[OLSR-users] MIPS crash problem solved

[Thomas Lopatic]

Hi there,

We've finally found the culprit. Thanks to Sven-Ola and Bruno for their 
help!

Also thanks to the people on the binutils mailing list for not screaming 
at me and patiently directing me to the relevant section of the binutils 
documentation! :-)

The problem is that olsrd contains some functions and variables that 
have the same names as some functions and variables in the plugin. On 
Intel Linux this does not lead to any problems, as the functions and 
variables in the plugin are separated from the functions and variables 
in olsrd.

On MIPS, however, this is different, for whatever reason. The functions 
and variables in olsrd interfere with the functions and variables in the 
plugin. The resulting confusion makes olsrd crash on (some versions of) 
the MIPS platform when plugins are loaded.

Although the Linux manual page makes you think otherwise at first sight, 
it is never guaranteed that the functions and variables in olsrd (i.e. 
the executable) do not interfere with the functions and variables in 
plugins (i.e. shared objects loaded by the executable). It's just a 
lucky coincidence that it works like this on Intel.

So, after all, this is not a bug in the MIPS toolchain. True, the MIPS 
toolchain behaves differently from the Intel toolchain. However, this 
behaviour is by design and not a bug, as I've just learned on the 
binutils mailing list.

There is a way to force the functions and variables in the plugin to 
remain separate from the functions and variables in olsrd. This works 
either via GCC by assigning an attribute to functions and variables in 
olsrd, as in

int i __attribute__ ((visibility ("hidden")));

The "hidden" attribute limits the scope of symbols in olsrd to olsrd. No 
interference with shared objects, i.e. plugins.

Alternatively this works by means of "version scripts" that can be 
passed to the linker and which also specify which functions and 
variables from olsrd are to be seen by the plugin and vice versa. The 
nice thing about version scripts is that we do not have to apply the 
"hidden" attribute to each and every function and variable in olsrd.

So, while the traditional "static" limits the scope of a symbol to a 
single object file, "hidden" visibility limits the scope of a symbol to 
a single executable or shared object file.

A version script that sets the scope of all symbols to hidden would look 
as follows.

VERS_1.0 {
   local: *;
};

The "*" means "all symbols" and "local" means "hidden" visibility. Could 
somebody try this out, save this version script as "version.txt", and 
pass it to the link step via "-Wl,--version-script=version.txt"? I'm 
pretty curious whether this will work...

-Thomas