Ubuntu Manpage: kexec_load, kexec_file_load - load a new kernel for later execution

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 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 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 CONFIG_KEXEC_JUMP, and is effective only if nr_segments is
greater than 0.

The high-order bits (corresponding to the mask 0xffff0000) of flags contain 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 address 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 configured 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 initrd (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 argument 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 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 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 contain 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.

VERSIONS

       The kexec_load() system call first appeared in Linux 2.6.13.  The kexec_file_load() system
       call first appeared in Linux 3.17.

STANDARDS

       These system calls are Linux-specific.