Errno Libretto

Unix Insider –

Opera is something I do not appreciate fully. The costumes are exquisite, the music is emotional, but without understanding Italian the plots are hard to follow. Pavarotti could be performing a free-form exploration of the UUCP source code and I would have trouble distinguishing it from Madama Butterfly. Bugs Bunny making a fruit salad on Elmer Fudd's head is the most comprehensible opera I've witnessed. "Wait," my cultured friends tell me, "use the libretto to grasp the story." While it's not quite a set of Cliff Notes, the libretto (text of the opera) helps you build a framework for understanding the action on stage.

What does this have to do with the world of system administration? If the error messages, user questions, system-call errors, and other cryptic failures you encounter sometimes make as much sense La Traviata, then you need a libretto -- a framework for understanding what the system is trying to tell you. We'll look at the various ways in which system calls fail, and the symptoms by which those failures manifest themselves. Starting with general file permission issues, we'll then dive down into NFS failures, and close with some comments on the importance of vigilance in enforcing system programming guidelines. You may not understand Puccini any better than before, but such help is easier to find.

Trap defense

System calls represent the boundary between user processes and operating system (kernel) services. When a process executes a system call, the associated wrapper in the libc.so library is called to perform some basic argument checking. If the call is syntactically acceptable, the wrapper executes a privileged instruction to force a trap into the kernel. From there, the operating system takes over by copying arguments, performing extensive checking, and completing the service request. If you dump out the code for a system call in libc.so, you'll see a "ta 8" instruction to issue trap 0x08, which is a system call (see /usr/include/sys/trap.h for trap types):

<font face="Courier">huey% adb /usr/lib/libc.so.*
_read,4?ia
_read:		st	%o0, [%sp + 0x44]
read+4:		mov	0x3, %g1
read+8:		ta	0x8
read+0xc:	bgeu	read + 0x40
</font>

Nearly every system call returns a single value, ranging from a pointer or an address, such as from

<font face="Courier">shmat()</font>
or
<font face="Courier">brk()</font>
, to the size of a data transfer from
<font face="Courier">read()</font>
and
<font face="Courier">write()</font>
, to a standard system type like a UID returned by
<font face="Courier">getuid()</font>
. System calls that return integers often use negative return values to flag a failure, but this rule doesn't apply to calls that return addresses, which are usually set to NULL if the call fails. Simple, inconsistent indicators of success or failure don't give you (and your process) enough information to determine what went wrong and how to repair the situation, so the system call return value is supplemented by the error number, or errno value.

If an exception is encountered while processing the system call, errno is set to one of the values in /usr/include/sys/errno.h. A successful call sets errno to zero. Most applications include the errno.h header file, containing the possible values of errno. Insert a

<font face="Courier">extern int errno;</font>
in your code, and it is accessible as an integer variable.

In theory, your code should check the value of errno after each system call, including those that should "never" fail like

<font face="Courier">close()</font>
, because these system calls can report failures deferred from other requests -- a topic we'll visit later. Of course, not all code does such paranoid checking, and you can't modify commercial applications to make them fit your quality standards ex post facto. So, how do you start tracking down a user issue when all you have is an error message?

The system call return value

is supplemented by the error number,

or errno value.

Trace amounts

The first thing to do is to become familiar with the various kinds of errors reported back through the errno mechanism. Your best source of information is the introduction to section 2 of the manual pages:

<font face="Courier">huey% man -s 2 intro
</font>

It explains the possible error values and associates them with the cryptic error messages like "address already in use" printed by the

<font face="Courier">perror()</font>
library routine. The descriptions aren't exhaustive and some of the errors are entirely non-obvious. Once you have a feel for the target, examine the routine in question (in this case, probproc) using
<font face="Courier">trace</font>
or
<font face="Courier">truss</font>
:

<font face="Courier">huey% truss -o /tmp/tr.out probproc -a -X -i90
</font>

<font face="Courier">truss</font>
dumps its output into the file named by the
<font face="Courier">-o</font>
option.
<font face="Courier">trace</font>
, the SunOS 4.1.x equivalent, doesn't follow forks or trace child processes, but
<font face="Courier">truss</font>
will chase down a thread of execution until it has exited. Every system call is shown in the
<font face="Courier">truss</font>
output, along with the arguments passed (or at least the first few bytes of them), the return value, and the value of errno, if it was set. Here's an edited
<font face="Courier">truss</font>
spiel from an attempt to list a non-existent file:

<font face="Courier">execve("/usr/bin/ls", 0xEFFFFAE0, 0xEFFFFAEC)  argc = 2
open("/usr/lib/libintl.so.1", O_RDONLY, 035737754720) = 3
ioctl(1, TIOCGWINSZ, 0x00024C84)		Err#22 EINVAL
lstat("xyz", 0xEFFFF9A8)			Err#2 ENOENT
_exit(2)
</font>

Note that the process opens up the internationalization library, libintl.so.1, a good hint that it was linked with

<font face="Courier">-lintl</font>
.
<font face="Courier">ls</font>
attempts to get the current window size using the TIOCGWINSZ
<font face="Courier">ioctl()</font>
, but gets an "invalid argument" because the example was generated on a dial-in line, not a
<font face="Courier">shelltool</font>
or
<font face="Courier">xterm</font>
. Searching for the file information on "xyz" returns a "file not found" error, which is printed by
<font face="Courier">ls</font>
on its way to a non-zero exit.

