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author | Victor Stinner <vstinner@redhat.com> | 2018-11-22 17:15:37 +0100 |
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committer | GitHub <noreply@github.com> | 2018-11-22 17:15:37 +0100 |
commit | 95036ea25d47f0081bda2ba96ea327f3375cb6a4 (patch) | |
tree | 9f98c855fbf31c95258b8db4279aa583c3d3f8dd /Objects | |
parent | a5194115733f6ca8fc1ddbee43eabbde536900e6 (diff) | |
download | cpython-git-95036ea25d47f0081bda2ba96ea327f3375cb6a4.tar.gz |
[3.7] bpo-9263: _PyObject_Dump() detects freed memory (GH-10061) (GH-10662)
* bpo-9263: _PyObject_Dump() detects freed memory (GH-10061)
_PyObject_Dump() now uses an heuristic to check if the object memory
has been freed: log "<freed object>" in that case.
The heuristic rely on the debug hooks on Python memory allocators
which fills the memory with DEADBYTE (0xDB) when memory is
deallocated. Use PYTHONMALLOC=debug to always enable these debug
hooks.
(cherry picked from commit 82af0b63b07aa8d92b50098e382b458143cfc677)
* bpo-9263: Fix _PyObject_Dump() for freed object (#10661)
If _PyObject_Dump() detects that the object is freed, don't try to
dump it (exit immediately).
Enhance also _PyObject_IsFreed(): it now detects if the pointer
itself looks like freed memory.
(cherry picked from commit 2cf5d32fd9e61488e8b0be55a2e92a752ba8b06b)
Diffstat (limited to 'Objects')
-rw-r--r-- | Objects/object.c | 83 | ||||
-rw-r--r-- | Objects/obmalloc.c | 16 |
2 files changed, 76 insertions, 23 deletions
diff --git a/Objects/object.c b/Objects/object.c index 5535b7ee7e..138df44880 100644 --- a/Objects/object.c +++ b/Objects/object.c @@ -411,34 +411,71 @@ _Py_BreakPoint(void) } +/* Heuristic checking if the object memory has been deallocated. + Rely on the debug hooks on Python memory allocators which fills the memory + with DEADBYTE (0xDB) when memory is deallocated. + + The function can be used to prevent segmentation fault on dereferencing + pointers like 0xdbdbdbdbdbdbdbdb. Such pointer is very unlikely to be mapped + in memory. */ +int +_PyObject_IsFreed(PyObject *op) +{ + uintptr_t ptr = (uintptr_t)op; + if (_PyMem_IsFreed(&ptr, sizeof(ptr))) { + return 1; + } + int freed = _PyMem_IsFreed(&op->ob_type, sizeof(op->ob_type)); + /* ignore op->ob_ref: the value can have be modified + by Py_INCREF() and Py_DECREF(). */ +#ifdef Py_TRACE_REFS + freed &= _PyMem_IsFreed(&op->_ob_next, sizeof(op->_ob_next)); + freed &= _PyMem_IsFreed(&op->_ob_prev, sizeof(op->_ob_prev)); +#endif + return freed; +} + + /* For debugging convenience. See Misc/gdbinit for some useful gdb hooks */ void _PyObject_Dump(PyObject* op) { - if (op == NULL) - fprintf(stderr, "NULL\n"); - else { - PyGILState_STATE gil; - PyObject *error_type, *error_value, *error_traceback; - - fprintf(stderr, "object : "); - gil = PyGILState_Ensure(); - - PyErr_Fetch(&error_type, &error_value, &error_traceback); - (void)PyObject_Print(op, stderr, 0); - PyErr_Restore(error_type, error_value, error_traceback); - - PyGILState_Release(gil); - /* XXX(twouters) cast refcount to long until %zd is - universally available */ - fprintf(stderr, "\n" - "type : %s\n" - "refcount: %ld\n" - "address : %p\n", - Py_TYPE(op)==NULL ? "NULL" : Py_TYPE(op)->tp_name, - (long)op->ob_refcnt, - op); + if (op == NULL) { + fprintf(stderr, "<NULL object>\n"); + fflush(stderr); + return; + } + + if (_PyObject_IsFreed(op)) { + /* It seems like the object memory has been freed: + don't access it to prevent a segmentation fault. */ + fprintf(stderr, "<freed object>\n"); + return; } + + PyGILState_STATE gil; + PyObject *error_type, *error_value, *error_traceback; + + fprintf(stderr, "object : "); + fflush(stderr); + gil = PyGILState_Ensure(); + + PyErr_Fetch(&error_type, &error_value, &error_traceback); + (void)PyObject_Print(op, stderr, 0); + fflush(stderr); + PyErr_Restore(error_type, error_value, error_traceback); + + PyGILState_Release(gil); + /* XXX(twouters) cast refcount to long until %zd is + universally available */ + fprintf(stderr, "\n" + "type : %s\n" + "refcount: %ld\n" + "address : %p\n", + Py_TYPE(op)==NULL ? "NULL" : Py_TYPE(op)->tp_name, + (long)op->ob_refcnt, + op); + fflush(stderr); } PyObject * diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c index 1b8b5eefe2..3b0c35bcc9 100644 --- a/Objects/obmalloc.c +++ b/Objects/obmalloc.c @@ -2091,6 +2091,22 @@ _PyMem_DebugRawCalloc(void *ctx, size_t nelem, size_t elsize) } +/* Heuristic checking if the memory has been freed. Rely on the debug hooks on + Python memory allocators which fills the memory with DEADBYTE (0xDB) when + memory is deallocated. */ +int +_PyMem_IsFreed(void *ptr, size_t size) +{ + unsigned char *bytes = ptr; + for (size_t i=0; i < size; i++) { + if (bytes[i] != DEADBYTE) { + return 0; + } + } + return 1; +} + + /* The debug free first checks the 2*SST bytes on each end for sanity (in particular, that the FORBIDDENBYTEs with the api ID are still intact). Then fills the original bytes with DEADBYTE. |