[PATCH v1] kexec-tools: Perform run-time linking of libxenctrl.so

Eric DeVolder eric.devolder at oracle.com
Wed Nov 29 08:45:16 PST 2017

When kexec is utilized in a Xen environment, it has an explicit
run-time dependency on libxenctrl.so. This dependency occurs
during the configure stage and when building kexec-tools.

When kexec is utilized in a non-Xen environment (either bare
metal or KVM), the configure and build of kexec-tools omits
any reference to libxenctrl.so.

Thus today it is not currently possible to configure and build
a *single* kexec that will work in *both* Xen and non-Xen
environments, unless the libxenctrl.so is *always* present.

For example, a kexec configured for Xen in a Xen environment:

 # ldd build/sbin/kexec
        linux-vdso.so.1 =>  (0x00007ffdeba5c000)
        libxenctrl.so.4.4 => /usr/lib64/libxenctrl.so.4.4 (0x00000038d8000000)
        libz.so.1 => /lib64/libz.so.1 (0x00000038d6c00000)
        libc.so.6 => /lib64/libc.so.6 (0x00000038d6000000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00000038d6400000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00000038d6800000)
        /lib64/ld-linux-x86-64.so.2 (0x000055e9f8c6c000)
 # build/sbin/kexec -v
 kexec-tools 2.0.16

However, the *same* kexec executable fails in a non-Xen environment:

 # copy xen kexec to .
 # ldd ./kexec
         linux-vdso.so.1 =>  (0x00007fffa9da7000)
         libxenctrl.so.4.4 => not found
         liblzma.so.0 => /usr/lib64/liblzma.so.0 (0x0000003014e00000)
         libz.so.1 => /lib64/libz.so.1 (0x000000300ea00000)
         libc.so.6 => /lib64/libc.so.6 (0x000000300de00000)
         libpthread.so.0 => /lib64/libpthread.so.0 (0x000000300e200000)
         /lib64/ld-linux-x86-64.so.2 (0x0000558cc786c000)
 # ./kexec -v
 ./kexec: error while loading shared libraries:
 libxenctrl.so.4.4: cannot open shared object file: No such file or directory

At Oracle we "workaround" this by having two kexec-tools packages,
one for Xen and another for non-Xen environments. At Oracle, the
desire is to offer a single kexec-tools package that works in either
environment. To achieve this, kexec-tools would either have to ship
with libxenctrl.so (which we have deemed as unacceptable), or we can
make kexec perform run-time linking against libxenctrl.so.

This patch is one possible way to alleviate the explicit run-time
dependency on libxenctrl.so. This implementation utilizes a set of
macros to wrap calls into libxenctrl.so so that the library can
instead be dlopen() and obtain the function via dlsym() and then
make the call. The advantage of this implementation is that it
requires few changes to the existing kexec-tools code. The dis-
advantage is that it uses macros to remap libxenctrl functions
and do work under the hood.

Another possible implementation worth considering is the approach
taken by libvmi. Reference the following file:


The libxc_wrapper_t structure definition that starts at line ~33
has members that are function pointers into libxenctrl.so. This
structure is populated once and then later referenced/dereferenced
by the callers of libxenctrl.so members. The advantage of this
implementation is it is more explicit in managing the use of
libxenctrl.so and its versions, but the disadvantage is it would
require touching more of the kexec-tools code.

The following is a list libxenctrl members utilized by kexec:



These were identified by configuring and building kexec-tools
with Xen support, but omitting the -lxenctrl from the LDFLAGS
in the Makefile for an x86_64 build.

The above libxenctrl members were referenced via these source


This patch provides a wrapper around the calls to the above
functions in libxenctrl.so. Every libxenctrl call must pass a
xc_interface which it obtains from xc_interface_open().
So the existing code is already structured in a manner that
facilitates graceful dlopen()'ing of the libxenctrl.so and
the subsequent dlsym() of the required member.

The patch creates a wrapper function around xc_interface_open()
and xc_interface_close() to perform the dlopen() and dlclose().

