Re: [xml] Strange XPath crash

[Daniel Veillard <veillard redhat com>]

On Wed, Apr 07, 2004 at 03:28:17PM -0400, Trey Gruel wrote:
> I'm using XPath (in libxml2-2.6.8) to parse some XML configuration files
> for a set of servers and I've run across an annoying and baffling seg
> fault.  I've got 4 servers that I'm starting up which all use basically
> the same code for parsing their config files.  The only difference in the
> parsing is the actual paths they're looking for.  Setup and node
> processing is the same.  On 3, it works fine, but on the fourth, I get a
> seg fault when calling xmlXPathFreeObject.
>
> Here's what backtrace shows in gdb:
> #0  __pthread_mutex_lock (mutex=0x206ca) at mutex.c:99
> #1  0x407c93e8 in __libc_free (mem=0x4062dfb0) at malloc.c:3152
> #2  0x4069f447 in xmlXPathFreeObject () from /usr/lib/libxml2.so.2
> #3  0x08059687 in MyApp::parseConfigFile (this=0x8137ec0) at parsexml.cpp:73
> #4  0x0804ea06 in MyApp::appInitialize (this=0x8137ec0) at myapp.cpp:400
> #5  0x0804d6de in main (argc=6, argv=0xbffff184) at main.cpp:59
> #6  0x40764657 in __libc_start_main (main=0x804d660 <main>, argc=6, 
>     ubp_av=0xbffff184, init=0x804c9c0 <_init>, fini=0x81033f0 <_fini>, 
>     rtld_fini=0x4000dc54 <_dl_fini>, stack_end=0xbffff17c)
>     at ../sysdeps/generic/libc-start.c:129
>
> And here's the xmlXPathObjectPtr I'm trying to free:
> $1 = {type = 1407551829, nodesetval = 0xe810ec83, boolval = -60, 
>   floatval = 1.044512297805358e+27, 
>   stringval = 0x48508b08 <Address 0x48508b08 out of bounds>, 
>   user = 0xac7ae852, index = 1569456127, user2 = 0x10c483fc, 
>   index2 = -1017254775}
>
> I imagine it's related to stringval being out of bounds, but as this is 
> internal to libxml2, I'm not sure what to do about it.

  Hum, the type of the xmlXPathObjectPtr is clearly wrong, it's like the
memory got trashed and overwritten by something else.

> I had a similar problem to this occur in one of the other servers when
> using the libxml/parser API to manually parse the file that magically
> fixed itself.

  I would suggest to valgrind your application for memory access errors.

> For the time being, I've gone back to using the parser API (which works)  
> for this particular server, but I would like to have them all using the
> same method of reading the config files.
>
> Any ideas?

  not really.

Daniel

-- 
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