KSYMS(7D) Devices KSYMS(7D)


ksyms - kernel symbols




The file /dev/ksyms is a character special file that allows read-only
access to an ELF format image containing two sections: a symbol table
and a corresponding string table. The contents of the symbol table
reflect the symbol state of the currently running kernel. You can
determine the size of the image with the fstat() system call. The
recommended method for accessing the /dev/ksyms file is by using the
ELF access library. See elf(3ELF) for details. If you are not familiar
with ELF format, see a.out(4).

/dev/ksyms is an executable for the processor on which you are accessing
it. It contains ELF program headers which describe the text and data
segment(s) in kernel memory. Since /dev/ksyms has no text or data, the
fields specific to file attributes are initialized to NULL. The remaining
fields describe the text or data segment(s) in kernel memory.

Symbol table
The SYMTAB section contains the symbol table entries
present in the currently running kernel. This section is
ordered as defined by the ELF definition with locally-
defined symbols first, followed by globally-defined
symbols. Within symbol type, the symbols are ordered by
kernel module load time. For example, the kernel file
symbols are first, followed by the first module's
symbols, and so on, ending with the symbols from the last
module loaded.

The section header index (st_shndx) field of each symbol
entry in the symbol table is set to SHN_ABS, because any
necessary symbol relocations are performed by the kernel
link editor at module load time.

String table
The STRTAB section contains the symbol name strings that
the symbol table entries reference.


kernel(1M), stat(2), elf(3ELF), kvm_open(3KVM), a.out(4), mem(7D)


The kernel is dynamically configured. It loads kernel modules when
necessary. Because of this aspect of the system, the symbol information
present in the running system can vary from time to time, as kernel
modules are loaded and unloaded.

When you open the /dev/ksyms file, you have access to an ELF image
which represents a snapshot of the state of the kernel symbol information
at that instant in time. While the /dev/ksyms file remains open, kernel
module autounloading is disabled, so that you are protected from the
possibility of acquiring stale symbol data. Note that new modules can
still be loaded, however. If kernel modules are loaded while you have the
/dev/ksyms file open, the snapshot held by you will not be updated. In
order to have access to the symbol information of the newly loaded
modules, you must first close and then reopen the /dev/ksyms file. Be
aware that the size of the /dev/ksyms file will have changed. You will
need to use the fstat() function (see stat(2)) to determine the new size
of the file.

Avoid keeping the /dev/ksyms file open for extended periods of time,
either by using kvm_open(3KVM) of the default namelist file or with a
direct open. There are two reasons why you should not hold /dev/ksyms
open. First, the system's ability to dynamically configure itself is
partially disabled by the locking down of loaded modules. Second, the
snapshot of symbol information held by you will not reflect the symbol
information of modules loaded after your initial open of /dev/ksyms.

Note that the ksyms driver is a loadable module, and that the kernel
driver modules are only loaded during an open system call. Thus it is
possible to run stat(2) on the /dev/ksyms file without causing the ksyms
driver to be loaded. In this case, the file size returned is
UNKNOWN_SIZE. A solution for this behavior is to first open the
/dev/ksyms file, causing the ksyms driver to be loaded (if necessary).
You can then use the file descriptor from this open in a fstat() system
call to get the file's size.


The kernel virtual memory access library (libkvm) routines use /dev/ksyms
as the default namelist file. See kvm_open(3KVM) for details.

December 11, 2000 KSYMS(7D)