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


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.

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


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