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Date: Mon, 19 May 2025 12:37:42 -0700 (PDT)
X-X-Sender: jeremyd AT resin DOT csoft DOT net
To: Yuyi Wang <Strawberry_Str AT hotmail DOT com>
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Subject: Re: fork error when trying to call VirtualAlloc with size==0
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From: Jeremy Drake via Cygwin <cygwin AT cygwin DOT com>
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On Mon, 19 May 2025, Yuyi Wang via Cygwin wrote:

> Recently I'm trying to build rustc on Cygwin (actually MSYS2). Here is the error:
>
>       0 [main] rustc 3299 child_info_fork::abort: couldn't allocate memory 0x1FD100000(0) for '\??\D:\Straw\Documents\Git\rust\build\x86_64-pc-cygwin\stage1-rustc\release\deps\zerovec_derive-4b870d338ef8cbdd.dll' alignment, Win32 error 87
>       0 [main] rustc 3299 child_info_fork::abort: couldn't allocate memory 0x1FD100000(0) for '\??\D:\Straw\Documents\Git\rust\build\x86_64-pc-cygwin\stage1-rustc\release\deps\zerovec_derive-4b870d338ef8cbdd.dll' alignment, Win32 error 87
> error: could not exec the linker `x86_64-pc-msys-gcc`
>
> Seems that reserve_at() in file cygwin/winsup/cygwin/dll_init.cc is trying to call
> VirtualAlloc() with size == 0. That's why it returns 87 (ERROR_INVALID_PARAMETER).
> This error is rare and I'm not able to create an MWE now.
>
> I looked into the source code in dll_init.cc and confirmed that size is possible
> to be zero. I guess that end == dll_end in this case. (here != dll_base because of
> the conditions in callee.) So the dll straddles our left edge. Is this a bug of
> the code?

That doesn't make sense to me.  It just loaded the DLL at `here`, but
`here` != `dll_base`, so it unloaded it again and queried the address
space where it just was.  It seems impossible for the 'straddles our left
edge' case to be hit *and* end == dll_end unless
a) the dll has changed and is now smaller than it used to be, or
b) mb.BaseAddress != `here`?
c) some sort of race condition where the memory where the dll was just
loaded is now not free at VirtualQuery time?  BLODA?
d) I'm missing something

The only thing I can see that might potentially be a bug in the code is
maybe

size = mb.RegionSize - (here - mb.BaseAddress);

but that seems extremely unlikely.  Is there a possibility that the dll is
getting replaced while a process that's running is still using it, which
then subsequently forks?

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