MAP_SHARED isn't proper for anonymous mappings for VxWorks #80829

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1. LihuaZhao commented on Apr 17, 2019

   

   LihuaZhaomannequin

   on Apr 17, 2019

   MannequinAuthor

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   anonymous mappings is not part of the POSIX standard, python user just need to specified -1 as fd value when do anonymous map, for example:

   m = mmap.mmap(-1, 100)

   then python adapter module(mmapmodule.c) try to specify MAP_SHARED or MAP_PRIVATE based on operate system requirement, Linux require MAP_SHARED, VxWorks require MAP_PRIVATE, this different should be hidden by this module, and python user won't be affected.

   Currently, mmap is only adapted for the system which use MAP_SHARED when do anonymous map, VxWorks need be supported.

   https://en.wikipedia.org/wiki/Mmap
2. LihuaZhao

   added

   3.8 (EOL)end of lifeend of life

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-featureA feature request or enhancementA feature request or enhancement

   on Apr 17, 2019
3. vstinner commented on Apr 18, 2019

   

   vstinner

   on Apr 18, 2019

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   What is the current behavior of m = mmap.mmap(-1, 100)? Does it raise an exception?
4. vstinner commented on Apr 18, 2019

   

   vstinner

   on Apr 18, 2019

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   I don't understand why PR 12394 modifies flags afterwards, whereas "m = mmap.mmap(-1, 100)" doesn't specify explicitly flags. So the bug looks to be default flags set by Python, no?

       int flags = MAP_SHARED;
       ...

   if (!PyArg_ParseTupleAndKeywords(args, kwdict, "in|iii" _Py_PARSE_OFF_T, keywords,
                                    &fd, &map_size, &flags, &prot,
                                    &access, &offset))
   

   Is MAP_SHARED constant available in C on VxWorks?

   Is mmap.MAP_SHARED constant available in Python on VxWorks?
5. LihuaZhao commented on Apr 18, 2019

   

   LihuaZhaomannequin

   on Apr 18, 2019

   MannequinAuthor

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   >What is the current behavior of m = mmap.mmap(-1, 100)? Does it raise an exception?

   No, the following statement will return -1 without PR 12394

   m_obj->data = mmap(NULL, map_size,
                      prot, flags,
                      fd, offset);
   

   >I don't understand why PR 12394 modifies flags afterwards, whereas "m = mmap.mmap(-1, 100)" doesn't specify explicitly flags. So the bug looks to be default flags set by Python, no?

   Yes, this statement doesn't specify the flag, but as you said, the new_mmap_object() firstly set MAP_SHARED for flag, and in later code:

       if (fd == -1) {
           m_obj->fd = -1;
       .....
   #ifdef MAP_ANONYMOUS
           /* BSD way to map anonymous memory */
           flags |= MAP_ANONYMOUS;

   #else
   #endif

   This routine will pass (MAP_ANONYMOUS | MAP_SHARED) to mmap and fd is -1, this is true for Linux, but for VxWorks, if fd is -1, the flag type should be (MAP_ANONYMOUS | MAP_PRIVATE), and this behavior is not part of the POSIX standard, we can't say VxWorks isn't right.

   So my changes clear MAP_SHARED and set MAP_PRIVATE when the system type is VxWorks.

   >Is MAP_SHARED constant available in C on VxWorks?

   Yes, but for MAP_ANONYMOUS, it require set MAP_PRIVATE.

   This PR still is try to enhance VxWorks support, doesn't affect Python user, and only change the behavior of VxWorks, other system don't have this issue, and also won't be influenced
6. vstinner commented on May 21, 2019

   

   vstinner

   on May 21, 2019

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   New changeset 4fb1502 by Victor Stinner (Lihua Zhao) in branch 'master':
   bpo-36648: fix mmap issue for VxWorks (GH-12394)
   4fb1502
7. vstinner

   closed this as completedon May 21, 2019
8. ezio-melotti

   transferred this issue fromon Apr 10, 2022