: thoughts on reallocarray, explicit_bzero?

["Daniel Cegiełka" <daniel.cegielka@gmail.com>]

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2014-05-19 18:16 GMT+02:00 Rich Felker <dalias@libc.org>:
> On Mon, May 19, 2014 at 05:44:59PM +0200, Daniel Cegiełka wrote:

>> diff -urN musl.orig/src/string/explicit_bzero.c musl/src/string/explicit_bzero.c
>> --- musl.orig/src/string/explicit_bzero.c     Thu Jan  1 00:00:00 1970
>> +++ musl/src/string/explicit_bzero.c  Fri May  9 09:57:45 2014
>> @@ -0,0 +1,8 @@
>> +#include <string.h>
>> +
>> +static void *(*volatile explicit_memset)(void *, int, size_t) = memset;
>> +
>> +void explicit_bzero(void *b, size_t len)
>> +{
>> +     (*explicit_memset)(b, 0, len);
>> +}
>
> This is a nice trick, but IIRC I actually observed GCC optimizing out
> similar code before (instead of your static volatile, I used a
> volatile compound literal). At least the concept is right though: you
> want to prevent the compiler from being able to do any flow analysis
> at compile time, and making the function pointer volatile achieves
> this rather well. On the other hand, GCC will put the volatile pointer
> (if it even emits it) in non-constant memory, meaning it's an
> additional attack vector for function-pointer-overwrite attacks.

Linux kernel has similar functions and uses a barrier() here:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/lib/string.c?id=refs/tags/v3.19-rc6#n600

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/linux/compiler.h?id=refs/tags/v3.19-rc6#n162

Is such a solution is more correct (and still portable)?

Daniel

> Rich

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#include <string.h>

static void *(*volatile explicit_memset)(void *, int, size_t) = memset;

void explicit_bzero(void *b, size_t len)
{
	(*explicit_memset)(b, 0, len);
	__asm__ volatile ( "" : : "r" (b) : "memory" );
}

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#include <string.h>

void explicit_bzero(void *b, size_t len)
{
	memset(b, 0, len);
	__asm__ volatile ( "" : : "r" (b) : "memory" );
}