freebsd-dev/share/man/man4/ksyms.4
Mark Johnston 22e406c80b Rework and simplify the ksyms(4) implementation.
- Store the symbol table contents in an anonymous swap-backed object. Have
  mmap(/dev/ksyms) map that object, and stop mapping the symbol table into
  the calling process in ksyms_open(). Previously we would cache a pointer
  to the pmap of the opening process, and mmap(/dev/ksyms) would create a
  mapping using the physical address found by a pmap lookup at the initial
  mapping address. However, this assumes that the cached pmap is valid,
  which may not be the case. [1]
- Remove the ksyms ioctl interface. It appears to have been added to work
  around a limitation in libelf that no longer exists; see r321842.
  Moreover, the interface is difficult to support and isn't present in
  illumos. Since ksyms was added specifically to support lockstat(1), it
  is expected that this removal won't have any real impact.
- Simplify ksyms_read() to avoid unnecessary copying.
- Don't call the device handle destructor if we fail to capture a snapshot
  of the kernel's symbol table. devfs will do that for us.

Reported by:	Ilja van Sprundel <ivansprundel@ioactive.com> [1]
Reviewed by:	kib (previous revision)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D11789
2017-08-03 00:38:13 +00:00

136 lines
4.4 KiB
Groff

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.Dd August 2, 2017
.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 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 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.