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>

diff --git a/sysdeps/mach/hurd/bits/mman_ext.h b/sysdeps/mach/hurd/bits/mman_ext.h
index bbb94743e9c1658c39021c139663fd669d5110ef..9658cdd658105720ae4f14ed0a61e8aaece06d82 100644 (file)
--- 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 33672cf60ad6fbc5bb0a1e8050f7602bca6121cf..20264a775b38757350e2b2ae8b96f046aedffb78 100644 (file)
--- 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