Re: [PATCH] cppcheck,valgrind: Don't pass NULL to recv() with MSG_TRUNC

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 15 Nov 2023 15:41:24 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> Usually, of course, it's invalid to pass a NULL buffer to recv().  However,
> it's acceptable when using MSG_TRUNC, because that suppresses actually
> writing to the buffer.  So, we pass NULL in tcp_sock_consume().
> 
> Unfortunately, checker tools aren't always aware of that special case: we
> already have a suppression for cppcheck to cover this.  valgrind-3.22.0
> (present in Fedora 39) has a similar problem, generating a spurious warning
> here.

I haven't tried valgrind 3.22 yet, but... do you happen to know why
test/valgrind.supp doesn't cover this anymore?

> We could generate another suppression for valgrind, however, it so happens
> that we already have tcp_buf_discard ready to hand.  If we pass this
> instead of NULL it makes both cppcheck and valgrind happy.  We're still
> using the MSG_TRUNC flag, the kernel doesn't actually have to copy data,
> so we should still have the performance benefits of it.

I'm not enthusiastic about this, because using tcp_buf_discard there
might tell an optimising compiler that it's useful to prefetch it.

We would also pass the actual address of tcp_buf_discard to the kernel,
and I'm not sure if this has further subtle implications on possible
optimisations in the kernel implementation (even though as you said no
data is actually copied).

-- 
Stefano