On 04/29/2015 04:54 PM, mancha security wrote:
On Wed, Apr 29, 2015 at 04:01:19PM +0200, Daniel Borkmann wrote:
On 04/29/2015 03:08 PM, mancha security wrote:
...
By the way, has anyone been able to verify that __memory_barrier
provides DSE protection under various optimizations? Unfortunately, I
don't have ready access to ICC at the moment or I'd test it myself.
Never used icc, but it looks like it's free for open source projects;
I can give it a try, but in case you're faster than I am, feel free
to post results here.
Time permitting, I'll try setting this up and post my results.
So I finally got the download link and an eval license for icc, and
after needing to download gigbytes of bloat for the suite, I could
finally start to experiment a bit.
So __GNUC__ and __INTEL_COMPILER is definitely defined by icc, __ECC
not in my case, so that part is as expected for the kernel header
includes.
With barrier_data(), I could observe insns for an inlined memset()
being emitted in the disassembly, same with barrier(), same with
__memory_barrier(). In fact, even if I only use ...
static inline void memzero_explicit(void *s, size_t count)
{
memset(s, 0, count);
}
int main(void)
{
char buff[20];
memzero_explicit(buff, sizeof(buff));
return 0;
}
... icc will emit memset instrinsic insns (did you notice that as
well?) when using various optimization levels. Using f.e. -Ofast
-ffreestanding resp. -fno-builtin-memset will emit a function call,
presumably, icc is then not allowed to make any assumptions, so given
the previous result, then would then be expected.
So, crafting a stupid example:
static inline void
dumb_memset(char *s, unsigned char c, size_t n)
{
int i;
for (i = 0; i < n; i++)
s[i] = c;
}
static inline void memzero_explicit(void *s, size_t count)
{
dumb_memset(s, 0, count);
<barrier-variant>
}
int main(void)
{
char buff[20];
memzero_explicit(buff, sizeof(buff));
return 0;
}
With no barrier at all, icc optimizes all that away (using -Ofast),
with barrier_data() it inlines and emits additional mov* insns!
Just using barrier() or __memory_barrier(), we end up with the same
case as with clang, that is, it gets optimized away. So, barrier_data()
seems to be better here as well.
Cheers,
Daniel
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