Re: [Xen-devel] [PATCH v5 2/4] x86/ldt: Make modify_ldt synchronous

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Thu, Jul 30, 2015 at 11:35 AM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> wrote:
> On 07/30/2015 02:14 PM, Andy Lutomirski wrote:
>>
>> On Thu, Jul 30, 2015 at 10:56 AM, Boris Ostrovsky
>> <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>
>>> On 07/28/2015 01:29 AM, Andy Lutomirski wrote:
>>>
>>>> +
>>>> +static inline void load_mm_ldt(struct mm_struct *mm)
>>>> +{
>>>> +       struct ldt_struct *ldt;
>>>> +       DEBUG_LOCKS_WARN_ON(!irqs_disabled());
>>>
>>>
>>>
>>> I thought this was supposed to be checking preemptible()?
>>
>> v6 fixes that.  Check your future inbox :)  I'm goint to rework the
>> Xen bit too based on the long discussion.
>>
>> Is that the only failure you're seeing?
>
>
> Yes.
>
>> ldt_gdt_32 passes on 64-bit for me
>
>
> With your patch:
>
> root@haswell> uname -a
> Linux dhcp-burlington7-2nd-B-east-10-152-55-89.usdhcp.oraclecorp.com
> 4.2.0-rc4 #107 SMP Thu Jul 30 11:05:19 EDT 2015 x86_64 x86_64 x86_64
> GNU/Linux
> root@haswell> cd tmp/linux/tools/testing/selftests/x86/
> root@haswell> ls -l ldt_gdt_32
> -rwxr-xr-x 1 root root 25975 Jul 30 11:48 ldt_gdt_32
> root@haswell> ./ldt_gdt_32
> [OK]    LDT entry 0 has AR 0x0040FA00 and limit 0x0000000A
> [OK]    LDT entry 0 has AR 0x00C0FA00 and limit 0x0000AFFF
> [OK]    LDT entry 1 is invalid
> [OK]    LDT entry 2 has AR 0x00C0FA00 and limit 0x0000AFFF
> [OK]    LDT entry 1 is invalid
> [OK]    LDT entry 2 has AR 0x00C0FA00 and limit 0x0000AFFF
> [OK]    LDT entry 2 has AR 0x00D0FA00 and limit 0x0000AFFF
> [OK]    LDT entry 2 has AR 0x00D07A00 and limit 0x0000AFFF
> [OK]    LDT entry 2 has AR 0x00907A00 and limit 0x0000AFFF
> [OK]    LDT entry 2 has AR 0x00D07200 and limit 0x0000AFFF
> [OK]    LDT entry 2 has AR 0x00D07000 and limit 0x0000AFFF
> [OK]    LDT entry 2 has AR 0x00D07400 and limit 0x0000AFFF
> [OK]    LDT entry 2 has AR 0x00507600 and limit 0x0000000A
> [OK]    LDT entry 2 has AR 0x00507E00 and limit 0x0000000A
> [OK]    LDT entry 2 has AR 0x00507C00 and limit 0x0000000A
> [OK]    LDT entry 2 has AR 0x00507A00 and limit 0x0000000A
> [OK]    LDT entry 2 has AR 0x00507800 and limit 0x0000000A
> [RUN]   Test fork
> [OK]    LDT entry 2 has AR 0x00507800 and limit 0x0000000A
> [OK]    LDT entry 1 is invalid
> [OK]    Child succeeded
> [RUN]   Test size
> [DONE]  Size test
> [OK]    modify_ldt failure 22
> [OK]    LDT entry 0 has AR 0x0000F200 and limit 0x00000000
> [OK]    LDT entry 0 has AR 0x00007200 and limit 0x00000000
> [OK]    LDT entry 0 has AR 0x0000F000 and limit 0x00000000
> [OK]    LDT entry 0 has AR 0x00007200 and limit 0x00000000
> [OK]    LDT entry 0 has AR 0x00007000 and limit 0x00000001
> [OK]    LDT entry 0 has AR 0x00007000 and limit 0x00000000
> [OK]    LDT entry 0 is invalid
> [OK]    LDT entry 0 has AR 0x0040F200 and limit 0x00000000
> [OK]    LDT entry 0 is invalid
> [RUN]   Cross-CPU LDT invalidation
> Segmentation fault (core dumped)

That's not good.

Can you backtrace it?  (I.e. compite ldt_gdt_32 with -g and load the
core dumb in gdb?)  My best guesses are either a signal delivery
failure (although that shouldn't be a problem for 32-bit userspace on
any kernel) or an actual LDT access fault, and the latter would be
interesting.

I haven't been able to reproduce this.

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