getgrent.3: Add ENOENT to error list.

Michael,

It's possible to get ENOENT returned from getgrent if the backend, for example say SSSD, isn't configured or the daemon isn't running. The same can be said of any of the NSS backend.

As POSIX does not list ENOENT, we can list it ourselves and define it how we like.

I don't know how you handle errno values that are glibc specific, but here is the patch that enhances getgrent to make users aware of what ENOENT is intended to mean from glibc.

Patch against master. Pleas apply.

index f49c746..02f26bd 100644
@@ -141,6 +141,11 @@ The calling process already has too many open files.
 .B ENFILE
 Too many open files in the system.
 .TP
+.\" not in POSIX
+.B ENOENT
+A necessary input file cannot be found.
+For NSS backends in glibc this indicates the backend is not correctly configured.
+.TP
 .B ENOMEM
 .\" not in POSIX
 Insufficient memory to allocate

---

Cheers,
Carlos.

The daemon not running is internally a NSS_STATUS_TRYAGAIN + EAGAIN[1], i.e. that is what the sssd nss plugin should return to glibc. glibc then should return that as a NOTFOUND, which for getgrent is a NULL return without errno set. I don't see why ENOENT is necessary.

Siddhesh

[1] http://www.gnu.org/software/libc/manual/html_node/NSS-Modules-Interface.html

This is orthogonal to the discussion at hand.

At present glibc will return a NULL `struct group*' and errno set to ENOENT if the NSS plugin returns NSS_STATUS_UNAVAIL and errno ENOENT indicating it is incorrectly configured. This is a documented entry in the glibc manual, and is presently how SSSD behaves (until it gets fixed).

Wether we like it or not there is a present day distinction between "permanently unavailable until an admin fixes it" (NSS_STATUS_UNAVAIL,ENOENT), "temporarily unavailable" (NSS_STATUS_TRYAGAIN,EAGAIN), and the former may be seen by the user, and may be useful to act upon by a program that is interested in that behaviour. I do not think glibc should hide NSS_STATUS_TRYAGAIN from the user.

To be clear ENOENT is neccessary if you want to actually detect that something is wrong with your system and take evasive action. Simply getting back no results is not sufficient to take corrective action. In the case of sss however the intent of the inactive plugin is to operate as if it had no data. At least this is what I've been told by those working on SSSD at Red Hat.

SSSD should *not* use status==NSS_STATUS_TRYAGAIN and errno==EAGAIN because that will simply result in EAGAIN being returned to userspace from getgrent which is again a deviation from the entire philosophy behind SSSD wanting sss in nsswitch.conf. The point is to appear as a transparent plugin that is enabled at a later time by starting up the daemon.

For example if you fix SSSD to use status==NSS_STATUS_TRYAGIN errno==EAGAIN instead you get this still wrong behaviour from this test case:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <grp.h>

int main(int argc, char *argv[])

{
    struct group *p_group;

    setgrent();
    while (1) {
        errno = 0;  /* initialize for  getgrent() */
        p_group = getgrent();
        if (p_group == NULL) {
            if (errno == 0) {
                    break;   /* end of groups */
            } else {
                perror("getgrent");
                /* error occurs. */
                printf("getgrent error %d \n", errno);
                endgrent();
                exit(-2);
            }
        }
        printf("getgrent() OK group(%d) = %s \n",p_group->gr_gid, p_group->gr_name);
    }

    exit(0);
}

With SSSD using status==NSS_STATUS_TRYAGAIN errno==EAGAIN:

getgrent() OK group(0) = root
getgrent() OK group(1) = bin
getgrent() OK group(2) = daemon
...
getgrent: Resource temporarily unavailable
getgrent error 11

With SSSD using status==NSS_STATUS_UNAVAIL errno==ENOENT:

getgrent() OK group(0) = root
getgrent() OK group(1) = bin
getgrent() OK group(2) = daemon
...
getgrent: No such file or directory
getgrent error 2

With SSSD using status==NSS_STATUS_NOTFOUND errno==0:
getgrent() OK group(0) = root
getgrent() OK group(1) = bin
getgrent() OK group(2) = daemon
getgrent() OK group(3) = sys
...
getgrent() OK group(185) = wildfly

Which completes successfully and is the only way it should work for an installed SSSD nss module.

e.g.

diff -urN sssd-1.11.6/src/sss_client/nss_group.c sssd-1.11.6.mod/src/sss_client/nss_group.c
@@ -539,6 +539,11 @@
     if (nret != NSS_STATUS_SUCCESS) {
         errno = errnop;
     }
+    /* Always pretend we have no data.  */
+    if (nret == NSS_STATUS_UNAVAIL) {
+	nret = NSS_STATUS_NOTFOUND;
+	errno = 0;
+    }
 
     sss_nss_unlock();
     return nret;

@@ -639,6 +644,11 @@
     if (nret != NSS_STATUS_SUCCESS) {
         errno = errnop;
     }
+    /* Always pretend we have no data.  */
+    if (nret == NSS_STATUS_UNAVAIL) {
+	nret = NSS_STATUS_NOTFOUND;
+	errno = 0;
+    }
 
     sss_nss_unlock();
     return nret;

---

Please correct me if you think something I've said is wrong or doesn't make sense.

Cheers,
Carlos.

Yes but the entry in the libc manual documents the interface between the plugin and glibc, not the plugin and the user or glibc and the user.

This seems to me to be a case for the nss subsystem to clear errno if it does not. I'd read the errno list as the number of ways it is allowed to fail extraordinarily and a resource not being available is currently not considered as an extraordinary failure by POSIX. So in that context it is a bug in the nss subsystem.

My point is that we'll be deviating from the standard by supporting an extra way to fail and maybe we should get some kind of clarification from the Austin group before simply documenting it as the truth.

Siddhesh

While I'm fixing one I might as well fix the other.

v1
- Add ENOENT.
v2
- Add EAGAIN.

index f49c746..599b9fd 100644
@@ -129,6 +129,11 @@ or
 .BR free (3).)
 .SH ERRORS
 .TP
+.B EAGAIN
+The service was temporarily unavailable; try again later.
+For NSS backends in glibc this indicates a temporary error talking to the backend.
+The error may correct itself, retrying later is suggested.
+.TP
 .B EINTR
 A signal was caught.
 .TP

@@ -141,6 +146,11 @@ The calling process already has too many open files.
 .B ENFILE
 Too many open files in the system.
 .TP
+.\" not in POSIX
+.B ENOENT
+A necessary input file cannot be found.
+For NSS backends in glibc this indicates the backend is not correctly configured.
+.TP
 .B ENOMEM
 .\" not in POSIX
 Insufficient memory to allocate

---

Cheers,
Carlos.
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Hi Carlos,

Thanks. I've applied. Are similar fixes also required for, say, getpwent.3, getspent.3, and perhaps other pages (e.g., putgrent)?

Cheers,

Michael

Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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