read procfs with fsopen/fsmount (without actually attaching the mount to the visible vfs)

procread.c

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

#include <cstdint>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <linux/unistd.h>
#include <linux/mount.h>

namespace {

inline int fsopen(const char *fs_name, unsigned int flags)
{
	return syscall(__NR_fsopen, fs_name, flags);
}

inline int fsmount(int fsfd, unsigned int flags, unsigned int ms_flags)
{
	return syscall(__NR_fsmount, fsfd, flags, ms_flags);
}

inline int fsconfig(int fsfd, unsigned int cmd,
			   const char *key, const void *val, int aux)
{
	return syscall(__NR_fsconfig, fsfd, cmd, key, val, aux);
}

constexpr unsigned int MOUNT_FLAGS = MOUNT_ATTR_RDONLY
	| MOUNT_ATTR_NODEV
	| MOUNT_ATTR_NOSUID
	| MOUNT_ATTR_NOEXEC
	| MOUNT_ATTR_RELATIME;

}

int main() {
	int ret = 0;

	int pid = getpid();
	printf("pid=%d\n", pid);

	int fsfd = fsopen("proc", FSOPEN_CLOEXEC);
	if (fsfd == -1) {
		printf("fsopen=%d, errno=%d (%s)\n", fsfd, errno, strerror(errno));
		return 1;
	}

	ret = fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
	if (ret == -1) {
		printf("fsconfig=%d, errno=%d (%s)\n", fsfd, errno, strerror(errno));
		close(fsfd);
		return 1;
	}

	printf("fsfd=%d\n", fsfd);
	
	int mfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_FLAGS);
	if (mfd == -1) {
		printf("fsmount=%d, errno=%d (%s)\n", mfd, errno, strerror(errno));
		close(fsfd);
		return 1;
	}

	close(fsfd);

	printf("mfd=%d\n", mfd);

	int verfd = openat(mfd, "version", O_RDONLY | O_CLOEXEC);
	if (verfd == -1) {
		printf("openat, errno=%d (%s)\n", errno, strerror(errno));
		close(mfd);
		return 1;
	}

	printf("verfd=%d\n", verfd);

	uint8_t buf[255] {};
	int r = read(verfd, buf, sizeof(buf));

	if (r > 0) {
		buf[sizeof(buf) - 1] = 0;
		printf("%s\n", buf);
	} else {
		printf("read failed with %d errno=%d (%s)\n", r, errno, strerror(errno));
	}

	close(verfd);
	close(mfd);
}