freebsd/share/man/man4/ksyms.4

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.Dd April 5, 2009
.Dt KSYMS 4
.Os
.Sh NAME
.Nm ksyms
.Nd kernel symbol table interface
.Sh SYNOPSIS
.Cd "device ksyms"
.Sh DESCRIPTION
The
.Pa /dev/ksyms
character device provides a read-only interface to a snapshot of the kernel
symbol table.  The in-kernel symbol manager is designed to be able to handle
many types of symbols tables, however, only
.Xr elf 5
symbol tables are supported by this device.  The ELF format image contains two
sections: a symbol table and a corresponding string table.
.Bl -tag -width indent -offset indent
.It Dv Symbol Table
The SYMTAB section contains the symbol table entries present in the current
running kernel, including the symbol table entries of any loaded modules. The
symbols are ordered by the kernel module load time starting with kernel file
symbols first, followed by the first loaded module's symbols and so on.
.It Dv String Table
The STRTAB section contains the symbol name strings from the kernel and any
loaded modules that the symbol table entries reference.
.El
.Pp
Elf formatted symbol table data read from the
.Pa /dev/ksyms
file represents the state of the kernel at the time when the device is opened.
Since
.Pa /dev/ksyms
has no text or data, most of the fields are initialized to NULL.
The
.Nm
driver does not block the loading or unloading of modules into the kernel
while the
.Pa /dev/ksyms
file is open but may contain stale data.
.Sh IOCTLS
The
.Xr ioctl 2
command codes below are defined in
.Aq Pa sys/ksyms.h .
.Pp
The (third) argument to the
.Xr ioctl 2
should be a pointer to the type indicated.
.Bl -tag -width indent -offset indent
.It Dv KIOCGSIZE (size_t)
Returns the total size of the current symbol table.
This can be used when allocating a buffer to make a copy of
the kernel symbol table.
.It Dv KIOCGADDR (void *)
Returns the address of the kernel symbol table mapped in
the process memory.
.El
.Sh FILES
.Bl -tag -width /dev/ksymsX
.It Pa /dev/ksyms
.El
.Sh ERRORS
An
.Xr open 2
of
.Pa /dev/ksyms
will fail if:
.Bl -tag -width Er
.It Bq Er EBUSY
The device is already open.  A process must close
.Pa /dev/ksyms
before it can be opened again.
.It Bq Er ENOMEM
There is a resource shortage in the kernel.
.It Bq Er ENXIO
The driver was unsuccessful in creating a snapshot of the kernel symbol
table.  This may occur if the kernel was in the process of loading or
unloading a module.
.El
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr nlist 3 ,
.Xr elf 5 ,
.Xr kldload 8
.Sh HISTORY
A
.Nm
device exists in many different operating systems.
This implementation is similar in function to the Solaris and NetBSD
.Nm
driver.
.Pp
The
.Nm
driver first appeared in
.Fx 8.0
to support
.Xr lockstat 1 .
.Sh AUTHORS
The
.Nm
driver was written by
.An Stacey Son Aq Mt sson@FreeBSD.org .
.Sh BUGS
Because files can be dynamically linked into the kernel at any time the symbol
information can vary.  When you open the
.Pa /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. Keeping the device open does not block the loading or unloading of kernel modules.  To get a new snapshot you must close and re-open the device.
.Pp
A process is only allowed to open the
.Pa /dev/ksyms
file once at a time.  The process must close the
.Pa /dev/ksyms
before it is allowed to open it again.
.Pp
The
.Nm
driver uses the calling process' memory address space to store the snapshot.
.Xr ioctl 2
can be used to get the memory address where the symbol table is stored to
save kernel memory.
.Xr mmap 2
may also be used but it will map it to another address.