Re: [fuse-devel] Handling of 32/64 bit off_t by getdents64()

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, Nov 27, 2019 at 9:52 PM Nikolaus Rath <Nikolaus@rath.org> wrote:
>
> On Nov 27 2019, Miklos Szeredi <miklos@szeredi.hu> wrote:
> >> Is there a way for a 64 bit process (in this case the FUSE daemon) to
> >> ask for 32 bit d_off values from getdents64()?
> >
> > Looking at ext4 d_off encoding, it looks like the simple workaround is
> > to use the *high* 32 bits of the offset.
> >
> > Just tried, and this works.  The lower bits are the "minor" number of
> > the offset, and no issue with zeroing those bits out, other than
> > increasing the chance of hash collision from practically zero to very
> > close to zero.
> >
> >> Would it be feasible to extend the FUSE protocol to include information
> >> about the available bits in d_off?
> >
> > Yes.
> >
> > The relevant bits from ext4 are:
> >
> > static inline int is_32bit_api(void)
> > {
> > #ifdef CONFIG_COMPAT
> >     return in_compat_syscall();
> > #else
> >     return (BITS_PER_LONG == 32);
> > #endif
> > }
>
> Thanks for the quick response!
>
> Is there a way to do the same without relying on ext4 internals, i.e. by
> manually calling getdents64() in such a way that in_compat_syscall()
> gives true even if the caller is 64 bit?

Generally that's not doable.   Might be able to do a 32bit syscall
specifically on x86_64, but I don't know the details.

Thanks,
Miklos