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SubjectRe: [PATCH v2] x86: Fix x32 System V message queue syscalls
On 16 Nov 2020, at 00:55, Jessica Clarke <> wrote:
> On 1 Nov 2020, at 21:01, Rich Felker <> wrote:
>> On Sun, Nov 01, 2020 at 06:27:10PM +0000, Jessica Clarke wrote:
>>> On 1 Nov 2020, at 18:15, Jessica Clarke <> wrote:
>>>> On 1 Nov 2020, at 18:07, Andy Lutomirski <> wrote:
>>>>> On Sat, Oct 31, 2020 at 6:50 PM Rich Felker <> wrote:
>>>>>> On Sun, Nov 01, 2020 at 01:27:35AM +0000, Jessica Clarke wrote:
>>>>>>> On 1 Nov 2020, at 01:22, Rich Felker <> wrote:
>>>>>>>> On Sat, Oct 31, 2020 at 04:30:44PM -0700, Andy Lutomirski wrote:
>>>>>>>>> cc: some libc folks
>>>>>>>>> On Mon, Oct 12, 2020 at 6:45 AM Jessica Clarke <> wrote:
>>>>>>>>>> POSIX specifies that the first field of the supplied msgp, namely mtype,
>>>>>>>>>> is a long, not a __kernel_long_t, and it's a user-defined struct due to
>>>>>>>>>> the variable-length mtext field so we can't even bend the spec and make
>>>>>>>>>> it a __kernel_long_t even if we wanted to. Thus we must use the compat
>>>>>>>>>> syscalls on x32 to avoid buffer overreads and overflows in msgsnd and
>>>>>>>>>> msgrcv respectively.
>>>>>>>>> This is a mess.
>>>>>>>>> include/uapi/linux/msg.h has:
>>>>>>>>> /* message buffer for msgsnd and msgrcv calls */
>>>>>>>>> struct msgbuf {
>>>>>>>>> __kernel_long_t mtype; /* type of message */
>>>>>>>>> char mtext[1]; /* message text */
>>>>>>>>> };
>>>>>>>>> Your test has:
>>>>>>>>> struct msg_long {
>>>>>>>>> long mtype;
>>>>>>>>> char mtext[8];
>>>>>>>>> };
>>>>>>>>> struct msg_long_ext {
>>>>>>>>> struct msg_long msg_long;
>>>>>>>>> char mext[4];
>>>>>>>>> };
>>>>>>>>> and I'm unclear as to exactly what you're trying to do there with the
>>>>>>>>> "mext" part.
>>>>>>>>> POSIX says:
>>>>>>>>> The application shall ensure that the argument msgp points to a user-
>>>>>>>>> defined buffer that contains first a field of type long specifying the
>>>>>>>>> type of the message, and then a data portion that holds the data bytes
>>>>>>>>> of the message. The structure below is an example of what this user-de‐
>>>>>>>>> fined buffer might look like:
>>>>>>>>> struct mymsg {
>>>>>>>>> long mtype; /* Message type. */
>>>>>>>>> char mtext[1]; /* Message text. */
>>>>>>>>> }
>>>>>>>>> NTP has this delightful piece of code:
>>>>>>>>> 44 typedef union {
>>>>>>>>> 45 struct msgbuf msgp;
>>>>>>>>> 46 struct {
>>>>>>>>> 47 long mtype;
>>>>>>>>> 48 int code;
>>>>>>>>> 49 struct timeval tv;
>>>>>>>>> 50 } msgb;
>>>>>>>>> 51 } MsgBuf;
>>>>>>>>> bluefish has:
>>>>>>>>> struct small_msgbuf {
>>>>>>>>> long mtype;
>>>>>>>>> char mtext[MSQ_QUEUE_SMALL_SIZE];
>>>>>>>>> } small_msgp;
>>>>>>>>> My laptop has nothing at all in /dev/mqueue.
>>>>>>>>> So I don't really know what the right thing to do is. Certainly if
>>>>>>>>> we're going to apply this patch, we should also fix the header. I
>>>>>>>>> almost think we should *delete* struct msgbuf from the headers, since
>>>>>>>>> it's all kinds of busted, but that will break the NTP build. Ideally
>>>>>>>>> we would go back in time and remove it from the headers.
>>>>>>>>> Libc people, any insight? We can probably fix the bug without
>>>>>>>>> annoying anyone given how lightly x32 is used and how lightly POSIX
>>>>>>>>> message queues are used.
>>>>>>>> If it's that outright wrong and always has been, I feel like the old
>>>>>>>> syscall numbers should just be deprecated and new ones assigned.
>>>>>>>> Otherwise, there's no way for userspace to be safe against data
>>>>>>>> corruption when run on older kernels. If there's a new syscall number,
>>>>>>>> libc can just use the new one unconditionally (giving ENOSYS on
>>>>>>>> kernels where it would be broken) or have a x32-specific
>>>>>>>> implementation that makes the old syscall and performs translation if
>>>>>>>> the new one fails with ENOSYS.
>>>>>>> That doesn't really help broken code continue to work reliably, as
>>>>>>> upgrading libc will just pull in the new syscall for a binary that's
>>>>>>> expecting the broken behaviour, unless you do symbol versioning, but
>>>>>>> then it'll just break when you next recompile the code, and there's no
>>>>>>> way for that to be diagnosed given the *application* has to define the
>>>>>>> type. But given it's application-defined I really struggle to see how
>>>>>>> any code out there is actually expecting the current x32 behaviour as
>>>>>>> you'd have to go really out of your way to find out that x32 is broken
>>>>>>> and needs __kernel_long_t. I don't think there's any way around just
>>>>>>> technically breaking ABI whilst likely really fixing ABI in 99.999% of
>>>>>>> cases (maybe 100%).
>>>>>> I'm not opposed to "breaking ABI" here because the current syscall
>>>>>> doesn't work unless someone wrote bogus x32-specific code to work
>>>>>> around it being wrong. I don't particularly want to preserve any of
>>>>>> the current behavior.
>>>>>> What I am somewhat opposed to is making a situation where an updated
>>>>>> libc can't be safe against getting run on a kernel with a broken
>>>>>> version of the syscall and silently corrupting data. I'm flexible
>>>>>> about how avoiding tha tis achieved.
>>>>> If we're sufficiently confident that we won't regress anything by
>>>>> fixing the bug, I propose we do the following. First, we commit a fix
>>>>> that's Jessica's patch plus a fix to struct msghdr, and we mark that
>>>>> for -stable. Then we commit another patch that removes 'struct
>>>>> msghdr' from uapi entirely, but we don't mark that for -stable. If
>>>>> people complain about the latter, we revert it.
>>>> Thinking about this more, MIPS n32 is also affected by that header. In
>>>> fact the n32 syscalls currently do the right thing and use the compat
>>>> implementations, so the header is currently out-of-sync with the kernel
>>>> there*. This should be noted when committing the change to msg.h.
>>> Never mind, it seems MIPS n32 is weird and leaves __kernel_long_t as a
>>> normal long despite being an ILP32-on-64-bit ABI, I guess because it's
>>> inherited from IRIX rather than being invented by the GNU world.
>> Yes, the whole __kernel_long_t invention is largely x32-only (maybe
>> theoretically on aarch64-ilp32 too? if that even really exists?) and
>> is pretty much entirely a mistake from lacking the proper
>> infrastructure to do time64 when x32 was introduced (note that n32 has
>> 32-bit old-time_t). I hope effort will be made to keep the same
>> mistake from creeping into future ilp32-on-64 ABIs if there are any.
> Ping? Does anyone have further thoughts/are people happy for this to
> land?



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