segfault on 32bit cygwin snapshot
Thu Mar 4 09:05:14 GMT 2021
Corinna Vinschen via Cygwin wrote:
> [Ping Mark Geisert]
> On Mar 3 18:56, Takashi Yano via Cygwin wrote:
>> Hi Corinna,
>> On Tue, 2 Mar 2021 16:48:45 +0100
>> Corinna Vinschen wrote:
>>> On Mar 2 20:03, Takashi Yano via Cygwin wrote:
>>>>> The following check code does not work as expected if
>>>>> newly build exe file is linked with old dll which calls
>>>>> uname() as in this case.
>>>>> if (CYGWIN_VERSION_CHECK_FOR_UNAME_X)
>>>>> return uname_x (in_name);
>>>>> Any idea?
>>>> Ping Corinna?
>>> I have no idea how we could fix that, other than by rebuilding the DLLs
>>> which call uname, too. We can't check the Cygwin build of all DLLs an
>>> executable is linked to.
>> I have checked all /usr/bin/*.dll in my cygwin installation and found
>> following dlls call uname() rather than uname_x().
>> Do you think rebuilding all of these (or maybe more) dlls is only
>> the solution?
> No, we could also drop the above code snippet.
> Here's the problem: When we changed some datatypes in the early 2000s,
> the old entry points have been conserved for backward compatibility,
> while the new function using the new datatypes got a new name, e. g.,
> stat vs. _stat64.
> Next, libcygwin.a gets changed so that a newly built executable (using
> the new datatypes as defined in the standard headers) calling stat is
> redirected to _stat64.
> All is well for the next 15+ years or so.
> Then we discover that the exact same mechanism fails to work for
> uname vs. the new uname_x in python. What happened?
> It turned out that python called uname dynamically Rather then just
> calling uname, it calls dlopen();dlsym("uname");
> This actually fetches the symbol for uname, not the symbol for uname_x.
> The good old mechanism used for ages on standard function, fails as soon
> as the caller uses dynamic loading of symbols. Surprise, surprise!
> It was just never taken into consideration that standard libc functions
> might be called dynamically, given it usually doesn't make sense.
> Given that this affects *all* of these redirected functions, we're in a
> bit of a fix. Fortunately, for all other functions this only affects 32
> bit Cygwin, because the 64 bit version never had this backward
> compatibility problem.
> Therefore, uname vs. uname_x is the only function affecting 64 bit
> Cygwin as well, and that's why I added the above crude redirection only
> to uname in the first place.
> So what we can do is this:
> - Either all old DLLs calling uname must be rebuilt.
.. or patched (somehow). Also, IIUC you're both talking about Cygwin-supplied
DLLs, not user builds of Cygwin or private DLLs. Dunno how we'd find every last one.
> - Or we remove the above code snippet again and behave like for all
> other redirected functions on 32 bit as well. Python's os.uname is
> broken, but all the affected DLL sstart working again.
I think you need to keep the above code snippet to handle old exe running with
current Cygwin DLL, no?
> Is there a way around that? I'm not quite sure, so let's brain storm
> a bit, ok?
> - One thing we could try is to remove the above code, but add a python
> hack to dlsym instead. This would let the "old" DLLs work again as
> before and for python we could add a hack to dlsym, along these lines:
> if (CYGWIN_VERSION_CHECK_FOR_UNAME_X
> && modulehandle == cygwin1.dll
> && strcmp (symname, "uname"))
> symname = "uname_x";
> Thoughts? Other ideas?
That's a sly fix, but it seems that it would do the job. That's good!
On a different tack, I was thinking about how run time code could tell the
difference between the versions of uname() being called. It can't. I looked at
glibc and FreeBSD libc to see if they had to deal with this compatibility issue.
It seems they don't, or I couldn't locate it if they do.
But FreeBSD has an approach that looked interesting in another way. They have the
standard uname() function taking the one usual arg. But it's just a wrapper on a
worker function that takes the original arg plus another arg specifying the size
of the fields in struct utsname. Paraphrasing, using Cygwin names:
int uname(struct utsname *uptr)
return uname_x((int) _UTSNAME_LENGTH, uptr);
They allow the user to override what we call _UTSNAME_LENGTH. That seems like an
invitation to exploit so I don't care for that. But what I was thinking is if we
make that first arg to uname_x() be a uintptr_t, we could tell (with pretty good
confidence) at run time inside uname_x() if we were passed a length (from new code
passing two args) or a ptr-to-buf (from old code just passing one arg).
I am unsure if this is useful. I am trying to consider the cross-product of exes
and dlls we care about and whether we can identify which combination we have at
run time. I think I need some time with a pad of paper, and maybe a drink.
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