i386 Go programs crash on amd64

[Neel Natu]

Hi Konstantin,

On Thu, May 29, 2014 at 2:27 PM, Konstantin Belousov
<kostikbel at gmail.com> wrote:
> On Thu, May 29, 2014 at 02:19:06PM -0700, Neel Natu wrote:
>> Hi,
>>
>> On Thu, May 29, 2014 at 3:44 AM, Peter Jeremy <peter at rulingia.com> wrote:
>> > On 2014-May-29 04:38:18 +0300, Konstantin Belousov <kostikbel at gmail.com> wrote:
>> >>Hm, I think I know what is going on.  Try this, please.
>> >>At least, your test binary worked for me.
>> >
>> > Thank you.  That seems to fix the problem even on non-trivial code.
>> >
>>
>> Occasionally, I run into a problem where sys arch(I386_SET_LDT) fails
>> with a ENOSPC. The golang runtime isn't checking the error return
>> correctly and proceeds to load the bogus selector value into %gs.
>>
>> ./write
>> pid 2571 comm write: signal 10 err e4 code 3 type 9 addr 0x8062446 rsp
>> 0xffffdae8 rip 0x8062446 <8e e8 83 c4 20 c3 00 00>
>> Bus error (core dumped)
>>
>> Here is the output of kdump:
>> $ kdump
>> capability mode sandbox enabled
>>   1934 ktrace   RET   ktrace 0
>>   1934 ktrace   CALL  execve(0x7fffffffed2f,0x7fffffffeae0,0x7fffffffeaf0)
>>   1934 ktrace   NAMI  "./write"
>>   1934 write    RET   execve 0
>>   1934 write    CALL  sysarch(0x1,0xffffdb38)
>>   1934 write    RET   sysarch -1 errno 28 No space left on device
>>   1934 write    PSIG  SIGBUS SIG_DFL code=BUS_OBJERR
>>   1934 write    NAMI  "write.core"
>>
>> In any case this is a problem because we aren't zeroing the LDT after
>> allocation. Here is a patch that fixes it:
>>
>> Index: sys/amd64/amd64/sys_machdep.c
>> ===================================================================
>> --- sys/amd64/amd64/sys_machdep.c (revision 266856)
>> +++ sys/amd64/amd64/sys_machdep.c (working copy)
>> @@ -462,7 +462,7 @@
>>   new_ldt = malloc(sizeof(struct proc_ldt), M_SUBPROC, M_WAITOK);
>>   new_ldt->ldt_base = (caddr_t)kmem_malloc(kernel_arena,
>>       max_ldt_segment * sizeof(struct user_segment_descriptor),
>> -     M_WAITOK);
>> +     M_WAITOK | M_ZERO);
>>   if (new_ldt->ldt_base == NULL) {
>>   FREE(new_ldt, M_SUBPROC);
>>   mtx_lock(&dt_lock);
>>
>> Ok to commit?
> Sure,
> this also should be merged back to at least stable/8.

Thanks, will do.

best
Neel