Re: [patch 0/3] no MAX_ARG_PAGES -v2

Peter Zijlstra
2007-06-14T21:18:34Z

On Thu, 2007-06-14 at 13:58 -0700, Ollie Wild wrote:
> > @@ -1385,6 +1401,10 @@ int do_execve(char * filename,
> > goto out;
> > bprm->argv_len = env_p - bprm->p;
> >
> > + retval = expand_arg_vma(bprm);
> > + if (retval < 0)
> > + goto out;
> > +
> > retval = search_binary_handler(bprm,regs);
> > if (retval >= 0) {
> > /* execve success */
>
> At this point bprm->argc hasn't been finalized yet. For example, the
> script binfmt reads the script header and adds additional arguments.
> The flush_old_exec() function is a better place to call this.

Sure, but at this time most of it is there, so when there are many, this
allocates the most of it.

> I'm not 100% sure this is the right way to handle this, though. The
> problem isn't as simple as ensuring the stack doesn't overflow during
> argument allocation. We also need to ensure the program has
> sufficient stack space to run subsequently. Otherwise, the observable
> behavior is identical.

Well, not identical, but similar indeed.

> Since we can't realistically predict
> acceptable stack availability requirements, some amount of uncertainty
> is always going to exist.

> A good heuristic, though, might be to limit
> argument size to a percentage (say 25%) of maximum stack size and
> validate this inside copy_strings().

Right, this seems a much simpler approach. I like it.