linux-kernel - Re: Commit fcd8843c40 breaks old compilers

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Date:   Sat, 18 Nov 2017 18:12:32 +0000
From:   Trond Myklebust <trondmy@...marydata.com>
To:     "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "Anna.Schumaker@...app.com" <Anna.Schumaker@...app.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: Commit fcd8843c40 breaks old compilers

On Sat, 2017-11-18 at 13:07 -0500, Boris Ostrovsky wrote:
> 
> On 11/18/2017 12:39 PM, Trond Myklebust wrote:
> > On Sat, 2017-11-18 at 12:19 -0500, Boris Ostrovsky wrote:
> > > Commit fcd8843c406b46433857ae45e5e9d84b01a7d20b breaks on older
> > > compilers which cannot process initializers for anonymous
> > > structures:
> > > 
> > > +const nfs4_stateid invalid_stateid = {
> > > +       {
> > > +               .seqid = cpu_to_be32(0xffffffffU),
> > > +               .other = { 0 },
> > > +       },
> > > +       .type = NFS4_INVALID_STATEID_TYPE,
> > > +};
> > > 
> > > 
> > > /home/build/linux-linus/fs/nfs/nfs4state.c:74: error: unknown
> > > field
> > > ‘seqid’ specified in initializer
> > > /home/build/linux-linus/fs/nfs/nfs4state.c:74: warning: missing
> > > braces
> > > around initializer
> > > /home/build/linux-linus/fs/nfs/nfs4state.c:74: warning: (near
> > > initialization for ‘invalid_stateid.<anonymous>.data’)
> > > /home/build/linux-linus/fs/nfs/nfs4state.c:74: warning: overflow
> > > in
> > > implicit constant conversion
> > > /home/build/linux-linus/fs/nfs/nfs4state.c:75: error: unknown
> > > field
> > > ‘other’ specified in initializer
> > > /home/build/linux-linus/fs/nfs/nfs4state.c:75: error: extra brace
> > > group
> > > at end of initializer
> > > /home/build/linux-linus/fs/nfs/nfs4state.c:75: error: (near
> > > initialization for ‘invalid_stateid.<anonymous>’)
> > > /home/build/linux-linus/fs/nfs/nfs4state.c:75: warning: excess
> > > elements
> > > in union initializer
> > > /home/build/linux-linus/fs/nfs/nfs4state.c:75: warning: (near
> > > initialization for ‘invalid_stateid.<anonymous>’)
> > > make[4]: *** [fs/nfs/nfs4state.o] Error 1
> > > make[3]: *** [fs/nfs] Error 2
> > > 
> > > 
> > > FC-64 <build@...ld-mk2:~> gcc --version
> > > gcc (GCC) 4.4.4 20100503 (Red Hat 4.4.4-2)
> > > 
> > > 
> > > A similar bug was fixed by
> > > e0714ec4f9efe7b86828b0dcc077fd8f5d8e5e91
> > > but
> > > I don't think the same approach can work here.
> > 
> > 
> > I don't have any setups with gcc 4.4.4. What is it expecting here?
> > Is
> > it expecting an extra set of braces due to the anonymous "struct"?
> > 
> 
> No, that won't work (at least I couldn't get it to work) because the 
> solution from e0714ec4f9e assumes that the anonymous struct is the
> first 
> one in the enveloping struct.
> 
> It worked only if I (this is a small C program with equivalent
> structs):
> 
> struct nfs4_stateid_struct {
>          union {
>                  //char data[4];
>                  struct {
>                          unsigned seqid;
>                          char other[6];
>                  } __attribute__ ((packed));
> 		char data[4];
>          };
> and then
> 
> const nfs4_stateid invalid_stateid = {
>          {
>                    {.seqid = 0xffffffffU,
>                    .other = { 0 } },
>          },
>          .type = NFS4_INVALID_STATEID_TYPE,
> };
> 
> If I keep data[4] where it is now I get compiler error
> 
> an.c:35:20: error: field name not in record or union initializer
>                     {.seqid = 0xffffffffU,
>                      ^
> an.c:35:20: note: (near initialization for 
> 'invalid_stateid.<anonymous>.data')
> an.c:35:29: warning: overflow in implicit constant conversion [-
> Woverflow]
>                     {.seqid = 0xffffffffU,
>                               ^~~~~~~~~~~
> an.c:36:19: error: field name not in record or union initializer
>                     .other = { 0 } },
>                     ^
> an.c:36:19: note: (near initialization for 
> 'invalid_stateid.<anonymous>.data')
> an.c:36:19: warning: braces around scalar initializer
> an.c:36:19: note: (near initialization for 
> 'invalid_stateid.<anonymous>.data[1]')
> 
> I don't know if you want to change public header file just to get
> around 
> this problem.

Sigh.... OK, how about something like the following then:

{ .data = { 0xff, 0xff, 0xff, 0xff, 0 }, }

-- 
Trond Myklebust
Linux NFS client maintainer, PrimaryData
trond.myklebust@...marydata.com