Re: SYSV IPC broken for no-legacy syscall kernels (was Re: [RFC PATCHv1 26/31] ARC: Build system: Makefiles, Kconfig, Linker script)

[Vineet Gupta]

Hi James,

On Monday 11 March 2013 06:14 PM, James Hogan wrote:
> Hi Vineet,
>
> On 11/03/13 12:29, Vineet Gupta wrote:
>> On Wednesday 07 November 2012 07:43 PM, Arnd Bergmann wrote:
>>> On Wednesday 07 November 2012, Vineet Gupta wrote:
>>>
>>>> +
>>>> +config ARC
>>>> +	def_bool y
>>>> +	select ARCH_WANT_IPC_PARSE_VERSION
>>>> +	# ARC Busybox based initramfs absolutely relies on DEVTMPFS for /dev
>>>> +	# DEVTMPS in turn needs HOTPLUG
>>>> +	select DEVTMPFS if !INITRAMFS_SOURCE=""
>>>> +	select GENERIC_ATOMIC64
>>>> +	select GENERIC_CLOCKEVENTS
>>>> +	select GENERIC_FIND_FIRST_BIT
>>>> +	# for now, we don't need GENERIC_IRQ_PROBE, CONFIG_GENERIC_IRQ_CHIP
>>>> +	select GENERIC_IRQ_SHOW
>>>> +	select GENERIC_PENDING_IRQ if SMP
>>>> +	select GENERIC_SMP_IDLE_THREAD
>>>> +	select HAVE_GENERIC_HARDIRQS
>>>> +	select HOTPLUG if !INITRAMFS_SOURCE=""
>>>> +	select MODULES_USE_ELF_RELA
>>> You should not need ARCH_WANT_IPC_PARSE_VERSION
>> It seems, we have a small bug in sysV IPC.
>> A lot of LTP tests have started failing on ARC 3.9-rc1 kernel due to absence of
>> ARCH_WANT_IPC_PARSE_VERSION.
>>
>> I'm assuming that for newer arches, msgctl(2) and friends need to have  need to
>> have @cmd of type 0x01nn where 0x0100 is IPC_64.
>> However for ! ARCH_WANT_IPC_PARSE_VERSION, ipc/util.h seems to be defining a dummy
>> version of ipc_parse_version(), which doesn't strip out the 0x0100 from @cmds,
>> causing the code to not enter the switch block in SYSCALL_DEFINE3(msgctl) making
>> it fail.
>> Adding the above item to arch/arc/Kconfig obviously fixes the breakage.
>>
>> It seems there is some bogosity in the way ipc_parse_version() is defined. Given
>> the way it is used, to strip out the 0x0100 bit, it seems we always need the
>> function variant - the IA-64 ish comment in there seems to be bit-rot.
>>
>> What do you think ?
> FYI, for metag we assumed this was intentional and that IPC_64 shouldn't
> be passed since it is the default behaviour for new arches.
> You may find the following uClibc commit relevant:
> http://git.uclibc.org/uClibc/commit/?id=6c2fa3451f35f8d78a31689044b2d19efa4eaca2

I've been using uClibc trunk - ported to ARC - for my testing, so I already have
that change. Looking at Markos' uClibc branch he is indeed defining __IPC_64 to 0
in metag port.
However, looking at the kernel code, it seems newer style is to do the other way
round - i.e. pass 0x0100 in those cmds - unless the comments in there are bogus.

include/uapi/linux/ipc.h

/*
 * Version flags for semctl, msgctl, and shmctl commands
 * These are passed as bitflags or-ed with the actual command
 */
#define IPC_OLD 0    /* Old version (no 32-bit UID support on many
               architectures) */
#define IPC_64  0x0100  /* New version (support 32-bit UIDs, bigger
               message sizes, etc. */

So someone with more ABI wisdom needs to suggest what is the right approach.

-Vineet
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