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gh-152433: Windows: fix ctypes error in UWP build #154700

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zooba merged 3 commits into from
Sep 17, 2026
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gh-152433: Windows: fix ctypes error in UWP build #154700

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gh-152433: Windows: fix ``ctypes`` MemoryError in UWP build
thexai Jul 25, 2026
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zooba Sep 17, 2026
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zooba Sep 17, 2026
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14 changes: 14 additions & 0 deletions Modules/_ctypes/malloc_closure.c
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Original file line number Diff line number Diff line change
Expand Up @@ -68,10 +68,24 @@ static void more_core(void)

/* allocate a memory block */
#ifdef MS_WIN32
#ifdef MS_WINDOWS_DESKTOP
item = (ITEM *)VirtualAlloc(NULL,
count * sizeof(ITEM),
MEM_COMMIT,
PAGE_EXECUTE_READWRITE);
#else // UWP
/* Due security restrictions, UWP not allows request Read-Write-Execute permissions at once.
The correct flow in UWP for execute dynamic code in memmory is:
1. Alloate as Read-Write (PAGE_READWRITE) and write dynamic code to memmory.
2. Change to Executable (PAGE_EXECUTE_READ) with 'VirtualProtectFromApp'.
3. Flush cache with 'FlushInstructionCache' to ensure CPU instruction cache coherency
before executing the generated code.
!@todo implement points 2. 3. in the appropriate places */
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item = (ITEM*)VirtualAllocFromApp(NULL,
count * sizeof(ITEM),
MEM_COMMIT | MEM_RESERVE,
PAGE_READWRITE);

@zooba zooba Sep 7, 2026

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If this works without EXECUTE, then do we need EXECUTE in the alternative VirtualAlloc call?

@thexai thexai Sep 11, 2026

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More work is needed for ctypes to have full functionality in UWP.

The EXECUTE flag is required, but in UWP, it is not allowed alongside READWRITE. This PR addresses some of the issues that recently arose after adding this code:

cpython/Modules/_ctypes/_ctypes.c

Lines 6390 to 6397 in e5d4fa2

_ctypes_mod_exec(PyObject *mod)
{
// See https://github.com/python/cpython/issues/128485
// This allocates some memory and then frees it to ensure that the
// the dlmalloc allocator initializes itself to avoid data races
// in free-threading.
void *codeloc = NULL;
void *ptr = Py_ffi_closure_alloc(sizeof(void *), &codeloc);

This causes errors immediately upon ctypes initialization (because reserves memory with execute permission).

But executing code in memory remains necessary for full ctypes functionality.

Let's say this PR only enables "limited" ctypes functionality on UWP but causes no harm on Windows Desktop.

@zooba zooba Sep 14, 2026

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Can we say that in a comment against this code, for next time one of us has to remember why we did it this way?

@thexai thexai Sep 17, 2026

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Added a code comment with more context and what remains to be implemented.

#endif // !MS_WINDOWS_DESKTOP
if (item == NULL)
return;
#else
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