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Cygwin: mmap: allow remapping part of an existing anonymous mapping

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Previously mmap with MAP_FIXED would fail with EINVAL on an attempt to
map an address range contained in the chunk of an existing mapping.
With this commit, mmap will succeed, provided the mappings are
anonymous, the MAP_SHARED/MAP_PRIVATE flags agree, and MAP_NORESERVE
is not set for either mapping.

Addresses: https://cygwin.com/pipermail/cygwin/2024-December/256901.html
Signed-off-by: Ken Brown <kbrown@cornell.edu>
This commit is contained in:
Ken Brown 2024-12-27 16:09:40 -05:00
parent 8adc45fdec
commit ad8d3195cc
2 changed files with 33 additions and 18 deletions
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45
winsup/cygwin/mm/mmap.cc
View File

@ -494,18 +494,24 @@ mmap_record::map_pages (caddr_t addr, SIZE_T len, int new_prot)
off_t off = addr - get_address ();
off /= wincap.page_size ();
len = PAGE_CNT (len);
/* First check if the area is unused right now. */
for (SIZE_T l = 0; l < len; ++l)
if (MAP_ISSET (off + l))
{
set_errno (EINVAL);
return false;
}
if (!noreserve ()
&& !VirtualProtect (get_address () + off * wincap.page_size (),
len * wincap.page_size (),
::gen_protect (new_prot, get_flags ()),
&old_prot))
/* VirtualProtect can only be called on committed pages, so it's not
clear how to change protection in the noreserve case. In this
case we will therefore require that the pages are unmapped, in
order to keep the behavior the same as it was before new_prot was
introduced. FIXME: Is there a better way to handle this? */
if (noreserve ())
{
for (SIZE_T l = 0; l < len; ++l)
if (MAP_ISSET (off + l))
{
set_errno (EINVAL);
return false;
}
}
else if (!VirtualProtect (get_address () + off * wincap.page_size (),
len * wincap.page_size (),
::gen_protect (new_prot, get_flags ()),
&old_prot))
{
__seterrno ();
return false;
@ -625,8 +631,9 @@ mmap_list::try_map (void *addr, size_t len, int new_prot, int flags, off_t off)
if (off == 0 && !fixed (flags))
{
/* If MAP_FIXED isn't given, check if this mapping matches into the
chunk of another already performed mapping. */
/* If MAP_FIXED isn't given, try to satisfy this mapping request
by recycling unmapped pages in the chunk of an existing
mapping. */
SIZE_T plen = PAGE_CNT (len);
LIST_FOREACH (rec, &recs, mr_next)
if (rec->find_unused_pages (plen) != (SIZE_T) -1)
@ -640,9 +647,10 @@ mmap_list::try_map (void *addr, size_t len, int new_prot, int flags, off_t off)
}
else if (fixed (flags))
{
/* If MAP_FIXED is given, test if the requested area is in an
unmapped part of an still active mapping. This can happen
if a memory region is unmapped and remapped with MAP_FIXED. */
/* If MAP_FIXED is given, test if the requested area is
contained in the chunk of an existing mapping. If so, and if
the flags of that mapping are compatible with those in the
request, try to reset the protection on the requested area. */
caddr_t u_addr;
SIZE_T u_len;
@ -1058,7 +1066,8 @@ go_ahead:
LIST_WRITE_LOCK ();
map_list = mmapped_areas.get_list_by_fd (fd, &st);
/* Test if an existing anonymous mapping can be recycled. */
/* Try to satisfy the request by resetting the protection on part of
an existing anonymous mapping. */
if (map_list && anonymous (flags))
{
caddr_t tried = map_list->try_map (addr, len, prot, flags, off);

6
winsup/cygwin/release/3.6.0
View File

@ -78,3 +78,9 @@ What changed:
- Raise maximum pid from 65536 to 4194304 to account for scenarios
with lots of CPUs and lots of tasks.
Addresses: https://cygwin.com/pipermail/cygwin/2024-December/256927.html
- Allow mmap with MAP_FIXED to succeed on an address range contained
in the chunk of an existing anonymous mapping, provided the
MAP_SHARED/MAP_PRIVATE flags agree and MAP_NORESERVE is not set for
either mapping.
Addresses: https://cygwin.com/pipermail/cygwin/2024-December/256901.html