For the remaing xc_ functions, this patch defines a macro
of the same name which performs the dlsym() and then invokes
the function. See the _xc_call() macro for details.

There was one data item in libxenctrl.so that presented a
unique problem, HYPERCALL_BUFFER_NULL. It was only utilized
once, as

    set_xen_guest_handle(xen_segs[s].buf.h, HYPERCALL_BUFFER_NULL);

I tried a variety of techniques but could not find a general
macro-type solution without modifying xenctrl.h. So the
solution was to declare a local HYPERCALL_BUFFER_NULL, and
this appears to work. I admit I am not familiar with libxenctrl
to state if this is a satisfactory workaround, so feedback
here welcome. I can state that this allows kexec to load/unload/kexec
on Xen and non-Xen environments that I've tested without issue.

With this patch applied, kexec-tools can be built with Xen
support and yet there is no explicit run-time dependency on
libxenctrl.so. Thus it can also be deployed in non-Xen
environments where libxenctrl.so is not installed.

 # ldd build/sbin/kexec
        linux-vdso.so.1 =>  (0x00007fff7dbcd000)
        liblzma.so.0 => /usr/lib64/liblzma.so.0 (0x00000038d9000000)
        libz.so.1 => /lib64/libz.so.1 (0x00000038d6c00000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00000038d6400000)
        libc.so.6 => /lib64/libc.so.6 (0x00000038d6000000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00000038d6800000)
        /lib64/ld-linux-x86-64.so.2 (0x0000562dc0c14000)
 # build/sbin/kexec -v
 kexec-tools 2.0.16

Currently this feature is enabled with the following:

 ./configure --with-xen-dl --with-xen=no

This is a bit clunky. I welcome feedback such as better names
and/or usage of --with, as well as if we might make this feature
the default.

Signed-off-by: Eric DeVolder <eric.devolder at oracle.com>
 configure.ac                       | 18 ++++++++++++++
 kexec/Makefile                     |  1 +
 kexec/arch/i386/crashdump-x86.c    |  4 +---
 kexec/arch/i386/kexec-x86-common.c |  4 +---
 kexec/crashdump-xen.c              |  4 +---
 kexec/kexec-xen.c                  | 43 +++++++++++++++++++++++++++++++++-
 kexec/kexec-xen.h                  | 48 ++++++++++++++++++++++++++++++++++++++
 7 files changed, 112 insertions(+), 10 deletions(-)
 create mode 100644 kexec/kexec-xen.h

