On 08/08/2012 05:51, Chris LeBlanc wrote:
I'm trying to help a coworker get Cygwin running on her Windows 7
(64b) machine, but we're seeing a segmentation fault with XWin.exe.
Previous versions of X11 worked fine on this machine, but recently it
has been unable to fork processes. It's been giving the error fork:
child -1 - forked process died unexpectedly ..., which is why I've
been trying to upgrade to the most recent version of Cygwin.
Thanks for the detailed bug report.
Given you were getting fork problems before this upgrade, this might well be a
problem with some other software interfering with cygwin's operation. So you
should take a look at [1], and probably try running XWin with CYGWIN including
detect_bloda [2]
Any advice on how to get more debug info? How hard is it to build
XWin from source? If it's a minor strcpy() issue it could be a pretty
easy fix. Any other advice?
Instructions on building the X server from source are in [3], but with the
debug info package installed you have source and symbols, so you should be
able to set a breakpoint on main and step through the code anyhow.
0 [main] XWin 6164 exception::handle: Exception: STATUS_ACCESS_VIOLATION
454 [main] XWin 6164 open_stackdumpfile: Dumping stack trace to
XWin.exe.stackdump
Program received signal SIGSEGV, Segmentation fault.
strcpy (dst0=0x40010006 Address 0x40010006 out of bounds, src0=0x0)
at /usr/src/debug/cygwin-1.7.16-1/newlib/libc/string/strcpy.c:86
86while (!DETECTNULL(*aligned_src))
(gdb) bt full
#0 strcpy (dst0=0x40010006 Address 0x40010006 out of bounds, src0=0x0)
at /usr/src/debug/cygwin-1.7.16-1/newlib/libc/string/strcpy.c:86
dst = 0x40010006 Address 0x40010006 out of bounds
src = 0x0
aligned_dst = 0x40010006
aligned_src = 0x28a026
#1 0x751d2ad6 in OutputDebugStringA () from
/cygdrive/c/Windows/syswow64/KERNELBASE.dll
No symbol table info available.
#2 0x40010006 in ?? ()
No symbol table info available.
#3 0x in ?? ()
No symbol table info available.
(gdb)
This stack trace looks highly suspicious:
- The presence of the invalid address 0x40010006 as src for strcpy and in the
backtrace
- I think cygwin is supposed to notice it is being debugged and give control
to the debugger and not write a stackdump file
- I don't think OutputDebugStringA() should ever end up calling cygwin's
strcpy()
So all this suggests to me some kind of stack corruption has happened.
Output from XWin.exe.stackdump:
Exception: STATUS_ACCESS_VIOLATION at eip=61129F8B
eax=0028D624 ebx= ecx= edx= esi=0028D624 edi=0028AB5C
ebp=0028A968 esp=0028A95C program=C:\cygwin\bin\XWin.exe, pid 6164, thread
main
cs=0023 ds=002B es=002B fs=0053 gs=002B ss=002B
Stack trace:
Frame Function Args
0028A968 61129F8B (0028D624, , 0028FF14, 771170D5)
0028AA28 6108E2BA (0028AA58, 611DF23B, 60FE8510, 0068)
0028AB78 6108EB57 (60FE000C, 0005, 0028ABC4, 005C2DE9)
0028AB98 61091F56 (005C2DCE, 005C2DCC, , )
0028ABF8 610D5F05 (6C34, 0001, 0028FD24, 0002)
0028AC38 00528F43 (0002, 0028AC60, 800280E8, 8003A441)
0028ACF8 61007535 (, 0028CD78, 61006B20, )
End of stack trace
Processing this with awk '/^[0-9]/{print $2}' XWin.exe.stackdump | addr2line
-asf -e /usr/bin/cygwin1.dll, and cleaning up the output by hand a bit,
gives...
0x61129f8b strcpy strcpy.c:86
0x6108e2ba fillout_mntent mount.cc:1636
0x6108eb57 cygdrive_getmntent mount.cc:1720
0x61091f56 getmntent mount.cc:1900
0x610d5f05 ?? ??:0
0x00528f43 mainmain.c:145
0x61007535 _Z10dll_crt0_1Pvdcrt0.cc:982
... which looks a bit more reasonable, and suggests something in the cygwin
DLL is exploding when XWin uses getmntent() in winCheckMount() to check for
FAT and textmode mounts.
[1] http://cygwin.com/faq/faq.using.html#faq.using.bloda
[2] http://cygwin.com/ml/cygwin/2012-02/msg00797.html
[2] http://x.cygwin.com/docs/cg/
--
Jon TURNEY
Volunteer Cygwin/X X Server maintainer
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