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Cygwin: mmap: fix mmap_is_attached_or_noreserve

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This commit fixes two problems.  The first is that
mmap_is_attached_or_noreserve would sometimes call VirtualAlloc with
MEM_COMMIT on address ranges that were not known to have MEM_RESERVE
status.  These calls could fail, causing SIGBUS to be raised
incorrectly.  See

  https://cygwin.com/pipermail/cygwin-developers/2024-December/012725.html

for details.  Fix this by calling VirtualAlloc only on the part of the
address range that's contained in the current mmap_record.

The second problem is that the code would sometimes break out of the
main loop without knowing whether attached mappings still occur later
in the mmap_list.  This could cause SIGBUS to not be raised when it
should be.  Fix this by using "continue" rather than "break".  For
efficiency, introduce a boolean variable "nocover" that's set to true
if we discover that the address range cannot be covered by noreserve
mmap regions.

Addresses:
https://cygwin.com/pipermail/cygwin-developers/2024-December/012725.html
Fixes: 70e476d27be8 ("* include/cygwin/version.h: Bump DLL version to
1.7.0")
Signed-off-by: Ken Brown <kbrown@cornell.edu>
This commit is contained in:
Ken Brown 2024-12-27 10:10:06 -05:00
parent 5e5e51f1dc
commit 8adc45fdec
1 changed files with 28 additions and 23 deletions
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51
winsup/cygwin/mm/mmap.cc
View File

@ -738,18 +738,18 @@ is_mmapped_region (caddr_t start_addr, caddr_t end_address)
/* This function is called from exception_handler when a segmentation
violation has occurred. It should also be called from all Cygwin
functions that want to support passing noreserve mmap page addresses
to Windows system calls. In that case, it should be called only after
a system call indicates that the application buffer passed had an
invalid virtual address to avoid any performance impact in non-noreserve
cases.
functions that want to support passing noreserve (anonymous) mmap
page addresses to Windows system calls. In that case, it should be
called only after a system call indicates that the application
buffer passed had an invalid virtual address to avoid any
performance impact in non-noreserve cases.
Check if the address range is all within noreserve mmap regions. If so,
call VirtualAlloc to commit the pages and return MMAP_NORESERVE_COMMITED
on success. If the page has __PROT_ATTACH (SUSv3 memory protection
extension), or if VirtualAlloc fails, return MMAP_RAISE_SIGBUS.
Otherwise, return MMAP_NONE if the address range is not covered by an
attached or noreserve map.
on success. If some page in the address range has __PROT_ATTACH
(SUSv3 memory protection extension), or if VirtualAlloc fails,
return MMAP_RAISE_SIGBUS. Otherwise, return MMAP_NONE if the
address range is not covered by noreserve maps.
On MAP_NORESERVE_COMMITED, the exeception handler should return 0 to
allow the application to retry the memory access, or the calling Cygwin
@ -768,12 +768,16 @@ mmap_is_attached_or_noreserve (void *addr, size_t len)
len += ((caddr_t) addr - start_addr);
len = roundup2 (len, pagesize);
if (map_list == NULL)
goto out;
mmap_record *rec;
caddr_t u_addr;
SIZE_T u_len;
/* nocover is set to true if we discover that our address range
cannot be covered by noreserve mmap regions. */
bool nocover = false;
size_t remaining = len;
if (map_list == NULL)
goto out;
LIST_FOREACH (rec, &map_list->recs, mr_next)
{
@ -784,23 +788,24 @@ mmap_is_attached_or_noreserve (void *addr, size_t len)
ret = MMAP_RAISE_SIGBUS;
break;
}
if (!rec->noreserve ())
break;
if (nocover || !rec->noreserve ())
{
/* We need to continue in case we encounter an attached mmap
later in the list. */
nocover = true;
continue;
}
size_t commit_len = u_len - (start_addr - u_addr);
if (commit_len > len)
commit_len = len;
if (!VirtualAlloc (start_addr, commit_len, MEM_COMMIT,
rec->gen_protect ()))
/* The interval determined by u_addr and u_len is the part of
our address range contained in the mmap region of rec. */
if (!VirtualAlloc (u_addr, u_len, MEM_COMMIT, rec->gen_protect ()))
{
ret = MMAP_RAISE_SIGBUS;
break;
}
start_addr += commit_len;
len -= commit_len;
if (!len)
remaining -= u_len;
if (!remaining)
{
ret = MMAP_NORESERVE_COMMITED;
break;