There seems to be a dearth of information about the System.map file. It's really nothing mysterious, and in the scheme of things, it's really not that important. But a lack of documentation makes it shady. It's like an earlobe; we all have one, but nobody really knows why. This is a little web page I cooked up that explains the why.

Note, I'm not out to be 100[percnt] correct. For instance, it's possible for a system to not have /proc filesystem support, but most systems do. I'm going to assume you "go with the flow" and have a fairly typical system.

Some of the stuff on oopses comes from Alessandro Rubini's "Linux Device Drivers" which is where I learned most of what I know about kernel programming.

In the context of programming, a symbol is the building block of a program: it is a variable name or a function name. It should be of no surprise that the kernel has symbols, just like the programs you write. The difference is, of course, that the kernel is a very complicated piece of coding and has many, many global symbols.

The kernel doesn't use symbol names. It's much happier knowing a variable or function name by the variable or function's address. Rather than using size_t BytesRead, the kernel prefers to refer to this variable as (for example) c0343f20.

Humans, on the other hand, do not appreciate names like c0343f20. We prefer to use something like size_t BytesRead. Normally, this doesn't present much of a problem. The kernel is mainly written in C, so the compiler/linker allows us to use symbol names when we code and allows the kernel to use addresses when it runs. Everyone is happy.

There are situations, however, where we need to know the address of a symbol (or the symbol for an address). This is done by a symbol table, and is very similar to how gdb can give you the function name from a address (or an address from a function name). A symbol table is a listing of all symbols along with their address. Here is an example of a symbol table:

	   c03441a0 B dmi_broken
	   c03441a4 B is_sony_vaio_laptop
	   c03441c0 b dmi_ident
	   c0344200 b pci_bios_present
	   c0344204 b pirq_table
	   c0344208 b pirq_router
	   c034420c b pirq_router_dev
	   c0344220 b ascii_buffer
	   c0344224 b ascii_buf_bytes
          

You can see that the variable named dmi_broken is at the kernel address c03441a0.

There are 2 files that are used as a symbol table:

There. You now know what the System.map file is.

Every time you compile a new kernel, the addresses of various symbol names are bound to change.

/proc/ksyms is a "proc file" and is created on the fly when a kernel boots up. Actually, it's not really a file; it's simply a representation of kernel data which is given the illusion of being a disk file. If you don't believe me, try finding the filesize of /proc/ksyms. Therefore, it will always be correct for the kernel that is currently running..

However, System.map is an actual file on your filesystem. When you compile a new kernel, your old System.map has wrong symbol information. A new System.map is generated with each kernel compile and you need to replace the old copy with your new copy.

What is the most common bug in your homebrewed programs? The segfault. Good ol' signal 11.

What is the most common bug in the Linux kernel? The segfault. Except here, the notion of a segfault is much more complicated and can be, as you can imagine, much more serious. When the kernel dereferences an invalid pointer, it's not called a segfault -- it's called an "oops". An oops indicates a kernel bug and should always be reported and fixed.

Note that an oops is not the same thing as a segfault. Your program cannot recover from a segfault. The kernel doesn't necessarily have to be in an unstable state when an oops occurs. The Linux kernel is very robust; the oops may just kill the current process and leave the rest of the kernel in a good, solid state.

An oops is not a kernel panic. In a panic, the kernel cannot continue; the system grinds to a halt and must be restarted. An oops may cause a panic if a vital part of the system is destroyed. An oops in a device driver, for example, will almost never cause a panic.

When an oops occurs, the system will print out information that is relevent to debugging the problem, like the contents of all the CPU registers, and the location of page descriptor tables. In particular, the contents of the EIP (instruction pointer) is printed. Like this:

	   EIP: 0010:[<00000000>]
	   Call Trace: [<c010b860>]
          

You can agree that the information given in EIP and Call Trace is not very informative. But more importantly, it's really not informative to a kernel developer either. Since a symbol doesn't have a fixed address, c010b860 can point anywhere.

To help us use this cryptic oops output, Linux uses a daemon called klogd, the kernel logging daemon. klogd intercepts kernel oopses and logs them with syslogd, changing some of the useless information like c010b860 with information that humans can use. In other words, klogd is a kernel message logger which can perform name-address resolution. Once klogd tranforms the kernel message, it uses whatever logger is in place to log system wide messages, usually syslogd.

