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hurd: Implement MAP_EXCL

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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>
This commit is contained in:
Sergey Bugaev 2023-06-26 02:17:51 +03:00 committed by Samuel Thibault
parent 19c3b31812
commit 27cb2bb93d
2 changed files with 23 additions and 10 deletions
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7
sysdeps/mach/hurd/bits/mman_ext.h
View File

@ -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 */

26
sysdeps/mach/hurd/mmap.c
View File

@ -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