Hi,
I would like to be able to catch SIGSEGV in my Python code! So I started to
hack Python trunk to support this feature. The idea is to use a signal
handler which call longjmp(), and add setjmp() at Py_EvalFrameEx() enter.
See attached ("small") patch: segfault.patch
Example read.py with the *evil* ctypes module of invalid memory read:
------------------- 8< --------------
from ctypes import string_at
def fault():
text = string_at(1, 10)
print("text = {0!r}".format(text))
def test():
print("test: 1")
try:
fault()
except MemoryError, err:
print "ooops!"
print err
print("test: 2")
try:
fault()
except MemoryError, err:
print "ooops!"
print err
print("test: end")
def main():
test()
if __name__ == "__main__":
main()
------------------- 8< --------------
Result:
------------------- 8< --------------
$ python read.py
test: 1
sizeof()=160
ooops!
segmentation fault
test: 2
sizeof()=160
ooops!
segmentation fault
test: end
------------------- 8< --------------
Example bug1.py of a stack overflow:
----------
loop = None,
for i in xrange(10**5):
loop = loop, None
----------
Result:
----------
$ python -i bug1.py
>>> print loop
(((((((((...Traceback (most recent call last):
File "<stdin>", line 1, in <module>
MemoryError: segmentation fault
>>>
----------
Python is able to restore a valid state (stack/heap) after a segmentation
fault and raise a classical Python exception (I choosed MemoryError, but it
could be a specific exception).
On my computer (Ubuntu Gutsy/i386), each segfault_frame takes
sizeof(sigjmpbuf) + sizeof(void*) = 160 bytes, allocated on the stack. I
don't know if it's huge or not, but that will limit the number of recursive
calls. The feature can be optional if we add a configure option and some
#ifdef/#endif. A dedicated stack is needed to be call the signal handler on
stack overflow error. I choosed 4 KB, but since I only call longjmp(),
smaller stack might also works.
Does other VM support such feature? JVM, Mono, .NET, etc. ?
I had the idea of catching SIGSEGV after reading the issue 1069092 (stack
overflow because of too many recursive calls).
--
Victor Stinner aka haypo
http://www.haypocalc.com/blog/
Index: Include/segfault.h
===================================================================
--- Include/segfault.h (révision 0)
+++ Include/segfault.h (révision 0)
@@ -0,0 +1,37 @@
+
+/* Interface to execute compiled code */
+
+#ifndef Py_SEGFAULT_H
+#define Py_SEGFAULT_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <setjmp.h>
+#include <signal.h>
+
+/* Use a custom stack for signal handlers, especially the segfault handler */
+#define SEGFAULT_STACK
+
+typedef struct _segfault_frame_t {
+ sigjmp_buf env;
+ struct _segfault_frame_t *previous;
+} segfault_frame_t;
+
+typedef struct {
+ int init;
+ PyObject *text;
+ segfault_frame_t *current_frame;
+#ifdef SEGFAULT_STACK
+ char stack[4096];
+#endif
+} segfault_t;
+
+void segfault_enter(segfault_frame_t *frame);
+void segfault_exit(segfault_frame_t *frame);
+void segfault_set_error(void);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_SEGFAULT_H */
Modification de propriétés sur Include/segfault.h
___________________________________________________________________
Nom : svn:eol-style
+ native
Index: Python/segfault.c
===================================================================
--- Python/segfault.c (révision 0)
+++ Python/segfault.c (révision 0)
@@ -0,0 +1,81 @@
+/**
+ * Python segmentation fault handler
+ */
+
+#include "Python.h"
+#include "segfault.h"
+
+static segfault_t segfault;
+
+void
+segfault_set_error(void)
+{
+ PyErr_SetObject(PyExc_MemoryError, segfault.text);
+}
+
+static int segfault_install(void);
+
+static void
+segfault_handler(int sig_num)
+{
+ (void)segfault_install();
+ siglongjmp(segfault.current_frame->env, 1);
+}
+
+static int
+segfault_install()
+{
+ struct sigaction context, ocontext;
+ context.sa_handler = segfault_handler;
+ sigemptyset(&context.sa_mask);
+#ifdef SEGFAULT_STACK
+ context.sa_flags = SA_RESETHAND | SA_RESTART | SA_ONSTACK;
+#else
+ context.sa_flags = SA_RESETHAND | SA_RESTART;
+#endif
+ if (sigaction(SIGSEGV, &context, &ocontext) == -1)
+ return 1;
+ else
+ return 0;
+}
+
+void
+segfault_enter(segfault_frame_t *frame)
+{
+ frame->previous = segfault.current_frame;
+ segfault.current_frame = frame;
+}
+
+void
+segfault_exit(segfault_frame_t *frame)
+{
+ if (segfault.current_frame)
+ segfault.current_frame = segfault.current_frame->previous;
+}
+
+void
+PyOS_InitSegfault()
+{
+#ifdef SEGFAULT_STACK
+ stack_t ss;
+ ss.ss_sp = segfault.stack;
+ ss.ss_size = sizeof(segfault.stack);
+ ss.ss_flags = 0;
+ if( sigaltstack(&ss, NULL)) {
+ /* FIXME: catch this error */
+ }
+#endif
+
+ segfault.text = PyString_FromString("segmentation fault");
+ /* FIXME
+ if (!segfault.text) ???;
+ */
+ (void)segfault_install();
+}
+
+void
+PyOS_FiniSegfault()
+{
+ Py_DECREF(segfault.text);
+}
+
Modification de propriétés sur Python/segfault.c
___________________________________________________________________
Nom : svn:eol-style
+ native
Index: Python/ceval.c
===================================================================
--- Python/ceval.c (révision 66680)
+++ Python/ceval.c (copie de travail)
@@ -16,6 +16,7 @@
#include "eval.h"
#include "opcode.h"
#include "structmember.h"
+#include "segfault.h"
#include <ctype.h>
@@ -553,6 +554,7 @@
PyObject *retval = NULL; /* Return value */
PyThreadState *tstate = PyThreadState_GET();
PyCodeObject *co;
+ segfault_frame_t segfault_frame;
/* when tracing we set things up so that
@@ -801,6 +803,13 @@
goto on_error;
}
+ segfault_enter(&segfault_frame);
+ if (sigsetjmp(segfault_frame.env, 1)) {
+ segfault_set_error();
+ why = WHY_EXCEPTION;
+ goto on_error;
+ }
+
for (;;) {
#ifdef WITH_TSC
if (inst1 == 0) {
@@ -2697,6 +2706,8 @@
/* pop frame */
exit_eval_frame:
+ segfault_exit(&segfault_frame);
+
Py_LeaveRecursiveCall();
tstate->frame = f->f_back;
Index: Makefile.pre.in
===================================================================
--- Makefile.pre.in (révision 66680)
+++ Makefile.pre.in (copie de travail)
@@ -280,6 +280,7 @@
Python/pymath.o \
Python/pystate.o \
Python/pythonrun.o \
+ Python/segfault.o \
Python/structmember.o \
Python/symtable.o \
Python/sysmodule.o \
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