Re: Constructor of external lib called too early

[Dominik Vogt <vogt at linux dot vnet dot ibm dot com>]

On Mon, Jul 29, 2013 at 06:46:56PM +0530, Siddhesh Poyarekar wrote:
> On 29 July 2013 18:25, Dominik Vogt <vogt@linux.vnet.ibm.com> wrote:
> > An external library - say, libfoo.{a,so} - defines a costructor
> >
> >   static void __attribute__ ((constructor 65535))
> >   __foo_ctor (void)
> >   {
> >     const char *val;
> >
> >     val = getenv ("FOO_CFG");
> >     ...
> >   }
> >
> > and glibc is built and linked with -lfoo.
> >
> > A program is linked against that the modified glibc.  Now, when
> > the program is started, at the time __foo_ctor is called, the
> > environment is empty and FOO is never evaluated.  I guess this
> > means that the constructor is called before the environment is
> > initialized.
> 
> Yes, and it is because glibc depends on this library and is hence
> initialized after the library.  The glibc _init initializes __environ.

Understood.

> > Is there a way / what is the right way to have __foo_ctor called
> > _after_ the initialization of the environment?  (Note that calling
> > the constructor from the executable instead of from glibc is not
> > an option.)
> 
> The constructors are usually called like this from the linker:
> 
>   if (l->l_info[DT_INIT] != NULL)
>     {
>       init_t init = (init_t) DL_DT_INIT_ADDRESS
>         (l, l->l_addr + l->l_info[DT_INIT]->d_un.d_ptr);
> 
>       /* Call the function.  */
>       init (argc, argv, env);
>     }
> 
> So if your constructor accepts argc, argv and envp, you might be able
> to get the environment from its argument.  This is a NULL terminated
> array, so you will have to do your own traversal and matching.  I
> don't know if this is documented anywhere, so I don't know if it is
> the canonical way to do this.

Okay, that works nicely, thanks.

> You also have to make sure you don't traverse the environment if the
> executed binary is suid.

Ciao

Dominik ^_^  ^_^

-- 

Dominik Vogt
IBM Germany