Re: [PATCH] flex-array: allow arrays of unions with flexible members.

[Ilya Maximets <i.maximets@ovn.org>]

On 10/8/20 1:15 AM, Luc Van Oostenryck wrote:
> On Wed, Oct 07, 2020 at 01:52:34PM +0200, Ilya Maximets wrote:
>> The actual code in question that makes sparse fail
>> OVS build could be found here:
>>   https://github.com/openvswitch/ovs/blob/39fbd2c3f0392811689ec780f09baf90faceb877/lib/netdev-linux.c#L1238
> 
> I'm impressed and surprised you're using of includes just for Sparse.
> I also see that this is since 2011. Just for my curiosity, have
> you an idea for why exactly this was needed and if it is still
> really needed?

There are some Sparse-related headers that could be safely removed now
because required functionality is already supported by Sparse.  We need
to clean this up someday, e.g. OVS builds fine without
include/sparse/threads.h since Sparse 0.5.1.

However, there are headers that are necessary for successful build.
There are few classes of issues that these headers are targeted on:

1. Missing functionality in Sparse.
For example, it doesn't know about __builtin_ia32_pause, so we have to have
include/sparse/xmmintrin.h.
Sparse also doesn't know __atomic_load_n that comes from some DPDK headers.
OVS itself avoids using builtin atomics if __CHECKER__ defined.
DPDK library also has some issues with types in __sync_add_and_fetch, but I
do not remember exact problem.  We have include/sparse/rte_atomic.h for it.
These are from the top of my head.  I could go through our specific headers
and make a list of missing features someday if you're interested.

2. Sparse complains on standard libraries.
Complains on PTHREAD_MUTEX_INITIALIZER: 
    error: Using plain integer as NULL pointer
So, we have to have include/sparse/pthread.h.
Maybe some other examples, but I do not remember right now.

3. Issues inside external libraries.
Ex. numa.h header from libnuma contains non-ANSI function declarations.
So we have include/sparse/numa.h.

4. Issues with restricted types (these are heaviest).
OVS uses restricted types like 'ovs_be32' inside (with __attribute__((bitwise))),
but standard functions like htonl() operates with usual 'uint32_t'.  While it's
safe to use these exact functions, Sparse complains about type mismatch.  One
option here is to explicitly cast arguments with (OVS_FORCE ovs_be32) each time,
but that is impractical (too many occurrences, ~4K lines of code only for hton/ntoh
conversions) and will make code less readable.  Much easier to override these
functions just for Sparse.  Ex. include/sparse/netinet/in.h.

Similar issue with some data types that goes from external libraries and system
headers. e.g. DPDK library operates with its own types like rte_be32_t.  While
it's completely safe to mix them with ovs_be32, Sparse doesn't know about that,
because DPDK doesn't mark them as bitwise.  This issue might be fixed on DPDK
side, I guess.  But Sparse will complain about different types even if these types
defined in exactly same way.  e.g. following test fails:

diff --git a/validation/bitwise-cast.c b/validation/bitwise-cast.c
index 0583461c..9284bd05 100644
--- a/validation/bitwise-cast.c
+++ b/validation/bitwise-cast.c
@@ -35,6 +35,18 @@ static __be32 quuy(void)
        return (__attribute__((force)) __be32) 1730;
 }
 
+/* Implicit casts of equally defined types, should be legal? */
+typedef u32 __attribute__((bitwise)) __my_be32_1;
+typedef u32 __attribute__((bitwise)) __my_be32_2;
+
+static __my_be32_1 my_type(void)
+{
+       __my_be32_2 x = (__attribute__((force)) __my_be32_2) 0x2a;
+
+       return x;
+}
+
+
 /*
  * check-name: conversions to bitwise types
  * check-command: sparse -Wbitwise $file
---

bitwise-cast.c:46:16: warning: incorrect type in return expression (different base types)
bitwise-cast.c:46:16:    expected restricted __my_be32_1
bitwise-cast.c:46:16:    got restricted __my_be32_2 [usertype] x


This might be not a full list of issues we have, but this is what I can remember right now.

Best regards, Ilya Maximets.