How do values get into the %gs:0x0 location?

[Amittai Aviram]

Hi!  I have a minimal C program that prints a double-precision  
floating-point number to stdout:


#include <stdio.h>

int main(int argc, char ** argv) {
   printf("%f\n", 3.1416);
   return 0;
}

I've compiled it with GCC for an Intel i686 machine under Linux using  
the -static flag, so that I can see all the compiled glibc code.  The  
glibc printf, in this case, calls ___printf_fp.  Now, at one point in  
___printf_fp, I see an instruction that gets the value from the 0  
offset within the TLS segment, whose selector is stored in the %gs  
register:

mov %gs:0x0,%ebx

This value turns to be important: it's used to compute an address to  
find another value, which I think is the original floating-point  
expression to be printed.  I've determined from GDB that it holds a  
nonzero value that looks like a plausible memory address within my  
program's address space.  (I did that by checking the value of %ebx  
right after the above instruction.)  My question is how this value  
every got there.

I have searched through a representation of the object code that I had  
gotten with objdump -D, and there are many, many instances of moving  
the value from %gs:0x0 to a register, as above -- and some instances  
of moving a value from a register to some location at a nonzero offset  
from %gs, such as %gs:0xc -- but not a single instruction in the whole  
object code dump that moves a value into %gs:0x0.  Does anybody have  
any idea how this works?  It's important to me right now, because I'm  
debugging a virtual execution environment (a sort of lightweight  
virtual OS) with a code translator, and it's failing on the printing  
of floating-point numbers.  It looks as if the value is remaining 0 at  
my virtual %gs:0x0, whereas it should be a number that serves as an  
address a few instructions later.  Thanks!


Amittai Aviram
Graduate Student in Computer Science
Yale University
646 483 2639
amittai.aviram@yale.edu
http://www.amittai.com

Amittai Aviram
Graduate Student in Computer Science
Yale University
646 483 2639
amittai.aviram@yale.edu
http://www.amittai.com