[issue4751] Patch for better thread support in hashlib

[STINNER Victor]

STINNER Victor <victor.stinner at haypocalc.com> added the comment:

If view.len is negative, EVP_hash() may read invalid memory :-/ Be 
careful of integer overflow in this block:

   Py_ssize_t offset = 0, sublen = len;
   while (sublen) {
      unsigned int process = sublen > MUNCH_SIZE ? MUNCH_SIZE : 
sublen;
      ...
   }

You removed Py_SAFE_DOWNCAST(len, Py_ssize_t, unsigned int) which 
should be used (eg. on process?).

Note: you might modify len directly instead of using a second variable 
(sublen), and cp instead of using an offset.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue4751>
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