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SubjectRe: [PATCH v2] x86: Fix x32 System V message queue syscalls
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On 1 Nov 2020, at 21:01, Rich Felker <dalias@libc.org> wrote:
>
> On Sun, Nov 01, 2020 at 06:27:10PM +0000, Jessica Clarke wrote:
>> On 1 Nov 2020, at 18:15, Jessica Clarke <jrtc27@jrtc27.com> wrote:
>>>
>>> On 1 Nov 2020, at 18:07, Andy Lutomirski <luto@kernel.org> wrote:
>>>>
>>>> On Sat, Oct 31, 2020 at 6:50 PM Rich Felker <dalias@libc.org> wrote:
>>>>>
>>>>> On Sun, Nov 01, 2020 at 01:27:35AM +0000, Jessica Clarke wrote:
>>>>>> On 1 Nov 2020, at 01:22, Rich Felker <dalias@libc.org> 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 <jrtc27@jrtc27.com> 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?

Jess

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