code that went to process segment now goes to shared segment

code that went to process segment now goes to shared segment

[bmuller@inf.ufpr.br]

I wrote a program that uses some dynamic libraries I designed. Then, I
compiled and executed a program that prints the address of a lot of things
global variables, shared procedures and the procedure main():

long int main ()
{
 ...
  printf ("Procedure main: (Address=%p)\n", main);
...
}

After compiling with gcc 5.4.0 (gcc (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0
20160609),  the execution shows that the procedure "main" was at address
0x400776, that is, in the process segment (along the global variables).

After compiling with gcc 7.4.0 (gcc (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0),
the execution  shows that the procedure "main" went to address
0x55bbddb7e7fa, that is inside the dynamic segment área (along the global
variables).

I guess that this "functional" change occurred along the GCC 6.0 changes,
but I could not find the motivation for it in the wiki, faq or the 6.0
change docs.

My question is if there is some documentation that presents the motivation
for this changes: advantages, disadvantages and if some abnormal behavior
was reported.

I teach operating systems, and some students reported that the programs I
used for exams were not producing the former addresses generating unusual
results. I'll have to rewrite my code, but I'd like to know if I should be
prepared for something... strange.

Thanks,

Bruno

Re: code that went to process segment now goes to shared segment

[Alexander Monakov]

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On Fri, 6 Dec 2019, bmuller@inf.ufpr.br wrote:

> After compiling with gcc 5.4.0 (gcc (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0
> 20160609),  the execution shows that the procedure "main" was at address
> 0x400776, that is, in the process segment (along the global variables).
> 
> After compiling with gcc 7.4.0 (gcc (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0),
> the execution  shows that the procedure "main" went to address
> 0x55bbddb7e7fa, that is inside the dynamic segment área (along the global
> variables).
> 
> I guess that this "functional" change occurred along the GCC 6.0 changes,
> but I could not find the motivation for it in the wiki, faq or the 6.0
> change docs.
> 
> My question is if there is some documentation that presents the motivation
> for this changes: advantages, disadvantages and if some abnormal behavior
> was reported.

You're seeing that newer Ubuntu configures GCC to produce position-independent
executables by default (look for --enable-default-pie in output of 'gcc -v').

Each distribution decides for itself whether to enable PIE by default or not,
the motivation is security hardening; I think you can look for articles on
ASLR, address space layout randomization, for further info.

Upstream GCC did not change its defaults with regard to PIE-by-default, this
was and remains up to distributions.

Alexander