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* [LTP] Segmentation fault when running sched_setaffinity01 in RHEL5.10GA
@ 2014-07-25  5:38 Xiaoguang Wang
  2014-07-25  6:39 ` Stanislav Kholmanskikh
  2014-07-25  7:05 ` Jan Stancek
  0 siblings, 2 replies; 6+ messages in thread
From: Xiaoguang Wang @ 2014-07-25  5:38 UTC (permalink / raw)
  To: LTP

Hi,

When we run sched_setaffinity01 in RHEL5.10GA, it occurs a segmentation fault.
Below is the possible reason.

Hi Jan, would you please help to confirm this problem. I'm afraid RHEL5.10GA is an old
distribution, which many people may not use it now, thanks!


Glibc provides a encapsulation for the raw kernel sched_setaffinity(2) system call,
the corresponding code is below(The version of glibc I used is glibc-2.5-20061008T1257-RHEL5.11Beta):
I delete some code just for simple.

#######################################################################################

/* Size definition for CPU sets.  */
# define __CPU_SETSIZE  1024
# define __NCPUBITS     (8 * sizeof (__cpu_mask))

/* Type for array elements in 'cpu_set'.  */
typedef unsigned long int __cpu_mask;

/* Basic access functions.  */
# define __CPUELT(cpu)  ((cpu) / __NCPUBITS)

/* Data structure to describe CPU mask.  */
typedef struct
{
  __cpu_mask __bits[__CPU_SETSIZE / __NCPUBITS];
} cpu_set_t;


int __sched_setaffinity_new (pid_t pid, size_t cpusetsize, const cpu_set_t *cpuset)
{
  if (__builtin_expect (__kernel_cpumask_size == 0, 0))
    {
      int res;

      while (res = INTERNAL_SYSCALL (sched_getaffinity, err, 3, getpid (),
                                     psize, p),
             INTERNAL_SYSCALL_ERROR_P (res, err)
             && INTERNAL_SYSCALL_ERRNO (res, err) == EINVAL)
     	....

      __kernel_cpumask_size = res;
    }

  /* We now know the size of the kernel cpumask_t.  Make sure the user
     does not request to set a bit beyond that.  */
  for (size_t cnt = __kernel_cpumask_size; cnt < cpusetsize; ++cnt)
    if (((char *) cpuset)[cnt] != '\0')
      {
        /* Found a nonzero byte.  This means the user request cannot be
           fulfilled.  */
        __set_errno (EINVAL);
        return -1;
      }

  return INLINE_SYSCALL (sched_setaffinity, 3, pid, cpusetsize, cpuset);
}
#######################################################################################

Glibc in RHEL5.10GA does not provide CPU_ALLOC_SIZE marco, so in ltp testcases/kernel/syscalls/sched_setaffinity/sched_setaffinity.h,
we define one.
#######################################################################################
#ifndef CPU_ALLOC_SIZE
#define CPU_ALLOC_SIZE(size) sizeof(cpu_set_t)
#endif
#######################################################################################

Then CPU_ALLOC_SIZE would always return 128 in RHEL5.10GA, that is when we test EFAULT for sched_setaffinity(2),
the passed cpusetsize is 128. But look at __sched_setaffinity_new() above, it first call
raw sched_getaffinity(2) to get  the size of the kernel cpumask_t, In RHEL5.10GA,
this value depends on CONFIG_NR_CPUS, if CONFIG_NR_CPUS is 255, the raw sched_getaffinity(2) will return 32.
In this case, __kernel_cpumask_size would be 32, cpusetsize is 128. Give that we're testing 
EFAULT, cpuset is a invalid pointer, if cnt > 32, it will generate segmentation fault in glibc code,
so this case exits abnormally

As why this test case can run normally in RHEL6.5GA or RHEL7.0GA, it's because
sched_getaffinity(2) in old kernel(RHEL5.10GA) return sizeof(cpumask_t), which totally depends on CONFIG_NR_CPUS.
In newer kernel, sched_getaffinity(2) returns the smaller one between min_t(size_t, len, cpumask_size()), 
here len is the value passed to sched_getaffinity as cpusetsize, cpumask_size() is the max allowed length.
so we can ensure __kernel_cpumask_size will never smaller cpusetsize, so the segmentation fault won't occur.

So I also think CPU_ALLOC and CPU_ALLOC_SIZE is wrong in testcases/kernel/syscalls/sched_setaffinity/sched_setaffinity.h. We should
refer to the implementation in glibc. or we define CPU_ALLOC_SIZE using raw sched_getaffinity as a workaround in older kernel . See below code:
#########################################################################################

        int ret;
        cpu_set_t cst;

        memset(&cst, 0, sizeof(cst));

        ret = syscall(__NR_sched_getaffinity, getpid(),
                      sizeof(cpu_set_t), &cst);
        if (ret < 0) {
                fprintf(stderr, "sched_getaffinity failed: %s\n",
                        strerror(errno));
                return 1;
        } else {
                printf("length of bit mask the kernel uses to represent the CPU"
                       ": %d\n", ret);
        }
#########################################################################################

Regards,
Xiaoguang Wang

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^ permalink raw reply	[flat|nested] 6+ messages in thread
end of thread, other threads:[~2014-07-25  7:51 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2014-07-25  5:38 [LTP] Segmentation fault when running sched_setaffinity01 in RHEL5.10GA Xiaoguang Wang
2014-07-25  6:39 ` Stanislav Kholmanskikh
2014-07-25  7:32   ` Xiaoguang Wang
2014-07-25  7:50     ` Stanislav Kholmanskikh
2014-07-25  7:05 ` Jan Stancek
2014-07-25  7:28   ` Xiaoguang Wang

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