linux-kernel - Re: [PATCH] uapi: Make __{u,s}64 match {u,}int64

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Date:   Tue, 23 Nov 2021 10:14:15 +0100
From:   Cyril Hrubis <chrubis@...e.cz>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        LTP List <ltp@...ts.linux.it>,
        GNU C Library <libc-alpha@...rceware.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH] uapi: Make __{u,s}64 match {u,}int64_t in userspace

Hi!
> > +#include <asm/bitsperlong.h>
> > +
> >  /*
> > - * int-ll64 is used everywhere now.
> > + * int-ll64 is used everywhere in kernel now.
> >   */
> > -#include <asm-generic/int-ll64.h>
> > +#if __BITS_PER_LONG == 64 && !defined(__KERNEL__)
> > +# include <asm-generic/int-l64.h>
> > +#else
> > +# include <asm-generic/int-ll64.h>
> > +#endif
> 
> I don't think this is correct on all 64-bit architectures, as far as I
> remember the
> definition can use either 'long' or 'long long' depending on the user space
> toolchain.

As far as I can tell the userspace bits/types.h does exactly the same
check in order to define uint64_t and int64_t, i.e.:

#if __WORDSIZE == 64
typedef signed long int __int64_t;
typedef unsigned long int __uint64_t;
#else
__extension__ typedef signed long long int __int64_t;
__extension__ typedef unsigned long long int __uint64_t;
#endif

The macro __WORDSIZE is defined per architecture, and it looks like the
defintions in glibc sources in bits/wordsize.h match the uapi
asm/bitsperlong.h. But I may have missed something, the code in glibc is
not exactly easy to read.

> Out of the ten supported 64-bit architectures, there are four that already
> use asm-generic/int-l64.h conditionally, and six that don't, and I
> think at least
> some of those are intentional.
>
> I think it would be safer to do this one architecture at a time to make
> sure this doesn't regress on those that require the int-ll64.h version.

I'm still trying to understand what exactly can go wrong here. As long
as __BITS_PER_LONG is correctly defined the __u64 and __s64 will be
correctly sized as well. The only visible change is that one 'long' is
dropped from the type when it's not needed.

> There should also be a check for __SANE_USERSPACE_TYPES__
> to let userspace ask for the ll64 version everywhere.

That one is easy to fix at least.

-- 
Cyril Hrubis
chrubis@...e.cz