diff --git a/configure.ac b/configure.ac
index 208dc0a..4fc8aa0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -92,6 +92,9 @@ AC_ARG_WITH([lzma], AC_HELP_STRING([--without-lzma],[disable lzma support]),
 AC_ARG_WITH([xen], AC_HELP_STRING([--without-xen],
 	[disable extended xen support]), [ with_xen="$withval"], [ with_xen=yes ] )
+AC_ARG_WITH([xen-dl], AC_HELP_STRING([--without-xen-dl],
+	[link libxenctrl.so at run-time rather than build-time]), [ with_xen_dl="$withval"], [ with_xen_dl=no ] )
 		AC_HELP_STRING([--with-booke],[build for booke]),
@@ -174,6 +177,21 @@ if test "$with_xen" = yes ; then
 				AC_MSG_NOTICE([The kexec_status call is not available]))
+if test "$with_xen_dl" = yes ; then
+	if test "$with_xen" = yes ; then
+		AC_MSG_ERROR([Options --with-xen and --with-xen-dl are mutually exclusive])
+	fi
+	AC_DEFINE(CONFIG_LIBXENCTRL_DL, 1, [Define to 1 to link libxenctrl.so at run-time rather than build-time])
+	AC_CHECK_HEADER(dlfcn.h, , AC_MSG_ERROR([Dynamic library linking not available]))
+	AC_CHECK_LIB(dl, dlopen, , AC_MSG_ERROR([Dynamic library linking not available]))
+	AC_CHECK_HEADER(xenctrl.h,
+		AC_DEFINE(HAVE_LIBXENCTRL, 1, [Define to 1 to enable run-time linking of libxenctrl.so]),
+		AC_MSG_ERROR([Xen support not available]))
+dnl NOTE: Explicitly *NOT* performing AC_CHECK_LIB(xenctrl) as only need the header file to build
+	AC_CHECK_HEADER(xenctrl.h,
+		AC_DEFINE(HAVE_KEXEC_CMD_STATUS, 1, [Define to 1 so kexec_status call is available]),
+		AC_MSG_NOTICE([The kexec_status call is not available]))
 dnl ---Sanity checks
 if test "$CC"      = "no"; then AC_MSG_ERROR([cc not found]); fi
diff --git a/kexec/Makefile b/kexec/Makefile
index 2b4fb3d..8871731 100644
--- a/kexec/Makefile
+++ b/kexec/Makefile
@@ -36,6 +36,7 @@ dist += kexec/Makefile						\
 	kexec/kexec-elf-boot.h					\
 	kexec/kexec-elf.h kexec/kexec-sha256.h			\
 	kexec/kexec-zlib.h kexec/kexec-lzma.h			\
+	kexec/kexec-xen.h 								\
 	kexec/kexec-syscall.h kexec/kexec.h kexec/kexec.8
 dist				+= kexec/proc_iomem.c
diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
index 69a063a..a948d9f 100644
--- a/kexec/arch/i386/crashdump-x86.c
+++ b/kexec/arch/i386/crashdump-x86.c
@@ -44,9 +44,7 @@
 #include "kexec-x86.h"
 #include "crashdump-x86.h"
-#include <xenctrl.h>
-#endif /* HAVE_LIBXENCTRL */
+#include "../../kexec-xen.h"
 #include "x86-linux-setup.h"
diff --git a/kexec/arch/i386/kexec-x86-common.c b/kexec/arch/i386/kexec-x86-common.c
index be03618..b44c8b7 100644
--- a/kexec/arch/i386/kexec-x86-common.c
+++ b/kexec/arch/i386/kexec-x86-common.c
@@ -40,9 +40,7 @@
 #include "../../crashdump.h"
 #include "kexec-x86.h"
-#include <xenctrl.h>
-#endif /* HAVE_LIBXENCTRL */
+#include "../../kexec-xen.h"
 /* Used below but not present in (older?) xenctrl.h */
 #ifndef E820_PMEM
diff --git a/kexec/crashdump-xen.c b/kexec/crashdump-xen.c
index 60594f6..2e4cbdc 100644
--- a/kexec/crashdump-xen.c
+++ b/kexec/crashdump-xen.c
@@ -18,9 +18,7 @@
 #include "config.h"
-#include <xenctrl.h>
+#include "kexec-xen.h"
 struct crash_note_info {
 	unsigned long base;
diff --git a/kexec/kexec-xen.c b/kexec/kexec-xen.c
index 2b448d3..2b01fee 100644
--- a/kexec/kexec-xen.c
+++ b/kexec/kexec-xen.c
@@ -10,10 +10,51 @@
 #include "config.h"
-#include <xenctrl.h>
+#include "kexec-xen.h"
 #include "crashdump.h"
+void *xc_dlhandle = NULL;
+xc_interface *_xc_interface_open(xentoollog_logger *logger,
+                                xentoollog_logger *dombuild_logger,
+                                unsigned open_flags)
+    xc_interface *xch = NULL;
