From b1ce65684d5f92f281d678581f0569cb16fa58e3 Mon Sep 17 00:00:00 2001 From: Simon Marchi Date: Fri, 20 Jan 2017 21:02:05 -0500 Subject: [PATCH] Fix Py_DECREF being executed without holding the GIL When the gdbpy_ref objects get destroyed, they call Py_DECREF to decrement the reference counter of the python object they hold a reference to. Any time we call into the Python API, we should be holding the GIL. The gdbpy_enter object does that for us in an RAII-fashion. However, if gdbpy_enter is declared after a gdbpy_ref object in a function, gdbpy_enter's destructor will be called (and the GIL will be released) before gdbpy_ref's destructor is called. Therefore, we will end up calling Py_DECREF without holding the GIL. This became obvious with Python 3.6, where memory management functions have asserts to make sure that the GIL is held. This was exposed by tests py-as-string.exp, py-function.exp and py-xmethods. For example: (gdb) p $_as_string(enum_valid) Fatal Python error: Python memory allocator called without holding the GIL Current thread 0x00007f7f7b21c780 (most recent call first): [1] 18678 abort (core dumped) ./gdb -nx testsuite/outputs/gdb.python/py-as-string/py-as-string #0 0x00007ffff618bc37 in raise () from /lib/x86_64-linux-gnu/libc.so.6 #1 0x00007ffff618f028 in abort () from /lib/x86_64-linux-gnu/libc.so.6 #2 0x00007ffff6b104d6 in Py_FatalError (msg=msg@entry=0x7ffff6ba15b8 "Python memory allocator called without holding the GIL") at Python/pylifecycle.c:1457 #3 0x00007ffff6a37a68 in _PyMem_DebugCheckGIL () at Objects/obmalloc.c:1972 #4 0x00007ffff6a3804e in _PyMem_DebugFree (ctx=0x7ffff6e65290 <_PyMem_Debug+48>, ptr=0x24f8830) at Objects/obmalloc.c:1994 #5 0x00007ffff6a38e1d in PyMem_Free (ptr=) at Objects/obmalloc.c:442 #6 0x00007ffff6b866c6 in _PyFaulthandler_Fini () at ./Modules/faulthandler.c:1369 #7 0x00007ffff6b104bd in Py_FatalError (msg=msg@entry=0x7ffff6ba15b8 "Python memory allocator called without holding the GIL") at Python/pylifecycle.c:1431 #8 0x00007ffff6a37a68 in _PyMem_DebugCheckGIL () at Objects/obmalloc.c:1972 #9 0x00007ffff6a3804e in _PyMem_DebugFree (ctx=0x7ffff6e652c0 <_PyMem_Debug+96>, ptr=0x7ffff46b6040) at Objects/obmalloc.c:1994 #10 0x00007ffff6a38f55 in PyObject_Free (ptr=) at Objects/obmalloc.c:503 #11 0x00007ffff6a5f27e in unicode_dealloc (unicode=unicode@entry=0x7ffff46b6040) at Objects/unicodeobject.c:1794 #12 0x00007ffff6a352a9 in _Py_Dealloc (op=0x7ffff46b6040) at Objects/object.c:1786 #13 0x000000000063f28b in gdb_Py_DECREF (op=0x7ffff46b6040) at /home/emaisin/src/binutils-gdb/gdb/python/python-internal.h:192 #14 0x000000000063fa33 in gdbpy_ref_policy::decref (ptr=0x7ffff46b6040) at /home/emaisin/src/binutils-gdb/gdb/python/py-ref.h:35 #15 0x000000000063fa77 in gdb::ref_ptr<_object, gdbpy_ref_policy>::~ref_ptr (this=0x7fffffffcdf0, __in_chrg=) at /home/emaisin/src/binutils-gdb/gdb/common/gdb_ref_ptr.h:91 #16 0x000000000064d8b8 in fnpy_call (gdbarch=0x2b50010, language=0x115d2c0 , cookie=0x7ffff46b7468, argc=1, argv=0x7fffffffcf48) at /home/emaisin/src/binutils-gdb/gdb/python/py-function.c:145 The fix is to place the gdbpy_enter first in the function. I also cleaned up the comments a bit and removed the unnecessary initialization of the value variable. gdb/ChangeLog: * python/py-function.c (fnpy_call): Reorder declarations to have the gdbpy_enter object declared first. * python/py-xmethods.c (gdbpy_get_xmethod_arg_types): Likewise. --- gdb/ChangeLog | 6 ++++++ gdb/python/py-function.c | 11 +++++------ gdb/python/py-xmethods.c | 5 +++-- 3 files changed, 14 insertions(+), 8 deletions(-) diff --git a/gdb/ChangeLog b/gdb/ChangeLog index 6311d91a0b6..6bd0a232586 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,9 @@ +2017-01-20 Simon Marchi + + * python/py-function.c (fnpy_call): Reorder declarations to have + the gdbpy_enter object declared first. + * python/py-xmethods.c (gdbpy_get_xmethod_arg_types): Likewise. + 2017-01-20 Simon Marchi PR python/21068 diff --git a/gdb/python/py-function.c b/gdb/python/py-function.c index 13c7a114854..6762a6d56e0 100644 --- a/gdb/python/py-function.c +++ b/gdb/python/py-function.c @@ -59,14 +59,13 @@ static struct value * fnpy_call (struct gdbarch *gdbarch, const struct language_defn *language, void *cookie, int argc, struct value **argv) { - struct value *value = NULL; - /* 'result' must be set to NULL, this initially indicates whether - the function was called, or not. */ - gdbpy_ref result; - + /* The gdbpy_enter object needs to be placed first, so that it's the last to + be destroyed. */ gdbpy_enter enter_py (gdbarch, language); - + struct value *value; + gdbpy_ref result; gdbpy_ref args (convert_values_to_python (argc, argv)); + /* convert_values_to_python can return NULL on error. If we encounter this, do not call the function, but allow the Python -> error code conversion below to deal with the Python exception. diff --git a/gdb/python/py-xmethods.c b/gdb/python/py-xmethods.c index c3ca75a644b..6505d066e74 100644 --- a/gdb/python/py-xmethods.c +++ b/gdb/python/py-xmethods.c @@ -308,6 +308,9 @@ gdbpy_get_xmethod_arg_types (const struct extension_language_defn *extlang, struct xmethod_worker *worker, int *nargs, struct type ***arg_types) { + /* The gdbpy_enter object needs to be placed first, so that it's the last to + be destroyed. */ + gdbpy_enter enter_py (get_current_arch (), current_language); struct gdbpy_worker_data *worker_data = (struct gdbpy_worker_data *) worker->data; PyObject *py_worker = worker_data->worker; @@ -319,8 +322,6 @@ gdbpy_get_xmethod_arg_types (const struct extension_language_defn *extlang, an invalid/unusable number of arg types. */ *nargs = -1; - gdbpy_enter enter_py (get_current_arch (), current_language); - gdbpy_ref get_arg_types_method (PyObject_GetAttrString (py_worker, get_arg_types_method_name)); if (get_arg_types_method == NULL) -- 2.30.2