Re: [PATCH 2/2] jmpbuf: Add paddings for target specific usage

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Nov 13, 2017 at 8:34 AM, Florian Weimer <fweimer@redhat.com> wrote:
> On 11/13/2017 03:05 PM, H.J. Lu wrote:
>>
>> On Mon, Nov 13, 2017 at 5:09 AM, Florian Weimer <fweimer@redhat.com>
>> wrote:
>>>
>>> On 11/08/2017 07:27 PM, H.J. Lu wrote:
>>>>
>>>>
>>>> +/* The biggest signal number + 1  */
>>>> +#define _JUMP_BUF_SIGSET_NSIG  257
>>>> +/* Number of longs to hold all signals.  */
>>>> +#define _JUMP_BUF_SIGSET_NWORDS \
>>>> +  ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
>>>
>>>
>>>
>>> Where does 257 come from?  65 or 129 I would understand considering the
>>> kernel sources, but 257 is odd.
>
>
> Oh.  I'm not sure if we should put this into installed header.
>
> Maybe we can use a different approach?  Something similar to the pthread
> types?  Or just not change the external type at all and just ad some
> internal space reuse mechanism?

I will see what I can do.

> We had problems with people poking at supposedly invisible jmpbuf contents
> in the past, and I'm worried that adding even __ members will encourage
> that.
>
>>> I think it would be clearer to hard-code the array sizes and explain why
>>> the
>>> values where chosen in that way.
>>>
>>> We also need a test that setprocmask does not read from the previously
>>
>>
>> Did you mean "sigprocmask"?
>
>
> Right.
>
>>> unused part.  I can move the existing next_to_fault bits to support/ if
>>> that
>>> would help.
>>
>>
>> Yes, please.
>
>
> Okay, I'll move it to support/ soon.
>

Here is what the test will look like

static int
do_test (void)
{
  struct __jmp_buf_tag *sj = memory from next_to_fault
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
"SJ" will point to the end of page in such a way that sigprocmask
will fail if it reads beyond the actual __saved_mask.

  sigset_t m;

  sigemptyset (&m);
  sigprocmask (SIG_SETMASK, &m, NULL);
  if (sigsetjmp (sj, 0) == 0)
    {
      sigaddset (&m, SIGUSR1);
      sigprocmask (SIG_SETMASK, &m, NULL);
      siglongjmp (sj, 1);
      return EXIT_FAILURE;
    }
  sigprocmask (SIG_SETMASK, NULL, &m);
  return sigismember (&m, SIGUSR1) ? EXIT_SUCCESS : EXIT_FAILURE;
}

#define TEST_FUNCTION do_test ()
#include "../test-skeleton.c"

Thanks.

-- 
H.J.