linux-kernel - Re: [PATCH v2 1/1] samples/seccomp: eliminate two compile warnings in user-trap.c

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Date:   Tue, 8 Sep 2020 16:42:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] samples/seccomp: eliminate two compile warnings
 in user-trap.c

On Wed, Sep 02, 2020 at 09:33:06AM +0800, Leizhen (ThunderTown) wrote:
> On 2020/9/1 16:39, Zhen Lei wrote:
> > samples/seccomp/user-trap.c is compiled with $(userccflags), and the
> > latter does not contain -fno-strict-aliasing, so the warnings reported as
> > below. Due to add "userccflags += -fno-strict-aliasing" will impact other
> > files, so use __attribute__((__may_alias__)) to suppress it exactly.
> > 
> > My gcc version is 5.5.0 20171010.
> > 
> > ----------
> > samples/seccomp/user-trap.c: In function ‘send_fd’:
> > samples/seccomp/user-trap.c:50:2: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
> >   *((int *)CMSG_DATA(cmsg)) = fd;
> >   ^
> > samples/seccomp/user-trap.c: In function ‘recv_fd’:
> > samples/seccomp/user-trap.c:83:2: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
> >   return *((int *)CMSG_DATA(cmsg));
> >   ^
>
> Doesn't anyone care about this? Or is it that everyone hasn't encountered this problem?
> Why do these two warnings occur every time I compiled?

Hi!

I think the samples have been a bit ignored lately because they have a
lot of weird build issues with regard to native vs compat and needing
the kernel headers to be built first, etc.

That said, yes, I'd like to fix warnings. However, I can't reproduce
this. How are you building? I tried x86_64 and cross-compiled to i386.

-- 
Kees Cook