Mail Archives: cygwin-developers/2002/07/19/22:09:02

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Date: Fri, 19 Jul 2002 22:08:59 -0400
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From: Joe Buehler <jbuehler AT hekimian DOT com>
Reply-To: joseph DOT buehler AT spirentcom DOT com
Organization: Spirent Communications
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To: Cygwin Developers <cygwin-developers AT cygwin DOT com>
Subject: [bug found] Re: cygwin hang problem
References: <3D32FC00 DOT 5090108 AT hekimian DOT com> <20020719050925 DOT GA24259 AT redhat DOT com> <3D37F0E5 DOT 50F3669B AT yahoo DOT com> <20020719141242 DOT GB27697 AT redhat DOT com> <3D38949C DOT 3090200 AT hekimian DOT com>

OK, I think I see what the problem may be.  In the dll_func_load
code (assembly language), the dll linkage code is patched (rewritten)
once the address of the loaded dll function is known.  The problem
is that there is a race -- the new opcode and its argument
are written separately.  What happens is this:

1. a mov instruction is overwritten with 0xe9 to become a jmp
2. another thread executes the jmp before step 3
3. the newly written jmp instruction gets the proper offset written

Since the mov instruction uses an offset from the beginning of the segment,
and the jmp uses an EIP-relative offset, the net effect is that the jmp
goes off in the weeds.  The data in the dll linkage code is what causes
the observed behavior of a jump to twice the value of the linkage data --
the mov instruction references memory just a few bytes further down.

In the core that I am looking at, here is what is at win32_CopySid AT 12:

0x610f00b8: 0xa1 0xbf 0x00 0x0f 0x61 # mov 0x610f00bf,%eax

This becomes -- at just the wrong moment:

0x610f00b8: 0xe9 0xbf 0x00 0x0f 0x61 # jmp %eip+0x610f00bf

So the locking needs some changing in the dll linkage code.  There is in fact
a comment above dll_func_load that the code may not be thread safe!

Joe Buehler

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