Add a new file descriptor type for IPC shared memory objects and use it to

implement shm_open(2) and shm_unlink(2) in the kernel:
- Each shared memory file descriptor is associated with a swap-backed vm
  object which provides the backing store.  Each descriptor starts off with
  a size of zero, but the size can be altered via ftruncate(2).  The shared
  memory file descriptors also support fstat(2).  read(2), write(2),
  ioctl(2), select(2), poll(2), and kevent(2) are not supported on shared
  memory file descriptors.
- shm_open(2) and shm_unlink(2) are now implemented as system calls that
  manage shared memory file descriptors.  The virtual namespace that maps
  pathnames to shared memory file descriptors is implemented as a hash
  table where the hash key is generated via the 32-bit Fowler/Noll/Vo hash
  of the pathname.
- As an extension, the constant 'SHM_ANON' may be specified in place of the
  path argument to shm_open(2).  In this case, an unnamed shared memory
  file descriptor will be created similar to the IPC_PRIVATE key for
  shmget(2).  Note that the shared memory object can still be shared among
  processes by sharing the file descriptor via fork(2) or sendmsg(2), but
  it is unnamed.  This effectively serves to implement the getmemfd() idea
  bandied about the lists several times over the years.
- The backing store for shared memory file descriptors are garbage
  collected when they are not referenced by any open file descriptors or
  the shm_open(2) virtual namespace.

Submitted by:	dillon, peter (previous versions)
Submitted by:	rwatson (I based this on his version)
Reviewed by:	alc (suggested converting getmemfd() to shm_open())

1 files changed, 53 insertions, 3 deletions

diff --git a/sys/vm/vm_mmap.c b/sys/vm/vm_mmap.c
index 4820a1442987..e6c82dcd1513 100644
--- a/sys/vm/vm_mmap.c
+++ b/sys/vm/vm_mmap.c
@@ -118,6 +118,8 @@ static int vm_mmap_vnode(struct thread *, vm_size_t, vm_prot_t, vm_prot_t *,
     int *, struct vnode *, vm_ooffset_t, vm_object_t *);
 static int vm_mmap_cdev(struct thread *, vm_size_t, vm_prot_t, vm_prot_t *,
     int *, struct cdev *, vm_ooffset_t, vm_object_t *);
+static int vm_mmap_shm(struct thread *, vm_size_t, vm_prot_t, vm_prot_t *,
+    int *, struct shmfd *, vm_ooffset_t, vm_object_t *);
 
 /*
  * MPSAFE
@@ -300,16 +302,29 @@ mmap(td, uap)
 		pos = 0;
 	} else {
 		/*
-		 * Mapping file, get fp for validation. Obtain vnode and make
-		 * sure it is of appropriate type.
-		 * don't let the descriptor disappear on us if we block
+		 * Mapping file, get fp for validation and
+		 * don't let the descriptor disappear on us if we block.
 		 */
 		if ((error = fget(td, uap->fd, &fp)) != 0)
 			goto done;
+		if (fp->f_type == DTYPE_SHM) {
+			handle = fp->f_data;
+			handle_type = OBJT_SWAP;
+			maxprot = VM_PROT_NONE;
+
+			/* FREAD should always be set. */
+			if (fp->f_flag & FREAD)
+				maxprot |= VM_PROT_EXECUTE | VM_PROT_READ;
+			if (fp->f_flag & FWRITE)
+				maxprot |= VM_PROT_WRITE;
+			goto map;
+		}
 		if (fp->f_type != DTYPE_VNODE) {
 			error = ENODEV;
 			goto done;
 		}
+#if defined(COMPAT_FREEBSD7) || defined(COMPAT_FREEBSD6) || \
+    defined(COMPAT_FREEBSD5) || defined(COMPAT_FREEBSD4)
 		/*
 		 * POSIX shared-memory objects are defined to have
 		 * kernel persistence, and are not defined to support
@@ -320,6 +335,7 @@ mmap(td, uap)
 		 */
 		if (fp->f_flag & FPOSIXSHM)
 			flags |= MAP_NOSYNC;
+#endif
 		vp = fp->f_vnode;
 		/*
 		 * Ensure that file and memory protections are
@@ -360,6 +376,7 @@ mmap(td, uap)
 		handle = (void *)vp;
 		handle_type = OBJT_VNODE;
 	}
+map:
 
 	/*
 	 * Do not allow more then a certain number of vm_map_entry structures
@@ -1291,6 +1308,35 @@ vm_mmap_cdev(struct thread *td, vm_size_t objsize,
 }
 
 /*
+ * vm_mmap_shm()
+ *
+ * MPSAFE
+ *
+ * Helper function for vm_mmap.  Perform sanity check specific for mmap
+ * operations on shm file descriptors.
+ */
+int
+vm_mmap_shm(struct thread *td, vm_size_t objsize,
+    vm_prot_t prot, vm_prot_t *maxprotp, int *flagsp,
+    struct shmfd *shmfd, vm_ooffset_t foff, vm_object_t *objp)
+{
+	int error;
+
+	if ((*maxprotp & VM_PROT_WRITE) == 0 &&
+	    (prot & PROT_WRITE) != 0)
+		return (EACCES);
+#ifdef MAC
+	error = mac_posixshm_check_mmap(td->td_ucred, shmfd, prot, *flagsp);
+	if (error != 0)
+		return (error);
+#endif
+	error = shm_mmap(shmfd, objsize, foff, objp);
+	if (error)
+		return (error);
+	return (0);
+}
+
+/*
  * vm_mmap()
  *
  * MPSAFE
@@ -1354,6 +1400,10 @@ vm_mmap(vm_map_t map, vm_offset_t *addr, vm_size_t size, vm_prot_t prot,
 		error = vm_mmap_vnode(td, size, prot, &maxprot, &flags,
 		    handle, foff, &object);
 		break;
+	case OBJT_SWAP:
+		error = vm_mmap_shm(td, size, prot, &maxprot, &flags,
+		    handle, foff, &object);
+		break;
 	case OBJT_DEFAULT:
 		if (handle == NULL) {
 			error = 0;