Edited kexec_load(2) [kexec_file_load()] man page for review

Michael Kerrisk (man-pages) mtk.manpages at gmail.com
Wed Jan 28 07:49:34 PST 2015 

[Dropping Andi into CC, which I should have done to start with, since
he wrote the original page, and might also have some comments]

Hello Vivek,

>> I've made various adjustments to the page in the light of your comments
>> above. Thanks!
>
> Thank you for following it up and improving kexec man page.

You're welcome. So, by now, I've made quite a lot of changes
(including adding a number of cases under ERRORS). I think the revised
kexec_load/kexec_file_load page is pretty much ready to go, but would
you be willing to give the text below a check over first?

Thanks

Michael

====

.\" Copyright (C) 2010 Intel Corporation, Author: Andi Kleen
.\" and Copyright 2014, Vivek Goyal <vgoyal at redhat.com>
.\" and Copyright (c) 2015, Michael Kerrisk <mtk.manpages at gmail.com>
.\"
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.\" entire resulting derived work is distributed under the terms of a
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.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
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.\"
.TH KEXEC_LOAD 2 2014-08-19 "Linux" "Linux Programmer's Manual"
.SH NAME
kexec_load, kexec_file_load \- load a new kernel for later execution
.SH SYNOPSIS
.nf
.B #include <linux/kexec.h>

.BI "long kexec_load(unsigned long " entry ", unsigned long " nr_segments ","
.BI "                struct kexec_segment *" segments \
", unsigned long " flags ");"

.BI "long kexec_file_load(int " kernel_fd ", int " initrd_fd ","
.br
.BI "                    unsigned long " cmdline_len  \
", const char *" cmdline ","
.BI "                    unsigned long " flags ");"

.fi
.IR Note :
There are no glibc wrappers for these system calls; see NOTES.
.SH DESCRIPTION
The
.BR kexec_load ()
system call loads a new kernel that can be executed later by
.BR reboot (2).
.PP
The
.I flags
argument is a bit mask that controls the operation of the call.
The following values can be specified in
.IR flags :
.TP
.BR KEXEC_ON_CRASH " (since Linux 2.6.13)"
Execute the new kernel automatically on a system crash.
This "crash kernel" is loaded into an area of reserved memory that
is determined at boot time using the
.I craskkernel
kernel command-line parameter.
The location of this reserved memory is exported to user space via the
.I /proc/iomem
file, in an entry labeled "Crash kernel".
A user-space application can parse this file and prepare a list of
segments (see below) that specify this reserved memory as destination.
If this flag is specified, the kernel checks that the
target segments specified in
.I segments
fall within the reserved region.
.TP
.BR KEXEC_PRESERVE_CONTEXT " (since Linux 2.6.27)"
Preserve the system hardware and
software states before executing the new kernel.
This could be used for system suspend.
This flag is available only if the kernel was configured with
.BR CONFIG_KEXEC_JUMP ,
and is effective only if
.I nr_segments
is greater than 0.
.PP
The high-order bits (corresponding to the mask 0xffff0000) of
.I flags
contain the architecture of the to-be-executed kernel.
Specify (OR) the constant
.B KEXEC_ARCH_DEFAULT
to use the current architecture,
or one of the following architecture constants
.BR KEXEC_ARCH_386 ,
.BR KEXEC_ARCH_68K ,
.BR KEXEC_ARCH_X86_64 ,
.BR KEXEC_ARCH_PPC ,
.BR KEXEC_ARCH_PPC64 ,
.BR KEXEC_ARCH_IA_64 ,
.BR KEXEC_ARCH_ARM ,
.BR KEXEC_ARCH_S390 ,
.BR KEXEC_ARCH_SH ,
.BR KEXEC_ARCH_MIPS ,
and
.BR KEXEC_ARCH_MIPS_LE .
The architecture must be executable on the CPU of the system.

The
.I entry
argument is the physical entry address in the kernel image.
The
.I nr_segments
argument is the number of segments pointed to by the
.I segments
pointer;
the kernel imposes an (arbitrary) limit of 16 on the number of segments.
The
.I segments
argument is an array of
.I kexec_segment
structures which define the kernel layout:
.in +4n
.nf