+    if (NULL == xc_dlhandle)
+        xc_dlhandle = dlopen("libxenctrl.so", RTLD_NOW | RTLD_NODELETE);
+    if (xc_dlhandle) {
+        typedef xc_interface *(*func_t)(xentoollog_logger *logger,
+                                xentoollog_logger *dombuild_logger,
+                                unsigned open_flags);
+        func_t func = (func_t)dlsym(xc_dlhandle, "xc_interface_open");
+        xch = func(logger, dombuild_logger, open_flags);
+    }
+    return xch;
+int _xc_interface_close(xc_interface *xch)
+    int rc = -1;
+    if (xc_dlhandle) {
+        typedef int (*func_t)(xc_interface *xch);
+        func_t func = (func_t)dlsym(xc_dlhandle, "xc_interface_close");
+        rc = func(xch);
+        xc_dlhandle = NULL;
+    }
+    return rc;
 int xen_kexec_load(struct kexec_info *info)
 	uint32_t nr_segments = info->nr_segments;
diff --git a/kexec/kexec-xen.h b/kexec/kexec-xen.h
new file mode 100644
index 0000000..6d2046e
--- /dev/null
+++ b/kexec/kexec-xen.h
@@ -0,0 +1,48 @@
+#ifndef KEXEC_XEN_H
+#define KEXEC_XEN_H
+#include <xenctrl.h>
+#include <dlfcn.h>
+/* The handle from dlopen(), needed by dlsym(), dlclose() */
+extern void *xc_dlhandle;
+/* Wrappers around xc_interface_open/close() to insert dlopen/dlclose() */
+xc_interface *_xc_interface_open(xentoollog_logger *logger,
+                                xentoollog_logger *dombuild_logger,
+                                unsigned open_flags);
+int _xc_interface_close(xc_interface *xch);
+/* GCC expression statements for evaluating dlsym() */
+#define _xc_call(DTYPE, NAME, ARGS...) ({ DTYPE value; typedef DTYPE (*func_t)(xc_interface *, ...); func_t func = dlsym(xc_dlhandle, #NAME); value = func(ARGS); value; } )
+#define _xc_data(DTYPE, NAME) ({ DTYPE *value = (DTYPE *)dlsym(xc_dlhandle, #NAME); value; } )
+/* The wrappers around utilized xenctrl.h functions */
+#define xc_interface_open(A,B,C)    _xc_interface_open(A,B,C)
+#define xc_interface_close(A)       _xc_interface_close(A)
+#define xc_version(ARGS...)         _xc_call(int, xc_version, ARGS)
+#define xc_get_max_cpus(ARGS...)    _xc_call(int, xc_get_max_cpus, ARGS)
+#define xc_get_machine_memory_map(ARGS...)  _xc_call(int, xc_get_machine_memory_map, ARGS)
+#define xc_kexec_get_range(ARGS...) _xc_call(int, xc_kexec_get_range, ARGS)
+#define xc_kexec_load(ARGS...)      _xc_call(int, xc_kexec_load, ARGS)
+#define xc_kexec_unload(ARGS...)    _xc_call(int, xc_kexec_unload, ARGS)
+#define xc_kexec_status(ARGS...)    _xc_call(int, xc_kexec_status, ARGS)
+#define xc_kexec_exec(ARGS...)      _xc_call(int, xc_kexec_exec, ARGS)
+#define xc_hypercall_buffer_array_create(ARGS...)   _xc_call(xc_hypercall_buffer_array_t *, xc_hypercall_buffer_array_create, ARGS)
+#define xc__hypercall_buffer_alloc(ARGS...) _xc_call(void *, xc__hypercall_buffer_alloc, ARGS)
+#define xc__hypercall_buffer_free(ARGS...)  _xc_call(void  , xc__hypercall_buffer_free, ARGS)
+#define xc__hypercall_buffer_alloc_pages(ARGS...)   _xc_call(void *, xc__hypercall_buffer_alloc_pages, ARGS)
+#define xc__hypercall_buffer_free_pages(ARGS...)    _xc_call(void  , xc__hypercall_buffer_free_pages, ARGS)
+#define xc__hypercall_buffer_array_alloc(ARGS...)   _xc_call(void *, xc__hypercall_buffer_array_alloc, ARGS)
+#define xc__hypercall_buffer_array_get(ARGS...)     _xc_call(void *, xc__hypercall_buffer_array_get, ARGS)
+#define xc_hypercall_buffer_array_destroy(ARGS...)  _xc_call(void *, xc_hypercall_buffer_array_destroy, ARGS)
+#endif /* HAVE_LIBXENCTRL */
+#endif /* KEXEC_XEN_H */

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