Re: gcc: optimizations, and stack traces

[Maxime Villard <max%m00nbsd.net@localhost>]

Le 09/02/2018 à 12:08, Valery Ushakov a écrit :
> On Fri, Feb 09, 2018 at 11:38:47 +0100, Martin Husemann wrote:
>
> > On Fri, Feb 09, 2018 at 11:23:17AM +0100, Maxime Villard wrote:
> >
> > > When I spotted this several months ago (while developing Live
> > > Kernel ASLR), I tried to look for GCC options that say "optimize
> > > with -O2, but keep the stack trace intact". I couldn't find one,
> > > and the only thing I ended up doing was disabling -O2 in the
> > > makefiles.
> >
> > -fno-omit-frame-pointer?
>
> That won't help.
>
>       `-O' also turns on `-fomit-frame-pointer' on machines where doing
>       so does not interfere with debugging.
>
> so it's not turned off in the first place.  The problem is that some
> of the later optimization passes may push frame pointer setup to some
> place later in function.  E.g. on -7
>
>      void
>      kernfs_get_rrootdev(void)
>      {
>              static int tried = 0;
>
>              if (tried) {
>                      /* Already did it once. */
>                      return;
>              }
>              tried = 1;
>
>              if (rootdev == NODEV)
>                      return;
>              rrootdev = devsw_blk2chr(rootdev);
>              if (rrootdev != NODEV)
>                      return;
>              rrootdev = NODEV;
>              printf("kernfs_get_rrootdev: no raw root device\n");
>      }
>
> is compiled to
>
>      c068f81b <kernfs_get_rrootdev>:
>      c068f81b:       mov    0xc0fc6b40,%eax
>      c068f820:       test   %eax,%eax
>      c068f822:       jne    c068f867 <kernfs_get_rrootdev+0x4c>
>      c068f824:       movl   $0x1,0xc0fc6b40
>      c068f82e:       mov    0xc0fde0b8,%edx
>      c068f834:       mov    0xc0fde0bc,%eax
>      c068f839:       mov    %edx,%ecx
>      c068f83b:       and    %eax,%ecx
>      c068f83d:       cmp    $0xffffffff,%ecx
>      c068f840:       je     c068f867 <kernfs_get_rrootdev+0x4c>
> ->  c068f842:       push   %ebp
> ->  c068f843:       mov    %esp,%ebp
>      c068f845:       sub    $0x8,%esp
>      c068f848:       mov    %edx,(%esp)
>      c068f84b:       mov    %eax,0x4(%esp)
>      c068f84f:       call   c091ce52 <devsw_blk2chr>

Yes, exactly. -fno-omit-frame-pointer doesn't change anything here, GCC
does not omit the frame pointer but moves the instructions a little later
in the function.

So we need to find a say to keep the two instructions at the beginning...

Maxime