Re: [PATCH] aarch64: Fix ipc_perm definition for ILP32

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 23/08/17 12:31, Yury Norov wrote:
> On Wed, Aug 23, 2017 at 11:37:32AM +0100, Szabolcs Nagy wrote:
>> On 23/08/17 11:10, Yury Norov wrote:
>>> On Wed, Aug 23, 2017 at 10:55:02AM +0100, Szabolcs Nagy wrote:
>>>> On 22/08/17 18:46, Steve Ellcey wrote:
>>>>> @@ -46,7 +46,12 @@ struct ipc_perm
>>>>>      __gid_t gid;			/* Owner's group ID.  */
>>>>>      __uid_t cuid;			/* Creator's user ID.  */
>>>>>      __gid_t cgid;			/* Creator's group ID.  */
>>>>> +#ifdef __LP64
>>>>>      unsigned int mode;			/* Read/write permission.  */
>>>>> +#else
>>>>> +    unsigned short int mode;		/* Read/write permission.  */
>>>>> +    unsigned short int __pad0;
>>>>> +#endif
>>>>
>>>> when did this happen?
>>>>
>>>> as far as i can tell staging/ilp32-4.12 branch in
>>>> git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git
>>>> still has unsigned int __kernel_mode_t in uapi, so this is
>>>> an abi change compared to that branch.
>>>>
>>>> i guess it's for 32bit compat, but i'd like to see the
>>>> kernel patch for this.
>>>
>>> It is there from the very beginning of ILP32 patches. The patch changes
>>> the ABI only for arm64/ilp32. LP64 remains untouched, and it works correct
>>> with and without ilp32 patches and this fix.
>>>
>>> To reproduce:
>>> struct semid_ds seminfo;
>>> static char name[] = "Hello";
>>>
>>> key_t key = ftok (name, 'G');
>>> semid = semget(key, 1, IPC_CREAT | IPC_EXCL | 0644);
>>> semctl (semid, 0, IPC_STAT, &seminfo);
>>> printf("%o\n", seminfo.sem_perm.mode); // bits 16-31 contain garbage
>>>
>>> On kernel side no fixes needed because arm64/ilp32 ipc requests are
>>> wired to compat layer, and the layout for structure is like this:
>>> arch/arm64/include/asm/compat.h:
>>> 246 struct compat_ipc64_perm {
>>> 247         compat_key_t key;
>>> 248         __compat_uid32_t uid;
>>> 249         __compat_gid32_t gid;
>>> 250         __compat_uid32_t cuid;
>>> 251         __compat_gid32_t cgid;
>>> 252         unsigned short mode;
>>> 253         unsigned short __pad1;
>>
>> we have to decide if mode_t is unsigned int or short on ilp32,
>> changing just the ipc_perm struct in libc is nonconforming:
>> http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_ipc.h.html
>>
>> (there are some existing conformance issues like that in
>> glibc/linux but we should try to avoid introducing new ones)
>>
>> i think the ilp32 linux uapi should typedef __kernel_mode_t
>> to unsigned short, but i don't know the effect of that on
>> the kernel, so please discuss this with the kernel folks.
> 
> The __mode_t is u32 for all glibc ports including arm64, but the problem is
> that the declaration of struct ipc_perm in sysdeps/unix/sysv/linux/bits/ipc.h 
> doesn't use __mode_t type, but unsigned long or short. 
> 
> On kernel side, __kernel_mode_t is already defined, and for arm64 it
> is u32. And like for glibc, compat layer doesn't use __kernel_mode_t,
> but unsigned short. 
> 
> (For arm, the __kernel_mode_t is unsigned short, while __MODE_T_TYPE
> on glibc side is still __U32_TYPE. I think this is wrong...)
> 
> So for me it looks like both glibc and kernel ignore POSIX standard in
> this case: declare mode not as mode_t type in the ipc_perm structure.
> And this is the root of problem. Fixing it is non-trivial task because
> mode_t is not unsigned short or long in general, and so other structures
> will be affected.
> 

ok, it seems it's one of the posix conformance issues
that affect other targets too

https://sourceware.org/bugzilla/show_bug.cgi?id=18231

so it's ok to have this issue on ilp32 too.

> For arm64/ilp32, we can continue as-is on glibc side, but it will
> require rework on kernel side. If it was only little-endian code, we
> would simply zero pad1, and it would be enough. But there is big-endian
> support for arm64/ilp32, and therefore we need to introduce wrappers
> to swap half-words.
> 
> I can prepare RFC that does it, but I think that the problem is
> generic, and so the generic fix required. 
> 
> Yury
>