struct kexec_segment {
    void   *buf;        /* Buffer in user space */
    size_t  bufsz;      /* Buffer length in user space */
    void   *mem;        /* Physical address of kernel */
    size_t  memsz;      /* Physical address length */
};
.fi
.in
.PP
The kernel image defined by
.I segments
is copied from the calling process into
the kernel either in regular
memory or in reserved memory (if
.BR KEXEC_ON_CRASH
is set).
The kernel first performs various sanity checks on the
information passed in
.IR segments .
If these checks pass, the kernel copies the segment data to kernel memory.
Each segment specified in
.I segments
is copied as follows:
.IP * 3
.I buf
and
.I bufsz
identify a memory region in the caller's virtual address space
that is the source of the copy.
The value in
.I bufsz
may not exceed the value in the
.I memsz
field.
.IP *
.I mem
and
.I memsz
specify a physical address range that is the target of the copy.
The values specified in both fields must be multiples of
the system page size.
.IP *
.I bufsz
bytes are copied from the source buffer to the target kernel buffer.
If
.I bufsz
is less than
.IR memsz ,
then the excess bytes in the kernel buffer are zeroed out.
.PP
In case of a normal kexec (i.e., the
.BR KEXEC_ON_CRASH
flag is not set), the segment data is loaded in any available memory
and is moved to the final destination at kexec reboot time (e.g., when the
.BR kexec (8)
command is executed with the
.I \-e
option).

In case of kexec on panic (i.e., the
.BR KEXEC_ON_CRASH
flag is set), the segment data is
loaded to reserved memory at the time of the call, and, after a crash,
the kexec mechanism simply passes control to that kernel.

The
.BR kexec_load ()
system call is available only if the kernel was configured with
.BR CONFIG_KEXEC .
.SS kexec_file_load()
The
.BR kexec_file_load ()
system call is similar to
.BR kexec_load (),
but it takes a different set of arguments.
It reads the kernel to be loaded from the file referred to by the descriptor
.IR kernel_fd ,
and the initrd (initial RAM disk)
to be loaded from file referred to by the descriptor
.IR initrd_fd .
The
.IR cmdline
argument is a pointer to a buffer containing the command line
for the new kernel.
The
.IR cmdline_len
argument specifies size of the buffer.
The last byte in the buffer must be a null byte (\(aq\\0\(aq).

The
.IR flags
argument is a bit mask which modifies the behavior of the call.
The following values can be specified in
.IR flags :
.TP
.BR KEXEC_FILE_UNLOAD
Unload the currently loaded kernel.
.TP
.BR KEXEC_FILE_ON_CRASH
Load the new kernel in the memory region reserved for the crash kernel
(as for
.BR KEXEC_ON_CRASH).
This kernel is booted if the currently running kernel crashes.
.TP
.BR KEXEC_FILE_NO_INITRAMFS
Loading initrd/initramfs is optional.
Specify this flag if no initramfs is being loaded.
If this flag is set, the value passed in
.IR initrd_fd
is ignored.
.PP
The
.BR kexec_file_load ()
.\" See also http://lwn.net/Articles/603116/
system call was added to provide support for systems
where "kexec" loading should be restricted to
only kernels that are signed.
This system call is available only if the kernel was configured with
.BR CONFIG_KEXEC_FILE .
.SH RETURN VALUE
On success, these system calls returns 0.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EADDRNOTAVAIL
.\" See kernel/kexec.::sanity_check_segment_list in the 3.19 kernel source
The
.B KEXEC_ON_CRASH
flags was specified, but the region specified by the
.I mem
and
.I memsz
fields of one of the
.I segments
entries lies outside the range of memory reserved for the crash kernel.
.TP
.B EADDRNOTAVAIL
The value in a
.I mem
or
.I memsz
field in one of the
.I segments
entries is not a multiple of the system page size.
.TP
.B EBADF
.I kernel_fd
or
.I initrd_fd
is not a valid file descriptor.
.TP
.B EBUSY
Another crash kernel is already being loaded
or a crash kernel is already in use.
.TP
.B EINVAL
.I flags
is invalid.
.TP
.B EINVAL
The value of a
.I bufsz
field in one of the
.I segments
entries exceeds the value in the corresponding
.I memsz
field.
.TP
.B EINVAL
.IR nr_segments
exceeds
.BR KEXEC_SEGMENT_MAX
(16).
.TP
.B EINVAL
Two or more of the kernel target buffers overlap.
.TP
.B EINVAL
The value in
.I cmdline[cmdline_len-1]
is not \(aq\\0\(aq.
.TP
.B EINVAL
The file referred to by
.I kernel_fd
or
.I initrd_fd
is empty (length zero).
.TP
.B ENOMEM
Could not allocate memory.
.TP
.B ENOEXEC
.I kernel_fd
does not refer to an open file, or the kernel can't load this file.
.TP
.B EPERM
The caller does not have the
.BR CAP_SYS_BOOT
capability.
.SH VERSIONS
The
.BR kexec_load ()
system call first appeared in Linux 2.6.13.
The
.BR kexec_file_load ()
system call first appeared in Linux 3.17.
.SH CONFORMING TO
These system calls are Linux-specific.
.SH NOTES
Currently, there is no glibc support for these system calls.
Call them using
.BR syscall (2).
.SH SEE ALSO
.BR reboot (2),
.BR syscall (2),
.BR kexec (8)

The kernel source files
.IR Documentation/kdump/kdump.txt
and
.IR Documentation/kernel-parameters.txt .

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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