[Bug libc/21536] New: sysconf(_SC_NPROCESSORS_ONLN) errno is set wrongly on success

["ptikhomirov at virtuozzo dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21536

            Bug ID: 21536
           Summary: sysconf(_SC_NPROCESSORS_ONLN) errno is set wrongly on
                    success
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: ptikhomirov at virtuozzo dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

In Virtuozzo containers we have access to sysfs prohibited for most of the
files for security reasons. Running the code below in container I get false
negative errno:

/# ls /sys/devices/system/cpu/online
ls: cannot access /sys/devices/system/cpu/online: No such file or directory

/# rpm -qa glibc
glibc-2.17-157.el7_3.2.x86_64

/# cat sysconf_nrproc_onln.c 
#include <stdio.h>
#include <unistd.h>
#include <errno.h>

int main()
{
        long nrproconln;

        printf("Before: errno=%d\n", errno);
        nrproconln = sysconf(_SC_NPROCESSORS_ONLN);
        printf("After: nrproc_onln=%ld errno=%d\n", nrproconln, errno);

        return 0;
}

/# gcc -o sysconf_nrproc_onln sysconf_nrproc_onln.c 
/# ./sysconf_nrproc_onln 
Before: errno=0
After: nrproc_onln=4 errno=2

In sysconf man we have:

RETURN VALUE
       If name is invalid, -1 is returned, and errno is set to EINVAL. 
Otherwise, the value returned is the value of the system  resource
       and  errno  is not changed.

But in my case errno is 2 but should stay zero accoring to man.

I can see with gdb:

Breakpoint 3, __get_nprocs () at ../sysdeps/unix/sysv/linux/getsysstats.c:151
151       int fd = open_not_cancel_2 ("/sys/devices/system/cpu/online", flags);
(gdb) p errno
$10 = 0
(gdb) c
Continuing.

Breakpoint 4, __get_nprocs () at ../sysdeps/unix/sysv/linux/getsysstats.c:154
154       if (fd != -1)
(gdb) p errno
$11 = 2
(gdb) c
Continuing.

Breakpoint 6, __get_nprocs () at ../sysdeps/unix/sysv/linux/getsysstats.c:199
199       fd = open_not_cancel_2 ("/proc/stat", flags);
(gdb) p errno
$12 = 2
(gdb) c
Continuing.

Breakpoint 7, __get_nprocs () at ../sysdeps/unix/sysv/linux/getsysstats.c:200
200       if (fd != -1)
(gdb) p errno
$13 = 2

Function __get_nprocs tries to read nrprocesses from sysfs, fails and sets
errno==2, fall-backs to read nrprocesses from proc, succeeds but never resets
errno back.

Our real problem is that docker(which we run in container) uses
C.sysconf(C._SC_NPROCESSORS_ONLN) and checks error using errno!=0 as according
to man it should not change from 0 by sysconf.

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