On 2019-12-15, Florian Weimer <fw@xxxxxxxxxxxxx> wrote:
> * Aleksa Sarai:
>
> > diff --git a/tools/testing/selftests/openat2/helpers.h b/tools/testing/selftests/openat2/helpers.h
> > index 43ca5ceab6e3..eb1535c8fa2e 100644
> > --- a/tools/testing/selftests/openat2/helpers.h
> > +++ b/tools/testing/selftests/openat2/helpers.h
> > @@ -32,17 +32,16 @@
> > * O_TMPFILE} are set.
> > *
> > * @flags: O_* flags.
> > - * @mode: O_CREAT/O_TMPFILE file mode.
> > * @resolve: RESOLVE_* flags.
> > + * @mode: O_CREAT/O_TMPFILE file mode.
> > */
> > struct open_how {
> > - __aligned_u64 flags;
> > + __u64 flags;
> > + __u64 resolve;
> > __u16 mode;
> > - __u16 __padding[3]; /* must be zeroed */
> > - __aligned_u64 resolve;
> > -};
> > +} __attribute__((packed));
> >
> > -#define OPEN_HOW_SIZE_VER0 24 /* sizeof first published struct */
> > +#define OPEN_HOW_SIZE_VER0 18 /* sizeof first published struct */
> > #define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER0
>
> A userspace ABI that depends on GCC extensions probably isn't a good
> idea. Even with GCC, it will not work well with some future
> extensions because it pretty much rules out having arrays or other
> members that are access through pointers. Current GCC does not carry
> over the packed-ness of the struct to addresses of its members.
Right, those are also good points.
Okay, I'm going to send a separate patch which changes the return value
for invalid __padding to -E2BIG, and moves the padding to the end of the
struct (along with open_how.mode). That should fix all of the warts I
raised, without running into the numerous problems with
__attribute__((packed)) of which I am now aware.
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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