RE: [PATCH][next] UAPI: nfsfh.h: Replace one-element array with flexible-array member

[David Laight <David.Laight@ACULAB.COM>]

From: Gustavo A. R. Silva
> Sent: 25 March 2021 21:12
> 
> On 3/25/21 10:29, David Laight wrote:
> 
> >>>
> >>> Could you use the simpler:
> >>>> struct nfs_fhbase_new {
> >>>>          __u8       fb_version;
> >>>>          __u8       fb_auth_type;
> >>>>          __u8       fb_fsid_type;
> >>>>          __u8       fb_fileid_type;
> >>>>          union {
> >>>>                 __u32      fb_auth[1];
> >>>>                 __u32      fb_auth_flex[0];
> >>>>          };
> >>>> };
> >>>
> >>> Although I'm not certain flexible arrays are supported
> >>> as the last element of a union.
> >>
> >> Nope; this is not allowed: https://godbolt.org/z/14vd4o8na
> >
> > Nothing an extra 'struct {__u32 fb_auth_flex[0]; }'; won't solve.
> 
> We don't want to introduce zero-length arrays [1].

I probably meant to write [] not [0] - doesn't affect the idea.

The real problem is that the compiler is likely to start rejecting
references to a flex array that go beyond the end of the outer
structure.

Thinking back, isn't fb_auth[] at least one entry long?
So it could be:

struct nfs_fhbase_new {
         __u8       fb_version;
         __u8       fb_auth_type;
         __u8       fb_fsid_type;
         __u8       fb_fileid_type;
         __u32      fb_auth[1];
         __u32      fb_auth_extra[];
};

(I've missed the 0 out this time...)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)