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posix_lchown(PyObject *self, PyObject *args)
{
 ...
 	int uid, gid;
 ...
 	if (!PyArg_ParseTuple(args, "etii:lchown",
 	 Py_FileSystemDefaultEncoding, &path,
 	 &uid, &gid))
 ...
}
uid and gid could cause over flow. A similar bug is issue 5705.
Patch attached. Any comment is appreciated!
Boya

posix modules contains a lot of function parsing uid_t / gid_t types. I would be
nice to factorize the code: create a function to get an uid_t, and another to
get a gid_t. I don't know the name of such callback, but it's used with:
PyArg_ParseTuple(args, "...O&...", ..., &uid, get_uid, ...)).
Such callbacks will be useful for: posix_chown(), posix_fchown(),
posix_lchown(), posix_setuid(), posix_seteuid(), posix_setreuid(),
posix_setegid(), posix_setregid(), posix_setgid().
And maybe also in: posix_setgroups().
In Python trunk, posix_set*id() function do check for uid_t/gid_d overflow, but
not the posix_*chown() functions. The patch only fixes posix_lchown().

The patch is incorrect. Why do you think there is an overflow? There is 
none in the call to ParseTuple: the i argument parser expects a signed 
int*; passing a long* will break on systems where 
sizeof(int)!=sizeof(long) (such as typical 64-bit Unix).
In addition, the *actual* overflow in the current code (casting to uid_t) 
is not handled in the patch.

Martin,
The reason why I think there is a possible overflow is that according to
issue 5705, uid/gid overflows are fixed in the following functions:
posix_setegid, posix_setreuid(), posix_setregid(), posix_setgid(). So I
think a similar fix should also be applied to the function posix_lchown.
Or did I misunderstand anything?
And you're right. The previous patch is incorrect. I now submitted
another patch that deals with the *actual* overflow of gid and uid. 
---
Victor,
I agree that all posix_*chown() functions should also be fixed for the
same overflow problem, and it's a good idea to create callback functions
as you described. But if nobody does that, I can at least created more
patches to fix other posix_*chown() functions.

I think the new patch is still incorrect. You now pass long variables into 
the i argument parser. Also, I would expect that compilers prefer to see 
an explicit cast from long to uid_t, in case it's a truncating cast.
Can you try your patch on a system where all this is an actual problem?

@loewis: I don't think that the explicit cast is required. posix_setuid() has no
explicit cast. But I would also prefer an explicit cast (just for the readability).

Martin,
I am sorry that I do not have a system where this code actually
triggered a problem, since this bug is discovered by a *static* analysis
tool that is recently developed by our research group, which finds code
segments that are similar to a previously fixed bugs as potential bugs.
 You are saying that if I pass a long to the i argument parser it will
cause a problem. But if I passed a int, it will be same as before and
overflow will not be detected at all. 
---
Victor,
Do you also agree that it will cause a problem if I pass a long to the i
argument parser? If so, I think maybe the overflow problem cannot be
solved by the patch I submitted.
Boya

> You are saying that if I pass a long to the i argument parser it will
> cause a problem. But if I passed a int, it will be same as before and
> overflow will not be detected at all. 
Correct. So you should use the l argument parser.

Martin,
Corrected the patch accordingly. Can you verify whether the fix is
correct or not now? 
Boya

Yes, it looks correct now. I still wish it could be tested on a system 
where the problem actually occurs.

I applied the same fix that was applied to chown in trunk r77007 for 
lchown and fchown. Could you test it on a platform where it previously 
failed?
The existing code might still have issues if there are platforms where 
uid_t and gid_t are unsigned but not the same size as a long as at the 
moment it merely casts and does not test to see that the values are the 
same as the patch you have supplied here.

Gregory,
I discovered this bug by static analysis, so I do not have a system 
that this bug is actually triggered. But I am happy to see the fix 
applied since this makes code safer. It would be great if anyone could 
write a test case that cause uid and gid to overflow, then use the test 
case as a regression test on the fix.
Boya

a test would still be a good thing but this should be fixed regardless.