freebsd-src/lib/libkvm/kvm.3

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.Dd February 5, 2020
.Dt KVM 3
.Os
.Sh NAME
.Nm kvm
.Nd kernel memory interface
.Sh LIBRARY
.Lb libkvm
.Sh DESCRIPTION
The
.Nm
library provides a uniform interface for accessing kernel virtual memory
images, including live systems and crash dumps.
Access to live systems is via
.Xr sysctl 3
for some functions, and
.Xr mem 4
and
.Xr kmem 4
for other functions,
while crash dumps can be examined via the core file generated by
.Xr savecore 8 .
The interface behaves similarly in both cases.
Memory can be read and written, kernel symbol addresses can be
looked up efficiently, and information about user processes can
be gathered.
.Pp
The
.Fn kvm_open
function is first called to obtain a descriptor for all subsequent calls.
.Sh COMPATIBILITY
The kvm interface was first introduced in SunOS.
A considerable
number of programs have been developed that use this interface,
making backward compatibility highly desirable.
In most respects, the Sun kvm interface is consistent and clean.
Accordingly, the generic portion of the interface (i.e.,
.Fn kvm_open ,
.Fn kvm_close ,
.Fn kvm_read ,
.Fn kvm_write ,
and
.Fn kvm_nlist )
has been incorporated into the
.Bx
interface.
Indeed, many kvm
applications (i.e., debuggers and statistical monitors) use only
this subset of the interface.
.Pp
The process interface was not kept.
This is not a portability
issue since any code that manipulates processes is inherently
machine dependent.
.Pp
Finally, the Sun kvm error reporting semantics are poorly defined.
The library can be configured either to print errors to
.Dv stderr
automatically,
or to print no error messages at all.
In the latter case, the nature of the error cannot be determined.
To overcome this, the
.Bx
interface includes a
routine,
.Xr kvm_geterr 3 ,
to return (not print out) the error message
corresponding to the most recent error condition on the
given descriptor.
.Sh CROSS DEBUGGING
The
.Nm
library supports inspection of crash dumps from non-native kernels.
Only a limited subset of the kvm interface is supported for these dumps.
To inspect a crash dump of a non-native kernel,
the caller must provide a
.Fa resolver
function when opening a descriptor via
.Fn kvm_open2 .
In addition,
the kvm interface defines an integer type
.Pq Vt kvaddr_t
that is large enough to hold all valid addresses of all supported
architectures.
The interface also defines a new namelist structure type
.Pq Vt "struct kvm_nlist"
for use with
.Fn kvm_nlist2 .
To avoid address truncation issues,
the caller should use
.Fn kvm_nlist2
and
.Fn kvm_read2
in place of
.Fn kvm_nlist
and
.Fn kvm_read ,
respectively.
Finally, only a limited subset of operations are supported for non-native
crash dumps:
.Fn kvm_close ,
.Fn kvm_geterr ,
.Fn kvm_kerndisp ,
.Fn kvm_open2 ,
.Fn kvm_native ,
.Fn kvm_nlist2 ,
and
.Fn kvm_read2 .
.Sh SEE ALSO
.Xr kvm_close 3 ,
.Xr kvm_getargv 3 ,
.Xr kvm_getenvv 3 ,
.Xr kvm_geterr 3 ,
.Xr kvm_getloadavg 3 ,
.Xr kvm_getprocs 3 ,
.Xr kvm_getswapinfo 3 ,
.Xr kvm_kerndisp 3 ,
.Xr kvm_native 3 ,
.Xr kvm_nlist 3 ,
.Xr kvm_nlist2 3 ,
.Xr kvm_open 3 ,
.Xr kvm_open2 3 ,
.Xr kvm_openfiles 3 ,
.Xr kvm_read 3 ,
.Xr kvm_read2 3 ,
.Xr kvm_write 3 ,
.Xr sysctl 3 ,
.Xr kmem 4 ,
.Xr mem 4
.Sh HISTORY
The
.Fn kvm_native ,
.Fn kvm_nlist2 ,
.Fn kvm_open2 ,
and
.Fn kvm_read2
functions first appeared in
.Fx 11.0 .