linux-kernel - Re: [PATCH v3 01/17] y2038: asm-generic: Extend sysvipc data structures

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Date:   Thu, 19 Apr 2018 23:24:36 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     y2038 Mailman List <y2038@...ts.linaro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        GNU C Library <libc-alpha@...rceware.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Albert ARIBAUD <albert.aribaud@...ev.fr>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "open list:RALINK MIPS ARCHITECTURE" <linux-mips@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        sparclinux <sparclinux@...r.kernel.org>
Subject: Re: [PATCH v3 01/17] y2038: asm-generic: Extend sysvipc data structures

On Thu, Apr 19, 2018 at 5:20 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Thu, Apr 19, 2018 at 4:59 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> I suspect you want to use __kernel_ulong_t here instead of a raw
>> unsigned long.  If nothing else it seems inconsistent to use typedefs
>> in one half of the structure and no typedefs in the other half.
>
> Good catch, there is definitely something wrong here, but I think using
> __kernel_ulong_t for all members would also be wrong, as that
> still changes the layout on x32, which effectively is
>
> struct msqid64_ds {
>      ipc64_perm msg_perm;
>      u64 msg_stime;
>      u32 __unused1;
>      /* 32 bit implict padding */
>      u64 msg_rtime;
>      u32 __unused2;
>      /* 32 bit implict padding */
>      u64 msg_ctime;
>      u32 __unused3;
>      /* 32 bit implict padding */
>      __kernel_pid_t          shm_cpid;       /* pid of creator */
>      __kernel_pid_t          shm_lpid;       /* pid of last operator */
>      ....
> };
>
> The choices here would be to either use a mix of
> __kernel_ulong_t and unsigned long, or taking the x32
> version back into arch/x86/include/uapi/asm/ so the
> generic version at least makes some sense.
>
> I can't use __kernel_time_t for the lower half on 32-bit
> since it really should be unsigned.

After thinking about it some more, I conclude that the structure is simply
incorrect on x32: The __kernel_ulong_t usage was introduced in 2013
in commit b9cd5ca22d67 ("uapi: Use __kernel_ulong_t in struct
msqid64_ds") and apparently was correct initially as __BITS_PER_LONG
evaluated to 64, but it broke with commit f4b4aae18288 ("x86/headers/uapi:
Fix __BITS_PER_LONG value for x32 builds") that changed the value
of __BITS_PER_LONG and introduced the extra padding in 2015.

The same change apparently also broke a lot of other definitions, e.g.

$ echo "#include <linux/types.h>" | gcc -mx32 -E -xc - | grep -A3
__kernel_size_t
typedef unsigned int __kernel_size_t;
typedef int __kernel_ssize_t;
typedef int __kernel_ptrdiff_t;

Those used to be defined as 'unsigned long long' and 'long long'
respectively, so now all kernel interfaces using those on x32
became incompatible!

       Arnd