i386 Go programs crash on amd64

[Konstantin Belousov]

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.
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