linux-kernel - Re: [PATCH v2] kho: add test for kexec handover

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Message-ID: <fae1936d-3f45-44d4-a9b0-43b29115d5ac@t-8ch.de>
Date: Fri, 1 Aug 2025 13:00:03 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Mike Rapoport <rppt@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Alexander Graf <graf@...zon.com>, Changyuan Lyu <changyuanl@...gle.com>, 
	Pasha Tatashin <pasha.tatashin@...een.com>, Pratyush Yadav <pratyush@...nel.org>, 
	Shuah Khan <shuah@...nel.org>, kexec@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] kho: add test for kexec handover

On 2025-08-01 13:06:30+0300, Mike Rapoport wrote:
> From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> 
> Testing kexec handover requires a kernel driver that will generate some
> data and preserve it with KHO on the first boot and then restore that
> data and verify it was preserved properly after kexec.
> 
> To facilitate such test, along with the kernel driver responsible for
> data generation, preservation and restoration add a script that runs a
> kernel in a VM with a minimal /init. The /init enables KHO, loads a
> kernel image for kexec and runs kexec reboot. After the boot of the
> kexeced kernel, the driver verifies that the data was properly
> preserved.
> 
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>

Looking at it while awake brought up some more nitpicks.
In any case:
Reviewed-by: Thomas Weißschuh <linux@...ssschuh.net> # for the nolibc usage

> diff --git a/tools/testing/selftests/kho/init.c b/tools/testing/selftests/kho/init.c
> new file mode 100644
> index 000000000000..8044ca56fff5
> --- /dev/null
> +++ b/tools/testing/selftests/kho/init.c
> @@ -0,0 +1,98 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <errno.h>

Not actually used.

> +#include <stdio.h>
> +#include <unistd.h>
> +#include <fcntl.h>
> +#include <sys/syscall.h>
> +#include <sys/mount.h>
> +#include <sys/reboot.h>
> +
> +/* from arch/x86/include/asm/setup.h */
> +#define COMMAND_LINE_SIZE	2048
> +
> +/* from include/linux/kexex.h */

"kexec.h"

> +#define KEXEC_FILE_NO_INITRAMFS	0x00000004

This is actually part of the UAPI headers, why not use it from there?

> +
> +#define KHO_FINILIZE "/debugfs/kho/out/finalize"

KHO_FINALIZE

> +#define KERNEL_IMAGE "/kernel"
> +
> +static int mount_filesystems(void)
> +{
> +	if (mount("debugfs", "/debugfs", "debugfs", 0, NULL) < 0)
> +		return -1;
> +
> +	return mount("proc", "/proc", "proc", 0, NULL);
> +}
> +
> +static int kho_enable(void)
> +{
> +	const char enable[] = "1";
> +	int fd;
> +
> +	fd = open(KHO_FINILIZE, O_RDWR);
> +	if (fd < 0)
> +		return -1;
> +
> +	if (write(fd, enable, sizeof(enable)) != sizeof(enable))
> +		return 1;
> +
> +	close(fd);
> +	return 0;
> +}
> +
> +static long kexec_file_load(int kernel_fd, int initrd_fd,
> +			    unsigned long cmdline_len, const char *cmdline,
> +			    unsigned long flags)
> +{
> +	return syscall(__NR_kexec_file_load, kernel_fd, initrd_fd, cmdline_len,
> +		       cmdline, flags);
> +}
> +
> +static int kexec_load(void)
> +{
> +	char cmdline[COMMAND_LINE_SIZE];
> +	ssize_t len;
> +	int fd, err;
> +
> +	fd = open("/proc/cmdline", O_RDONLY);
> +	if (fd < 0)
> +		return -1;
> +
> +	len = read(fd, cmdline, sizeof(cmdline));
> +	close(fd);
> +	if (len < 0)
> +		return -1;
> +
> +	/* replace \n with \0 */
> +	cmdline[len - 1] = 0;
> +	fd = open(KERNEL_IMAGE, O_RDONLY);
> +	if (fd < 0)
> +		return -1;
> +
> +	err = kexec_file_load(fd, -1, len, cmdline, KEXEC_FILE_NO_INITRAMFS);
> +	close(fd);
> +
> +	return err ? : 0;
> +}
> +
> +int main(int argc, char *argv[])
> +{
> +	if (mount_filesystems())
> +		goto err_reboot;
> +
> +	if (kho_enable())
> +		goto err_reboot;
> +
> +	if (kexec_load())
> +		goto err_reboot;
> +
> +	if (reboot(RB_KEXEC))
> +		goto err_reboot;
> +
> +	return 0;
> +
> +err_reboot:
> +	reboot(RB_AUTOBOOT);
> +	return -1;
> +}
> diff --git a/tools/testing/selftests/kho/vmtest.sh b/tools/testing/selftests/kho/vmtest.sh
> new file mode 100755
> index 000000000000..3f6c17166846
> --- /dev/null
> +++ b/tools/testing/selftests/kho/vmtest.sh

(...)

> +function mkinitrd() {
> +	local kernel=$1
> +
> +	"$CROSS_COMPILE"gcc -s -static -Os -nostdinc -nostdlib \
> +			-fno-asynchronous-unwind-tables -fno-ident \
> +			-I "$headers_dir/include" \
> +			-I "$kernel_dir/tools/include/nolibc" \
> +			-o "$tmp_dir/init" "$test_dir/init.c"

You could build this as a userprog as part of kbuild.
Then it will also work automatically with clang.

> +
> +	cat > "$tmp_dir/cpio_list" <<EOF
> +dir /dev 0755 0 0
> +dir /proc 0755 0 0
> +dir /debugfs 0755 0 0
> +nod /dev/console 0600 0 0 c 5 1
> +file /init $tmp_dir/init 0755 0 0
> +file /kernel $kernel 0644 0 0
> +EOF
> +
> +	"$build_dir/usr/gen_init_cpio" "$tmp_dir/cpio_list" > "$initrd"

This could also be hooked up to kbuild.

> +}

(...)