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Protect getlist() walk with a critical section (free-threading, #9852) by espressolee · Pull Request #9854 · python-pillow/Pillow · GitHub

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Protect getlist() walk with a critical section (free-threading, #9852) #9854

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Protect getlist() walk with a critical section (free-threading, #9852) #9854

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15 changes: 14 additions & 1 deletion src/_imaging.c
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Original file line number Diff line number Diff line change
Expand Up @@ -97,6 +97,8 @@

#include "libImaging/Imaging.h"

#include "thirdparty/pythoncapi_compat.h"

#define _USE_MATH_DEFINES
#include <math.h>
#include <stddef.h>
Expand Down Expand Up @@ -461,7 +463,17 @@ getlist(PyObject *arg, Py_ssize_t *length, const char *wrong_length, int type) {
return NULL;
}

for (i = 0; i < n; i++) {
// On a free-threaded build PySequence_Fast returns the list itself for a list
// input, so the walk below aliases the caller's list. Hold a critical section on
// it and clamp to its current length, so a concurrent resize cannot make
// PySequence_Fast_GET_ITEM read out of bounds (#9852). `n` was read before this
// point and may be stale.
Py_BEGIN_CRITICAL_SECTION(seq);
Py_ssize_t count = PySequence_Fast_GET_SIZE(seq);
if (count > n) {
count = n;
}
for (i = 0; i < count; i++) {
op = PySequence_Fast_GET_ITEM(seq, i);
// DRY, branch prediction is going to work _really_ well
// on this switch. And 3 fewer loops to copy/paste.
Expand All @@ -484,6 +496,7 @@ getlist(PyObject *arg, Py_ssize_t *length, const char *wrong_length, int type) {
break;
}
}
Py_END_CRITICAL_SECTION();

Py_DECREF(seq);

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