kexec_load(2) System Calls Manual kexec_load(2)
NAME
kexec_load, kexec_file_load - load a new kernel for later execution
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <linux/kexec.h> /* Definition of KEXEC_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
long syscall(SYS_kexec_load, unsigned long entry,
unsigned long nr_segments, struct kexec_segment *segments,
unsigned long flags);
long syscall(SYS_kexec_file_load, int kernel_fd, int initrd_fd,
unsigned long cmdline_len, const char *cmdline,
unsigned long flags);
Note: glibc provides no wrappers for these system calls, necessitating
the use of syscall(2).
DESCRIPTION
The kexec_load() system call loads a new kernel that can be executed
later by reboot(2).
The flags argument is a bit mask that controls the operation of the
call. The following values can be specified in flags:
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 crashkernel kernel command-line
parameter. The location of this reserved memory is exported to
user space via the /proc/iomem file, in an entry labeled "Crash
kernel". A user-space application can parse this file and pre-
pare 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 segments fall within
the reserved region.
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 CON-
FIG_KEXEC_JUMP, and is effective only if nr_segments is greater
than 0.
The high-order bits (corresponding to the mask 0xffff0000) of flags con-
tain the architecture of the to-be-executed kernel. Specify (OR) the
constant KEXEC_ARCH_DEFAULT to use the current architecture, or one of
the following architecture constants KEXEC_ARCH_386, KEXEC_ARCH_68K,
KEXEC_ARCH_X86_64, KEXEC_ARCH_PPC, KEXEC_ARCH_PPC64, KEXEC_ARCH_IA_64,
KEXEC_ARCH_ARM, KEXEC_ARCH_S390, KEXEC_ARCH_SH, KEXEC_ARCH_MIPS, and
KEXEC_ARCH_MIPS_LE. The architecture must be executable on the CPU of
the system.
The entry argument is the physical entry address in the kernel image.
The nr_segments argument is the number of segments pointed to by the
segments pointer; the kernel imposes an (arbitrary) limit of 16 on the
number of segments. The segments argument is an array of kexec_segment
structures which define the kernel layout:
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 */
};
The kernel image defined by segments is copied from the calling process
into the kernel either in regular memory or in reserved memory (if
KEXEC_ON_CRASH is set). The kernel first performs various sanity checks
on the information passed in segments. If these checks pass, the kernel
copies the segment data to kernel memory. Each segment specified in
segments is copied as follows:
• buf and bufsz identify a memory region in the caller's virtual ad-
dress space that is the source of the copy. The value in bufsz may
not exceed the value in the memsz field.
• mem and 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.
• bufsz bytes are copied from the source buffer to the target kernel
buffer. If bufsz is less than memsz, then the excess bytes in the
kernel buffer are zeroed out.
In case of a normal kexec (i.e., the 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 kexec(8) command
is executed with the -e option).
In case of kexec on panic (i.e., the 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 kexec_load() system call is available only if the kernel was config-
ured with CONFIG_KEXEC.
kexec_file_load()
The kexec_file_load() system call is similar to kexec_load(), but it
takes a different set of arguments. It reads the kernel to be loaded
from the file referred to by the file descriptor kernel_fd, and the ini-
trd (initial RAM disk) to be loaded from file referred to by the file
descriptor initrd_fd. The cmdline argument is a pointer to a buffer
containing the command line for the new kernel. The cmdline_len argu-
ment specifies size of the buffer. The last byte in the buffer must be
a null byte ('\0').
The flags argument is a bit mask which modifies the behavior of the
call. The following values can be specified in flags:
KEXEC_FILE_UNLOAD
Unload the currently loaded kernel.
KEXEC_FILE_ON_CRASH
Load the new kernel in the memory region reserved for the crash
kernel (as for KEXEC_ON_CRASH). This kernel is booted if the
currently running kernel crashes.
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 initrd_fd is ignored.
The kexec_file_load() system call was added to provide support for sys-
tems where "kexec" loading should be restricted to only kernels that are
signed. This system call is available only if the kernel was configured
with CONFIG_KEXEC_FILE.
RETURN VALUE
On success, these system calls returns 0. On error, -1 is returned and
errno is set to indicate the error.
ERRORS
EADDRNOTAVAIL
The KEXEC_ON_CRASH flags was specified, but the region specified
by the mem and memsz fields of one of the segments entries lies
outside the range of memory reserved for the crash kernel.
EADDRNOTAVAIL
The value in a mem or memsz field in one of the segments entries
is not a multiple of the system page size.
EBADF kernel_fd or initrd_fd is not a valid file descriptor.
EBUSY Another crash kernel is already being loaded or a crash kernel is
already in use.
EINVAL flags is invalid.
EINVAL The value of a bufsz field in one of the segments entries exceeds
the value in the corresponding memsz field.
EINVAL nr_segments exceeds KEXEC_SEGMENT_MAX (16).
EINVAL Two or more of the kernel target buffers overlap.
EINVAL The value in cmdline[cmdline_len-1] is not '\0'.
EINVAL The file referred to by kernel_fd or initrd_fd is empty (length
zero).
ENOEXEC
kernel_fd does not refer to an open file, or the kernel can't
load this file. Currently, the file must be a bzImage and con-
tain an x86 kernel that is loadable above 4 GiB in memory (see
the kernel source file Documentation/x86/boot.txt).
ENOMEM Could not allocate memory.
EPERM The caller does not have the CAP_SYS_BOOT capability.
STANDARDS
Linux.
HISTORY
kexec_load()
Linux 2.6.13.
kexec_file_load()
Linux 3.17.
SEE ALSO
reboot(2), syscall(2), kexec(8)
The kernel source files Documentation/kdump/kdump.txt and Documenta-
tion/admin-guide/kernel-parameters.txt
Linux man-pages 6.9.1 2024-06-15 kexec_load(2)
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