Re: [PATCH 04/24] 32-bit userspace ABI: introduce ARCH_32BIT_OFF_T config option

[Yury Norov <ynorov@caviumnetworks.com>]

On Fri, Jun 08, 2018 at 03:33:51PM -0700, Palmer Dabbelt wrote:
> On Fri, 08 Jun 2018 10:32:07 PDT (-0700), catalin.marinas@arm.com wrote:
> > On Wed, May 16, 2018 at 11:18:49AM +0300, Yury Norov wrote:
> > > diff --git a/arch/Kconfig b/arch/Kconfig
> > > index 76c0b54443b1..ee079244dc3c 100644
> > > --- a/arch/Kconfig
> > > +++ b/arch/Kconfig
> > > @@ -264,6 +264,21 @@ config ARCH_THREAD_STACK_ALLOCATOR
> > >  config ARCH_WANTS_DYNAMIC_TASK_STRUCT
> > >  	bool
> > > 
> > > +config ARCH_32BIT_OFF_T
> > > +	bool
> > > +	depends on !64BIT
> > > +	help
> > > +	  All new 32-bit architectures should have 64-bit off_t type on
> > > +	  userspace side which corresponds to the loff_t kernel type. This
> > > +	  is the requirement for modern ABIs. Some existing architectures
> > > +	  already have 32-bit off_t. This option is enabled for all such
> > > +	  architectures explicitly. Namely: arc, arm, blackfin, cris, frv,
> > > +	  h8300, hexagon, m32r, m68k, metag, microblaze, mips32, mn10300,
> > > +	  nios2, openrisc, parisc32, powerpc32, score, sh, sparc, tile32,
> > > +	  unicore32, x86_32 and xtensa. This is the complete list. Any
> > > +	  new 32-bit architecture should declare 64-bit off_t type on user
> > > +	  side and so should not enable this option.
> > 
> > Do you know if this is the case for riscv and nds32, merged in the
> > meantime? If not, I suggest you drop this patch altogether and just
> > define force_o_largefile() for arm64/ilp32 as we don't seem to stick to
> > "all new 32-bit architectures should have 64-bit off_t".
> 
> We (RISC-V) don't have support for rv32i in glibc yet, so there really isn't
> a fixed ABI there yet.  From my understanding the rv32i port as it currently
> stands has a 32-bit off_t (via __kernel_off_t being defined as long), so
> this change would technically be a kernel ABI break.
> 
> Since we don't have rv32i glibc yet I'm not fundamentally opposed to an ABI
> break.  Is there a concrete advantage to this?

One obvious advantage is manipulating large files - if file is greater than
2G, you cannot easily mmap(), lseek() etc with 32-bit offset.

Another point is unification of layuots for structures like struct
stat between 32- and 64-bit worlds.

On glibc side it helps to unify 32-bit and 64-bit versions of syscalls.
Refer, for example this commit:
3c7f1f59cd161 (Consolidate lseek/lseek64/llseek implementations).

Yury