Re: How is info->cmap supposed to work?

Re: How is info->cmap supposed to work?

[Petr Vandrovec]

On 25 Jul 03 at 1:04, James Simmons wrote:
> 
> Hi!

Finally ;-) I just posted on lk that nobody answered...
  
> > (1) FBIOGETCMAP calls fb_copy_cmap(&info->cmap, &cmap, 0);
> >     Should not it use last argument of '2', as &cmap points
> >     to the userspace? It looks to me like that anybody can
> >     overwrite kernel currently... Only positive side is that
> >     there is no way to control info->cmap contents (see (2)),
> >     so you can only crash kernel with random code, you cannot 
> >     stuff some malicious code there.
> 
> I think I posted something about that some time ago and I didn't here 
> anything. Looking at the code I realized that yeap its broke. Its strange
> that both fb_set_cmap and fb_copy_cmap can get data from userland. I would
> think that we either have fb_set_cmap just set the video hardware or 
> have fb_set_cmap be able to grab data from userland and fb_copy_cmap send 
> data to userland. 

It looks reasonable.

> > (2) FBIOPUTCMAP calls fb_set_cmap, which in turn calls
> >     fb_setcolreg. 
> 
> True.
> 
> >     FBIOGETCMAP copies cmap entries from
> >     info->cmap (after fixing (1)). Does it mean that
> >     fb_setcolreg has to fill info->cmap itself? 
> 
> No. At present all the drivers initialize a default cmap. Then it
> doesn't matter which function gets called first. 

Problem is that if you do FBIOPUTCMAP to change (say) entry #0,
FBIOGETCMAP will retrieve default value of entry #0 instead of value
you just set with FBIOPUTCMAP, unless driver updates its info->cmap
itself... 

> > Is not it
> >     a bit ugly? And fb_set_cmap documentation is incorrect:
> >     kspc == 0 means copy from userspace, while
> >     kspc != 0 means copy "local", inside kernel-space. Documentation
> >     says that 0 is local, while 1 is get_user.
> 
> :-( I have to look to see if that has been around for a while. I have a 
> feeling it has been.

2.4.x does same. But I do not think that it is excuse ;-)
 
> The reason for the driver initalizing the default cmap is because 
> we don't know how big the actual colormap will be. I don't know if 
> the generic method of setting to color map to 2^bpp until above 8 bpp 
> mode in which case we only set 16 colors will always work. Perhaps we 
> could just set the cmap.len field and have the upper layer just generate 
> from that.

Ok, then there is a problem - as nothing in matroxfb uses info->cmap,
I did not saw any need to initialize it. I'll fix it.
                                                            Petr
                                                            

Re: How is info->cmap supposed to work?

[Geert Uytterhoeven]

On Fri, 25 Jul 2003, Petr Vandrovec wrote:
> On 25 Jul 03 at 1:04, James Simmons wrote:
> > 
> > Hi!
> 
> Finally ;-) I just posted on lk that nobody answered...

Sorry, I didn't notice your email because you CC'ed it to sourceforge.org
instead of .net ;-)

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds

How is info->cmap supposed to work?

[Petr Vandrovec]

Hi James,
  I have few problems with understanding how
info->cmap is supposed to work:

(1) FBIOGETCMAP calls fb_copy_cmap(&info->cmap, &cmap, 0);
    Should not it use last argument of '2', as &cmap points
    to the userspace? It looks to me like that anybody can
    overwrite kernel currently... Only positive side is that
    there is no way to control info->cmap contents (see (2)),
    so you can only crash kernel with random code, you cannot 
    stuff some malicious code there.

(2) FBIOPUTCMAP calls fb_set_cmap, which in turn calls
    fb_setcolreg. FBIOGETCMAP copies cmap entries from
    info->cmap (after fixing (1)). Does it mean that
    fb_setcolreg has to fill info->cmap itself? Is not it
    a bit ugly? And fb_set_cmap documentation is incorrect:
    kspc == 0 means copy from userspace, while
    kspc != 0 means copy "local", inside kernel-space. Documentation
    says that 0 is local, while 1 is get_user.

(3) And who is supposed to initialize info->cmap, and to
    what value? It looks to me like that fbdev driver is
    supposed to do:

      memset(&info->cmap, 0, sizeof(info->cmap));
      fb_alloc_cmap(&info->cmap, 256, 1);

    Is it right? What about fb_init_cmap() then? And why
    fbdev has to play with info->cmap, cannot generic
    layer take a care of this, if all info->cmap accesses
    go through generic layer, and fbdev driver itself has
    no need for this field?

    				Thanks,
    					Petr Vandrovec
    					vandrove@vc.cvut.cz

Re: How is info->cmap supposed to work?

[James Simmons]

> Hi James,
>   I have few problems with understanding how
> info->cmap is supposed to work:

Hi!
 
> (1) FBIOGETCMAP calls fb_copy_cmap(&info->cmap, &cmap, 0);
>     Should not it use last argument of '2', as &cmap points
>     to the userspace? It looks to me like that anybody can
>     overwrite kernel currently... Only positive side is that
>     there is no way to control info->cmap contents (see (2)),
>     so you can only crash kernel with random code, you cannot 
>     stuff some malicious code there.

I think I posted something about that some time ago and I didn't here 
anything. Looking at the code I realized that yeap its broke. Its strange
that both fb_set_cmap and fb_copy_cmap can get data from userland. I would
think that we either have fb_set_cmap just set the video hardware or 
have fb_set_cmap be able to grab data from userland and fb_copy_cmap send 
data to userland. 

> (2) FBIOPUTCMAP calls fb_set_cmap, which in turn calls
>     fb_setcolreg. 

True.

>     FBIOGETCMAP copies cmap entries from
>     info->cmap (after fixing (1)). Does it mean that
>     fb_setcolreg has to fill info->cmap itself? 

No. At present all the drivers initialize a default cmap. Then it
doesn't matter which function gets called first. 

> Is not it
>     a bit ugly? And fb_set_cmap documentation is incorrect:
>     kspc == 0 means copy from userspace, while
>     kspc != 0 means copy "local", inside kernel-space. Documentation
>     says that 0 is local, while 1 is get_user.

:-( I have to look to see if that has been around for a while. I have a 
feeling it has been.

> (3) And who is supposed to initialize info->cmap, and to
>     what value? It looks to me like that fbdev driver is
>     supposed to do:
> 
>       memset(&info->cmap, 0, sizeof(info->cmap));
>       fb_alloc_cmap(&info->cmap, 256, 1);
> 
>     Is it right? What about fb_init_cmap() then? And why
>     fbdev has to play with info->cmap, cannot generic
>     layer take a care of this, if all info->cmap accesses
>     go through generic layer, and fbdev driver itself has
>     no need for this field?

The reason for the driver initalizing the default cmap is because 
we don't know how big the actual colormap will be. I don't know if 
the generic method of setting to color map to 2^bpp until above 8 bpp 
mode in which case we only set 16 colors will always work. Perhaps we 
could just set the cmap.len field and have the upper layer just generate 
from that.