hurd: Implement MAP_EXCL

MAP_FIXED is defined to silently replace any existing mappings at the
address range being mapped over. This, however, is a dangerous, and only
rarely desired behavior.

Various Unix systems provide replacements or additions to MAP_FIXED:

* SerenityOS and Linux provide MAP_FIXED_NOREPLACE. If the address space
  already contains a mapping in the requested range, Linux returns
  EEXIST. SerenityOS returns ENOMEM, however that is a bug, as the
  MAP_FIXED_NOREPLACE implementation is intended to be compatible with
  Linux.

* FreeBSD provides the MAP_EXCL flag that has to be used in combination
  with MAP_FIXED. It returns EINVAL if the requested range already
  contains existing mappings. This is directly analogous to the O_EXCL
  flag in the open () call.

* DragonFly BSD, NetBSD, and OpenBSD provide MAP_TRYFIXED, but with
  different semantics. DragonFly BSD returns ENOMEM if the requested
  range already contains existing mappings. NetBSD does not return an
  error, but instead creates the mapping at a different address if the
  requested range contains mappings. OpenBSD behaves the same, but also
  notes that this is the default behavior even without MAP_TRYFIXED
  (which is the case on the Hurd too).

Since the Hurd leans closer to the BSD side, add MAP_EXCL as the primary
API to request the behavior of not replacing existing mappings. Declare
MAP_FIXED_NOREPLACE and MAP_TRYFIXED as aliases of (MAP_FIXED|MAP_EXCL),
so any existing software that checks for either of those macros will
pick them up automatically. For compatibility with Linux, return EEXIST
if a mapping already exists.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230625231751.404120-5-bugaevc@gmail.com>

2 files changed, 23 insertions, 10 deletions

diff --git a/sysdeps/mach/hurd/bits/mman_ext.h b/sysdeps/mach/hurd/bits/mman_ext.h
index bbb9474..9658cdd 100644
--- a/sysdeps/mach/hurd/bits/mman_ext.h
+++ b/sysdeps/mach/hurd/bits/mman_ext.h
@@ -22,5 +22,10 @@
 
 #ifdef __USE_GNU
 # define SHM_ANON	((const char *) 1)
-# define MAP_32BIT	0x1000
+
+# define MAP_32BIT	0x1000	/* Map in the lower 2 GB.  */
+# define MAP_EXCL	0x4000	/* With MAP_FIXED, don't replace existing mappings.  */
+
+# define MAP_TRYFIXED		(MAP_FIXED | MAP_EXCL)	/* BSD name.  */
+# define MAP_FIXED_NOREPLACE	(MAP_FIXED | MAP_EXCL)	/* Linux name.  */
 #endif /* __USE_GNU  */
diff --git a/sysdeps/mach/hurd/mmap.c b/sysdeps/mach/hurd/mmap.c
index 33672cf..20264a7 100644
--- a/sysdeps/mach/hurd/mmap.c
+++ b/sysdeps/mach/hurd/mmap.c
@@ -46,6 +46,9 @@ __mmap (void *addr, size_t len, int prot, int flags, int fd, off_t offset)
   if ((mapaddr & (__vm_page_size - 1)) || (offset & (__vm_page_size - 1)))
     return (void *) (long int) __hurd_fail (EINVAL);
 
+  if ((flags & MAP_EXCL) && ! (flags & MAP_FIXED))
+    return (void *) (long int) __hurd_fail (EINVAL);
+
   vmprot = VM_PROT_NONE;
   if (prot & PROT_READ)
     vmprot |= VM_PROT_READ;
@@ -156,15 +159,20 @@ __mmap (void *addr, size_t len, int prot, int flags, int fd, off_t offset)
     {
       if (err == KERN_NO_SPACE)
 	{
-	  /* XXX this is not atomic as it is in unix! */
-	  /* The region is already allocated; deallocate it first.  */
-	  err = __vm_deallocate (__mach_task_self (), mapaddr, len);
-	  if (! err)
-	    err = __vm_map (__mach_task_self (),
-			    &mapaddr, (vm_size_t) len, mask,
-			    0, memobj, (vm_offset_t) offset,
-			    copy, vmprot, max_vmprot,
-			    copy ? VM_INHERIT_COPY : VM_INHERIT_SHARE);
+	  if (flags & MAP_EXCL)
+	    err = EEXIST;
+	  else
+	    {
+	      /* The region is already allocated; deallocate it first.  */
+	      /* XXX this is not atomic as it is in unix! */
+	      err = __vm_deallocate (__mach_task_self (), mapaddr, len);
+	      if (! err)
+		err = __vm_map (__mach_task_self (),
+				&mapaddr, (vm_size_t) len, mask,
+				0, memobj, (vm_offset_t) offset,
+				copy, vmprot, max_vmprot,
+				copy ? VM_INHERIT_COPY : VM_INHERIT_SHARE);
+	    }
 	}
     }
   else