Re: [LTP] Segmentation fault when running sched_setaffinity01 in RHEL5.10GA

[Jan Stancek <jstancek@redhat.com>]

----- Original Message -----
> From: "Xiaoguang Wang" <wangxg.fnst@cn.fujitsu.com>
> To: "LTP" <ltp-list@lists.sourceforge.net>
> Cc: "Jan Stancek" <jstancek@redhat.com>
> Sent: Friday, 25 July, 2014 7:38:05 AM
> Subject: Segmentation fault when running sched_setaffinity01  in RHEL5.10GA
> 
> 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!

Hi,

RHEL5.10 GA was January 2013, it's an older codebase, but release is quite recent.

I ran this on RHEL5.3:

(gdb) r
Starting program: /root/ltp/testcases/kernel/syscalls/sched_setaffinity/sched_setaffinity01 
Detaching after fork from child process 7692.

Program received signal SIGSEGV, Segmentation fault.
__sched_setaffinity_new (pid=<value optimized out>, cpusetsize=<value optimized out>, cpuset=<value optimized out>)
    at ../sysdeps/unix/sysv/linux/sched_setaffinity.c:62
62	    if (((char *) cpuset)[cnt] != '\0')
(gdb) list
57	    }
58	
59	  /* We now know the size of the kernel cpumask_t.  Make sure the user
60	     does not request to set a bit beyond that.  */
61	  for (size_t cnt = __kernel_cpumask_size; cnt < cpusetsize; ++cnt)
62	    if (((char *) cpuset)[cnt] != '\0')
63	      {
64	        /* Found a nonzero byte.  This means the user request cannot be
65		   fulfilled.  */
66		__set_errno (EINVAL);
(gdb) p __kernel_cpumask_size
$1 = 32
(gdb) p cpusetsize
$2 = <value optimized out>


Looking at sources of glibc-2.5 (RHEL5.10) and glibc-2.17 (RHEL7), code leading
up to this place looks identical. So, I agree with your findings. I see 2 options:

1) find cpuset size in same way as glibc does it

2) call syscall directly (and don't use glibc wrapper):

diff --git a/testcases/kernel/syscalls/sched_setaffinity/sched_setaffinity01.c b/testcases/kernel/syscalls/sched_setaffinity/sched_setaffinity01.c
index 0ac4478..0c0488f 100644
--- a/testcases/kernel/syscalls/sched_setaffinity/sched_setaffinity01.c
+++ b/testcases/kernel/syscalls/sched_setaffinity/sched_setaffinity01.c
@@ -42,6 +42,7 @@
 #include "usctest.h"
 #include "safe_macros.h"
 #include "sched_setaffinity.h"
+#include "linux_syscall_numbers.h"
 
 char *TCID = "sched_setaffinity01";
 
@@ -151,7 +152,7 @@ int main(int argc, char *argv[])
        for (lc = 0; TEST_LOOPING(lc); lc++) {
                tst_count = 0;
                for (i = 0; i < TST_TOTAL; i++) {
-                       TEST(sched_setaffinity(*(test_cases[i].pid),
+                       TEST(ltp_syscall(__NR_sched_setaffinity, *(test_cases[i].pid),
                                                *(test_cases[i].mask_size),
                                                *(test_cases[i].mask)));


# ./sched_setaffinity01 
sched_setaffinity01    1  TPASS  :  expected failure with 'Bad address'
sched_setaffinity01    2  TPASS  :  expected failure with 'Invalid argument'
sched_setaffinity01    3  TPASS  :  expected failure with 'No such process'
sched_setaffinity01    4  TPASS  :  expected failure with 'Operation not permitted'

# uname -r
2.6.18-128.37.1.el5

Regards,
Jan

> 
> 
> 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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