[RFC PATCH v2 04/13] KVM: guest_mem: Add ioctl KVM_LINK_GUEST_MEMFD

[Ryan Afranji <afranji@google.com>]

From: Ackerley Tng <ackerleytng@google.com>

KVM_LINK_GUEST_MEMFD will link a gmem fd's underlying inode to a new
file (and fd).

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Co-developed-by: Ryan Afranji <afranji@google.com>
Signed-off-by: Ryan Afranji <afranji@google.com>
---
 include/uapi/linux/kvm.h |  8 ++++++
 virt/kvm/guest_memfd.c   | 57 ++++++++++++++++++++++++++++++++++++++++
 virt/kvm/kvm_main.c      | 10 +++++++
 virt/kvm/kvm_mm.h        |  7 +++++
 4 files changed, 82 insertions(+)

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index c6988e2c68d5..8f17f0b462aa 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1583,4 +1583,12 @@ struct kvm_pre_fault_memory {
 	__u64 padding[5];
 };
 
+#define KVM_LINK_GUEST_MEMFD	_IOWR(KVMIO,  0xd6, struct kvm_link_guest_memfd)
+
+struct kvm_link_guest_memfd {
+	__u64 fd;
+	__u64 flags;
+	__u64 reserved[6];
+};
+
 #endif /* __LINUX_KVM_H */
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index a3918d1695b9..d76bd1119198 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -555,6 +555,63 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 	return __kvm_gmem_create(kvm, size, flags);
 }
 
+int kvm_gmem_link(struct kvm *kvm, struct kvm_link_guest_memfd *args)
+{
+	static const char *name = "[kvm-gmem]";
+	u64 flags = args->flags;
+	u64 valid_flags = 0;
+	struct file *dst_file, *src_file;
+	struct kvm_gmem *gmem;
+	struct timespec64 ts;
+	struct inode *inode;
+	struct fd f;
+	int ret, fd;
+
+	if (flags & ~valid_flags)
+		return -EINVAL;
+
+	f = fdget(args->fd);
+	src_file = fd_file(f);
+	if (!src_file)
+		return -EINVAL;
+
+	ret = -EINVAL;
+	if (src_file->f_op != &kvm_gmem_fops)
+		goto out;
+
+	/* Cannot link a gmem file with the same vm again */
+	gmem = src_file->private_data;
+	if (gmem->kvm == kvm)
+		goto out;
+
+	ret = fd = get_unused_fd_flags(0);
+	if (ret < 0)
+		goto out;
+
+	inode = file_inode(src_file);
+	dst_file = kvm_gmem_alloc_view(kvm, inode, name);
+	if (IS_ERR(dst_file)) {
+		ret = PTR_ERR(dst_file);
+		goto out_fd;
+	}
+
+	ts = inode_set_ctime_current(inode);
+	inode_set_atime_to_ts(inode, ts);
+
+	inc_nlink(inode);
+	ihold(inode);
+
+	fd_install(fd, dst_file);
+	fdput(f);
+	return fd;
+
+out_fd:
+	put_unused_fd(fd);
+out:
+	fdput(f);
+	return ret;
+}
+
 int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 		  unsigned int fd, loff_t offset)
 {
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 1e3fd81868bc..a9b01841a243 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -5285,6 +5285,16 @@ static long kvm_vm_ioctl(struct file *filp,
 		r = kvm_gmem_create(kvm, &guest_memfd);
 		break;
 	}
+	case KVM_LINK_GUEST_MEMFD: {
+		struct kvm_link_guest_memfd params;
+
+		r = -EFAULT;
+		if (copy_from_user(&params, argp, sizeof(params)))
+			goto out;
+
+		r = kvm_gmem_link(kvm, &params);
+		break;
+	}
 #endif
 	default:
 		r = kvm_arch_vm_ioctl(filp, ioctl, arg);
diff --git a/virt/kvm/kvm_mm.h b/virt/kvm/kvm_mm.h
index dcacb76b8f00..85baf8a7e0de 100644
--- a/virt/kvm/kvm_mm.h
+++ b/virt/kvm/kvm_mm.h
@@ -71,6 +71,7 @@ static inline void gfn_to_pfn_cache_invalidate_start(struct kvm *kvm,
 int kvm_gmem_init(struct module *module);
 void kvm_gmem_exit(void);
 int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args);
+int kvm_gmem_link(struct kvm *kvm, struct kvm_link_guest_memfd *args);
 int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 		  unsigned int fd, loff_t offset);
 void kvm_gmem_unbind(struct kvm_memory_slot *slot);
@@ -82,6 +83,12 @@ static inline int kvm_gmem_init(struct module *module)
 
 static inline void kvm_gmem_exit(void) {};
 
+static inline int kvm_gmem_link(struct kvm *kvm,
+				struct kvm_link_guest_memfd *args)
+{
+	return -EOPNOTSUPP;
+}
+
 static inline int kvm_gmem_bind(struct kvm *kvm,
 					 struct kvm_memory_slot *slot,
 					 unsigned int fd, loff_t offset)
-- 
2.49.0.1101.gccaa498523-goog