On Fri, Jul 31, 2020 at 03:27:12PM -0300, Jason Gunthorpe wrote:

On Fri, Jul 31, 2020 at 07:19:24PM +0200, Greg Kroah-Hartman wrote:

quoted

quoted

I tried for a bit and didn't find a way to get even old gcc 4.4 to not
initialize the holes.

Odd, so it is just the "= {0};" that does not zero out the holes?

Nope, it seems to work fine too. I tried a number of situations and I
could not get the compiler to not zero holes, even back to gcc 4.4

It is not just accidental either, take this:

	struct rds_rdma_notify {
		unsigned long user_token;
		unsigned char status;
		unsigned long user_token1 __attribute__((aligned(32)));
	} foo = {0};

Which has quite a big hole, clang generates:

	movq	$0, 56(%rdi)
	movq	$0, 48(%rdi)
	movq	$0, 40(%rdi)
	movq	$0, 32(%rdi)
	movq	$0, 24(%rdi)
	movq	$0, 16(%rdi)
	movq	$0, 8(%rdi)
	movq	$0, (%rdi)

Deliberate extra instructions to fill both holes. gcc 10 does the
same, older gcc's do create a rep stosq over the whole thing.

Some fiddling with godbolt shows quite a variety of output, but I
didn't see anything that looks like a compiler not filling
padding. Even godbolt's gcc 4.1 filled the padding, which is super old.

In several cases it seems the aggregate initializer produced better
code than memset, in other cases it didn't

Without an actual example where this doesn't work right it is hard to
say anything more..

Here is the example that set off the recent patches:

https://lkml.org/lkml/2020/7/27/199

Another example is commit 5ff223e86f5a ("net: Zeroing the structure
ethtool_wolinfo in ethtool_get_wol()").  I tested this one with GCC 7.4
at the time and it was a real life bug.

The rest of these patches were based on static analysis from Smatch.
They're all "theoretical" bugs based on the C standard but it's
impossible to know if and when they'll turn into real life bugs.

It's not a super long list of code that's affected because we've known
that the bug was possible for a few years.  It was only last year when
I saw that it had become a real life bug.

regards,
dan carpenter