To perform name-address resolution, klogd uses System.map. Now you know what an oops has to do with System.map.

Fine print: There are actually two types of address resolution are performed by klogd.

System.map and is therefore not relevant to this discussion, but I'll describe it briefly anyhow.

Klogd Dynamic Translation

Suppose you load a kernel module which generates an oops. An oops message is generated, and klogd intercepts it. It is found that the oops occured at d00cf810. Since this address belongs to a dynamically loaded module, it has no entry in the System.map file. klogd will search for it, find nothing, and conclude that a loadable module must have generated the oops. klogd then queries the kernel for symbols that were exported by loadable modules. Even if the module author didn't export his symbols, at the very least, klogd will know what module generated the oops, which is better than knowing nothing about the oops at all.

There's other software that uses System.map, and I'll get into that shortly.

System.map should be located wherever the software that uses it looks for it. That being said, let me talk about where klogd looks for it. Upon bootup, if klogd isn't given the location of System.map as an argument, it will look for System.map in 3 places, in the following order:

System.map also has versioning information, and klogd intelligently searches for the correct map file. For instance, suppose you're running kernel 2.4.18 and the associated map file is /boot/System.map. You now compile a new kernel 2.5.1 in the tree /usr/src/linux. During the compiling process, the file /usr/src/linux/System.map is created. When you boot your new kernel, klogd will first look at /boot/System.map, determine it's not the correct map file for the booting kernel, then look at /usr/src/linux/System.map, determine that it is the correct map file for the booting kernel and start reading the symbols.

A few nota bene's:

	   # strace -f /sbin/klogd | grep 'System.map'
	   31208 open("/boot/System.map-2.4.18", O_RDONLY|O_LARGEFILE) = 2
          

Apparently, not only does klogd look for the correct version of the map in the 3 klogd search directories, but klogd also knows to look for the name "System.map" followed by "-kernelversion", like System.map-2.4.18. This is undocumented feature of klogd.

A few drivers will need System.map to resolve symbols (since they're linked against the kernel headers instead of, say, glibc). They will not work correctly without the System.map created for the particular kernel you're currently running. This is NOT the same thing as a module not loading because of a kernel version mismatch. That has to do with the kernel version, not the kernel symbol table which changes between kernels of the same version!

Don't think that System.map is only useful for kernel oopses. Although the kernel itself doesn't really use System.map, other programs such as klogd, lsof,

	   satan# strace lsof 2>&1 1> /dev/null | grep System
	   readlink("/proc/22711/fd/4", "/boot/System.map-2.4.18", 4095) = 23
          

and ps :

	   satan# strace ps 2>&1 1> /dev/null | grep System
	   open("/boot/System.map-2.4.18", O_RDONLY|O_NONBLOCK|O_NOCTTY) = 6
          

and many other pieces of software like dosemu require a correct System.map.

Suppose you have multiple kernels on the same machine. You need a separate System.map files for each kernel! If boot a kernel that doesn't have a System.map file, you'll periodically see a message like: System.map does not match actual kernel Not a fatal error, but can be annoying to see everytime you do a ps ax. Some software, like dosemu, may not work correctly (although I don't know of anything off the top of my head). Lastly, your klogd or ksymoops output will not be reliable in case of a kernel oops. See the manual pages by giving commands 'man ksymoops' and 'man klogd'.

The solution is to keep all your System.map files in /boot and rename them with the kernel version. Suppose you have multiple kernels like:

Then just rename your map files according to the kernel version and put them in /boot, like:

	   /boot/System.map-2.2.14
	   /boot/System.map-2.2.13
          

Now what if you have two copies of the same kernel? Like:

The best answer would be if all software looked for the following files:

	   /boot/System.map-2.2.14
	   /boot/System.map-2.2.14.nosound
          

You can also use symlinks:

	   System.map-2.2.14
	   System.map-2.2.14.sound
	   ln -s System.map-2.2.14.sound System.map     # Here System.map -> System.map-2.2.14.sound