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Re: Re: Debugging help for fork failure: resource temporarily unavailable

On 2:59 PM, Jon TURNEY wrote:
On 09/03/2011 17:04, Ryan Johnson wrote
BTW, while looking at the code I noticed a potential source of remap problems:
if B depends on A, and we remap A first, then only A's location will be
checked carefully; B will be pulled in wherever it happens to end up when we
do the full load of A. The code seems to assume that every DLL we try to remap
is currently not loaded.

I'm actually not sure what would happen when time came to remap B, because
loading it would just return the handle we didn't know we had, and closing
that handle wouldn't take its reference count to zero.
I too have idly mused that there might be an issue with dependent DLLs here.

But, since dll_list::load_after_fork() walks the dll list in the same order as
the dlopen() calls occur, I've never been able to convince myself there is a
real problem, barring esoteric scenarios like: B depends on A, C depends on A,
load B, load C (C collides with A so loads at non-preferred address), unload
B, fork
Oh, I see what you mean... in theory, asking Windows to load the same dlls in the same order should put them at the same addresses.
That doesn't match what really happens though: where problems are seen it's
often with python or perl, which dynamically load libraries when modules are
imported, but won't unload them in normal use.
All of this assumes Windows is consistent in choosing locations when conflicts are involved. IOW, consider the case that B depends on A, with A and B both conflicting with a later-loaded C. The first time A and C load Windows will choose alternate locations for them, and if that order changes in the child, it's totally possible that A ends up in the child where C was in the parent.

Incidentally, this
same problem would arise if a BLODA injected a DLL into the process -- that
DLL would be on the todo list for fork() to process (because it was also
injected into the parent process), but would already be loaded by the time we
try to remap it. Also, if we do want to force Windows not to put a dll in a
certain address, wouldn't it make more sense to reserve the (wrong) space it
went into on the first try? Right now if the offending location is higher than
the one we want, nothing stops Windows from just putting it right back in its
old spot because the code only reserves locations lower than the desired one.

Is this accurate or am I missing something here?
I'm not sure that particular scenario with injected DLLs is possible, as the
list traversed in dll_list::load_after_fork() is only of dynamically loaded
cygwin-based DLLs?
Oh, so injected dlls, though not statically linked in, still wouldn't be on this list?

BTW, I found a good way to identify, if not fix, BLODA: given an app which loads no libraries at runtime -- such as 'ls' -- any dlls mentioned in /proc/$$/maps which cygcheck does not mention are probably dodgy. In my case, Windows Live (which I didn't think was even installed on my machine) has injected a WLIDNSP.DLL ("Microsoft Windows Live ID Namespace Provider") in all my processes.

$ objdump -p /usr/bin/cygpyglib-2.0-python2.6-0.dll | grep ^ImageBase
ImageBase               6aa40000

$ objdump -p /usr/bin/cygglib-2.0-0.dll | grep ^ImageBase
ImageBase               6aa40000

C:\cygwin\bin\cygglib-2.0-0.dll @ 0x6AA40000 using DONT_RESOLVE_DLL_REFERENCES
    1263 [main] python 3008 dll_list::load_after_fork: reserve_upto 0x18C40000
to try to force it to load there
    1473 [main] python 3008 dll_list::load_after_fork: LoadLibrary
C:\cygwin\bin\cygglib-2.0-0.dll @ 0x6AA40000 using DONT_RESOLVE_DLL_REFERENCES
    1620 [main] python 3008 C:\cygwin\bin\python.exe: *** fatal error - unable
to remap C:\cygwin\bin\cygglib-2.0-0.dll to same address as parent: 0x18C40000
!= 0x6AA40000

and I've confirmed that in the parent, cygpyglib-2.0-python2.6-0.dll loads at
0x6AA40000 and cygglib-2.0-0.dll loads at 0x18C40000.

At a wild guess, it looks like LoadLibraryEx() maps DLLs into memory starting
from the top of the dependency chain, but then calls the DLL's entry point
starting from the bottom of the dependency chain (which makes all kinds of
sense, but leads to this inversion of the load order in the child)

So the problem basically arises because dlls in the child are not actually loaded in the same order as in the parent? In this case I assume that cygpyglib depends on cygglib, which suggests that we could avoid a lot of trouble by handling dependent children first.

Also, it looks like the above is exactly the case I suspected -- the offending dll attempts to load *higher* than where we want it, so reserving space below does nothing for us.
I assume there's a way to enumerate the dlls loaded in a given process; would
it make sense to use a three-step algorithm?
1. Unload all currently-loaded dlls, complaining loudly to stderr or a log
file (these are due to BLODA and deserve to be called out)
2. Load without deps every DLL and make sure it lands at the right address
(using memory reservation tricks if needed)
3. Reload with deps every DLL. Presumably once it has landed correctly once it
will do so thereafter (the current code assumes this, at least)
Doing 2&  3 is an interesting idea, the first call to let you pin it at a
particular address and the second to make it executable.

I've no idea what happens, but unfortunately, the comments in
dll_list::load_after_fork() seem to suggest this doesn't work, as the DLLs
entry point doesn't get called the second time it's loaded.
The code currently unloads the library completely and the reloads it normally, which I assumed was to ensure entry points get called.

In theory, the first step might allow cygwin to resist dll injection (maybe on
an opt-out basis?), though I don't know what the consequences of that choice
would be.

The third step would be significantly easier if we had a dependency graph so
that we could ensure dependencies always get processed before they're needed,
but I don't know if that's feasible. How expensive/embeddable is cygcheck?
Another idea (assuming my guess about LoadLibrary() behaviour above is
correct) would be to have dlopen() rather than simply call LoadLibrary() on a
DLL, construct the dependency tree of the DLL it's been asked to open and load
the DLLs starting from the bottom, so that the order of loading into memory
matches the order which entry points are called (and hence the order in
dll_list)? (This would have the advantage of not making fork() even more
Some variant of objdump -p $THE_DLL | grep 'DLL Name' ?

It might also make sense for the parent process to record some ordering information at dlopen time in case it forks later. Given that the dlls are opening anyway it would probably be cheap to do it then. Just build a tree of all dlls which the current dlopen() triggers dlopen() calls for. Alternatively (simpler?) just make dlopen() add dlls to its list just before it returns. That way, any recursive calls will add the dependencies to the list first. No special data structures needed. Only problem is, I can't see where in the source this magical list is generated in the first place :(

Alternatively, maybe all that is needed is a slightly more complex approach to
forcing the DLL to load at a particular address?  If reserve_upto() has been
called, but it loads higher than that, can we assume load order inversion has
occurred, and try to to block it from loading at it's preferred address by
VirtualAlloc()-ing there as well? I think I might even try to write a patch to
do that...
The second approach might be easier to hack together quickly, but the first would actually make fork() more efficient and eliminate a lot of code: it's likely that all the rebasing/remapping fallbacks could disappear.

A third alternative would be to traverse the remaining list of dlls and find the one that we should have loaded first. This would have to be recursive to handle the case where several dlls map to the same base, but might otherwise be workable.


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