qemu: allow selection of host qemu-user from menuconfig

Raise an error if the host is using an older kernel than the target. Since qemu-user passes emulated system calls to the host kernel, this prevents usage of qemu-user in situations where those system calls will fail.

This is based on an original patch from Frank Hunleth <[email protected]>, but completely rewritten in a different way:

* Instead of using shell based testing, we use pure make tests, which allows to detect the problem not when host-qemu starts to build, but at the very beginning of the entire Buildroot build.

* Instead of looking at $(STAGING_DIR)/usr/include/linux/version.h (which requires having a dependency on the 'toolchain' package, which is a bit unusual for a host package), we use the BR2_TOOLCHAIN_HEADERS_AT_LEAST Config.in option which tells us the version of the kernel headers used in the toolchain.

Signed-off-by Thomas Petazzoni <[email protected]>

---
 package/qemu/qemu.mk | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

package/qemu/qemu.mk +28 -1

index 9cff5ed..6c5ddf3 100644
@@ -59,6 +59,34 @@ HOST_QEMU_ARCH = ppc
 endif
 HOST_QEMU_TARGETS = $(HOST_QEMU_ARCH)-linux-user
 
+ifeq ($(BR2_PACKAGE_HOST_QEMU),y)
+HOST_QEMU_HOST_SYSTEM_TYPE = $(shell uname -s)
+ifneq ($(HOST_QEMU_HOST_SYSTEM_TYPE),Linux)
+$(error "qemu-user can only be used on Linux hosts")
+endif
+
+HOST_QEMU_HOST_SYSTEM_VERSION_MAJOR = $(shell uname -r | cut -f1 -d'.')
+HOST_QEMU_HOST_SYSTEM_VERSION_MINOR = $(shell uname -r | cut -f2 -d'.')
+HOST_QEMU_TARGET_SYSTEM_VERSION_MAJOR = $(shell echo $(BR2_TOOLCHAIN_HEADERS_AT_LEAST) | cut -f1 -d'.')
+HOST_QEMU_TARGET_SYSTEM_VERSION_MINOR = $(shell echo $(BR2_TOOLCHAIN_HEADERS_AT_LEAST) | cut -f2 -d'.')
+HOST_QEMU_COMPARE_VERSION_MAJOR = $(shell test $(HOST_QEMU_HOST_SYSTEM_VERSION_MAJOR) -ge $(HOST_QEMU_TARGET_SYSTEM_VERSION_MAJOR) && echo OK)
+HOST_QEMU_COMPARE_VERSION_MINOR = $(shell test $(HOST_QEMU_HOST_SYSTEM_VERSION_MINOR) -ge $(HOST_QEMU_TARGET_SYSTEM_VERSION_MINOR) && echo OK)
+
+#
+# The principle of qemu-user is that it emulates the instructions of
+# the target architecture when running the binary, and then when this
+# binary does a system call, it converts this system call into a
+# system call on the host machine. This mechanism makes an assumption:
+# that the target binary will not do system calls that do not exist on
+# the host. This basically requires that the target binary should be
+# built with kernel headers that are older or the same as the kernel
+# version running on the host machine.
+#
+ifneq ($(HOST_QEMU_COMPARE_VERSION_MAJOR)$(HOST_QEMU_COMPARE_VERSION_MINOR),OKOK)
+$(error "Refusing to build qemu-user: target Linux version newer than host's.")
+endif
+endif
+
 define HOST_QEMU_CONFIGURE_CMDS
        cd $(@D); $(HOST_CONFIGURE_OPTS) ./configure    \
                --target-list="$(HOST_QEMU_TARGETS)"    \
--

2.1.0

+ifneq ($(HOST_QEMU_COMPARE_VERSION_MAJOR)$(HOST_QEMU_COMPARE_VERSION_MINOR),OKO
K) +$(error "Refusing to build qemu-user: target Linux version newer than host's.") +endif

Quoted 4 lines

+ifneq ($(HOST_QEMU_COMPARE_VERSION_MAJOR)$(HOST_QEMU_COMPARE_VERSION_MINOR),OKO
K) +$(error "Refusing to build qemu-user: target Linux version newer than host's.") +endif

while testing an allyesconfig with "make source-check" this error was raised. I had to comment this line in order to run the source-check. Is it possible to run this check only during a build phase?

Frank Hunleth (1):
qemu: add to host utilities menu

Thomas Petazzoni (1):
qemu: add host/target Linux version check