Understanding errno isn't purely a serious business. One of the more popular contests at USENIX conferences has been creating new errno names.

Link dink post-shrink

One of the most frustrating exercises performed by system administrators is explaining (calmly) to users why applications that behave routinely on machine A suddenly fail or exhibit strange side effects on machine B. For well-known tools like the C shell, you can wade through .cshrc scripts and find minor environmental differences. But how do you deal with shrink-wrapped code? Use

<font face="Courier">truss</font>
to identify the configuration and initialization files opened by the application. On the good machine, grep out the list of files opened and then match it against the same list on the problem machine:

<font face="Courier">huey% fgrep  'open(' truss.out1 > /tmp/out1
huey% fgrep  'open(' truss.out2 > /tmp/out2
huey% diff /tmp/out1 /tmp/out2
</font>

Look for the string "Err#2 ENOENT" signaling a missing file. Double check automounter maps, environment variables, and installation processes that modify files local to each machine, in /etc or /usr/lib, for example. Some applications search for configuration files in several directories, and may find identical files on the two hosts but process them in a different order. Again, checking the sequence of the

<font face="Courier">open()</font>
calls and the ENOENT results will tell you if you have a configuration problem.

Use

<font face="Courier">truss</font>
to identify the

configuration and initialization

files opened by the application.

Also look for EACCESS errors, caused by insufficient file or directory permissions. If the file exists but can't be read by the user, ensure that user and group IDs are consistent between the machines in question. Group-readable files aren't effective unless you enforce group membership on all machines at which users may camp.

Here's a nastier version of the same problem: a user is panicking to set up a demo environment. Rather than create new users and their environments, he runs the demo as root, only to have it fail miserably. Even root gets slapped with EACCESS violations if the files being accessed are NFS mounted. Over the network, root becomes the anonymous user nobody, and relies on world read and execute permissions to open files and search directories. Any application that works for non-privileged users but fails for root is probably opening configuration or data files over NFS. If you suspect that NFS access is contributing to your problem, locate the filesystem for the file in question using

<font face="Courier">df</font>
:

<font face="Courier">huey% df `dirname /usr/lib/gfx/config.common`
Filesystem                 kbytes    used   avail capacity  Mounted on
bigboy:/export/home/stern 1952573  944377  812946    54%   /home/stern
</font>

Watch where you drop direct maps for the automounter, and where you use hierarchical maps that may deposit NFS mounts in the middle of someone's home directory. Applications that rely on making backup copies or renaming input data sets using hard links will fail if an NFS mount is introduced into the middle. For example, assume you are mounting home directories using the following hierarchical automounter map:

<font face="Courier">* \
	/	homeserv:/export/home/&
	/fxdata	dataserv:/export/datasets/fxdata
</font>

When /home/stern is mounted, /home/stern/fxdata is picked up from the machine dataserv. So far, so good. But an application may assume that it can create a hard link between files in /home/stern/fxdata and /home/stern/backup, since they appear to be on the same filesystem. The

<font face="Courier">link()</font>
system call fails, however, with EXDEV because the hard link would cross volume boundaries.

Trail of stale crumbs

NFS errors tend to be hard to resolve because you're assigning blame in more than one operating system and host environment. Here are some of the common pitfalls:

  • "File not found" or ENOENT problems, are almost always a client issue. Automounter maps, /etc/vfstab, or volume manager configuration files should be your starting points. If the client can't even find the file, the client is probably at fault.

  • EACCESS problems can be caused by inconsistent user and group numbering, or by the root-to-nobody mapping problems described above. While responsibility for presenting valid user and group values falls on the client, you're also bumping into name server and file server issues. Like ENOENT errors, these are reported by the calling process, showing up on the command line or in a dialog box created by the application.

  • "NFS write error" or "Stale file handle" errors are server-specific. An error occurred on the server while handling the NFS request, causing it to fail. You'll see these reported in the console window and in the /var/adm/messages log, since the errors are noticed by the kernel's NFS Remote Procedure Call (RPC) code.

NFS errors tend to be hard

to resolve because you're assigning

blame in more than one operating

system and host environment.

You have to follow the trail of network crumbs from the client back to the server to resolve server-specific errors. Your first step: get a general feeling for what went wrong using the NFS error number in the console message. NFS uses the standard errno values, so "NFS write error 28" is the same as ENOSPC, namely, the disk is full or the user exceeded his or her disk quota while writing a file. Many NFS errors have obvious explanations: bumping against quotas, filling up a disk, or a disk failure that results in a general I/O error. The more difficult one to chase is a stale file handle.

NFS file handles encode the server's filesystem ID and the file's inode number to uniquely identify each NFS-mounted file. Each inode also contains an inode generation number used to differentiate files that have re-used an inode. Delete a file, for example, /home/stern/summary, and then create a new file, say /home/stern/report on the same filesystem. The new file re-uses the same inode number as the previously deleted file (assuming no other file creation activity snuck in) but increments the inode generation number to distinguish it from the old, removed file.

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