Re: [Patch] Off by one in drivers/usb/serial/usb-serial.c

Re: [Patch] Off by one in drivers/usb/serial/usb-serial.c

[Mikael Pettersson]

On Wed, 21 Jun 2006 23:28:17 +0200, Eric Sesterhenn wrote:
> this fixes coverity id #554. since serial table
> is defines as serial_table[SERIAL_TTY_MINORS] we
> should make sure we dont acess with an index
> of SERIAL_TTY_MINORS.
> 
> Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
> 
> --- linux-2.6.17-git2/drivers/usb/serial/usb-serial.c.orig	2006-06-21 23:24:07.000000000 +0200
> +++ linux-2.6.17-git2/drivers/usb/serial/usb-serial.c	2006-06-21 23:25:12.000000000 +0200
> @@ -83,7 +83,7 @@ static struct usb_serial *get_free_seria
>  
>  		good_spot = 1;
>  		for (j = 1; j <= num_ports-1; ++j)
> -			if ((i+j >= SERIAL_TTY_MINORS) || (serial_table[i+j])) {
> +			if ((i+j >= SERIAL_TTY_MINORS-1)||(serial_table[i+j])) {
>  				good_spot = 0;
>  				i += j;
>  				break;

Where is the access coverity complained about? If it's the serial_table[i+j]
quoted above, then the original code is OK since i+j < SERIAL_TTY_MINORS is
an invariant in that subexpression.

And the other accesses to serial_table[] in get_free_serial() are also only
done when the index is < SERIAL_TTY_MINORS.

/Mikael

Re: [Patch] Off by one in drivers/usb/serial/usb-serial.c

[Eric Sesterhenn / Snakebyte]

* Mikael Pettersson (mikpe@it.uu.se) wrote:
> On Wed, 21 Jun 2006 23:28:17 +0200, Eric Sesterhenn wrote:
> > this fixes coverity id #554. since serial table
> > is defines as serial_table[SERIAL_TTY_MINORS] we
> > should make sure we dont acess with an index
> > of SERIAL_TTY_MINORS.
> > 
> > Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
> > 
> > --- linux-2.6.17-git2/drivers/usb/serial/usb-serial.c.orig	2006-06-21 23:24:07.000000000 +0200
> > +++ linux-2.6.17-git2/drivers/usb/serial/usb-serial.c	2006-06-21 23:25:12.000000000 +0200
> > @@ -83,7 +83,7 @@ static struct usb_serial *get_free_seria
> >  
> >  		good_spot = 1;
> >  		for (j = 1; j <= num_ports-1; ++j)
> > -			if ((i+j >= SERIAL_TTY_MINORS) || (serial_table[i+j])) {
> > +			if ((i+j >= SERIAL_TTY_MINORS-1)||(serial_table[i+j])) {
> >  				good_spot = 0;
> >  				i += j;
> >  				break;
> 
> Where is the access coverity complained about? If it's the serial_table[i+j]
> quoted above, then the original code is OK since i+j < SERIAL_TTY_MINORS is
> an invariant in that subexpression.
> 
> And the other accesses to serial_table[] in get_free_serial() are also only
> done when the index is < SERIAL_TTY_MINORS.

I'll check that again on sunday, when i am back home.

Greetings, Eric

Re: [Patch] Off by one in drivers/usb/serial/usb-serial.c

[Eric Sesterhenn / Snakebyte]

* Mikael Pettersson (mikpe@it.uu.se) wrote:
> On Wed, 21 Jun 2006 23:28:17 +0200, Eric Sesterhenn wrote:
> > this fixes coverity id #554. since serial table
> > is defines as serial_table[SERIAL_TTY_MINORS] we
> > should make sure we dont acess with an index
> > of SERIAL_TTY_MINORS.
> > 
> > Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
> > 
> > --- linux-2.6.17-git2/drivers/usb/serial/usb-serial.c.orig	2006-06-21 23:24:07.000000000 +0200
> > +++ linux-2.6.17-git2/drivers/usb/serial/usb-serial.c	2006-06-21 23:25:12.000000000 +0200
> > @@ -83,7 +83,7 @@ static struct usb_serial *get_free_seria
> >  
> >  		good_spot = 1;
> >  		for (j = 1; j <= num_ports-1; ++j)
> > -			if ((i+j >= SERIAL_TTY_MINORS) || (serial_table[i+j])) {
> > +			if ((i+j >= SERIAL_TTY_MINORS-1)||(serial_table[i+j])) {
> >  				good_spot = 0;
> >  				i += j;
> >  				break;
> 
> Where is the access coverity complained about? If it's the serial_table[i+j]
> quoted above, then the original code is OK since i+j < SERIAL_TTY_MINORS is
> an invariant in that subexpression.
> 
> And the other accesses to serial_table[] in get_free_serial() are also only
> done when the index is < SERIAL_TTY_MINORS.

guess i was too quick on that one, sorry. Here is the coverity
report for completeness.

Event assignment: Assigning "1" to "j"
Also see events: [overrun-local]
At conditional (11): "j <= (num_ports - 1)" taking true path
At conditional (16): "j <= (num_ports - 1)" taking true path

85   			for (j = 1; j <= num_ports-1; ++j)

Event overrun-local: Overrun of static array "serial_table" of size 255
at position 255 with index variable "(i + j)"
Also see events: [assignment]
At conditional (12): "(i + j) >= 255" taking true path
At conditional (17): "(i + j) >= 255" taking false path

86   				if ((i+j >= SERIAL_TTY_MINORS) ||
(serial_table[i+j])) {
87   					good_spot = 0;
88   					i += j;
89   					break;
90   				}




greetings, Eric

Re: [Patch] Off by one in drivers/usb/serial/usb-serial.c

[Greg KH]

On Mon, Jun 26, 2006 at 12:59:21AM +0200, Eric Sesterhenn / Snakebyte wrote:
> * Mikael Pettersson (mikpe@it.uu.se) wrote:
> > On Wed, 21 Jun 2006 23:28:17 +0200, Eric Sesterhenn wrote:
> > > this fixes coverity id #554. since serial table
> > > is defines as serial_table[SERIAL_TTY_MINORS] we
> > > should make sure we dont acess with an index
> > > of SERIAL_TTY_MINORS.
> > > 
> > > Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
> > > 
> > > --- linux-2.6.17-git2/drivers/usb/serial/usb-serial.c.orig	2006-06-21 23:24:07.000000000 +0200
> > > +++ linux-2.6.17-git2/drivers/usb/serial/usb-serial.c	2006-06-21 23:25:12.000000000 +0200
> > > @@ -83,7 +83,7 @@ static struct usb_serial *get_free_seria
> > >  
> > >  		good_spot = 1;
> > >  		for (j = 1; j <= num_ports-1; ++j)
> > > -			if ((i+j >= SERIAL_TTY_MINORS) || (serial_table[i+j])) {
> > > +			if ((i+j >= SERIAL_TTY_MINORS-1)||(serial_table[i+j])) {
> > >  				good_spot = 0;
> > >  				i += j;
> > >  				break;
> > 
> > Where is the access coverity complained about? If it's the serial_table[i+j]
> > quoted above, then the original code is OK since i+j < SERIAL_TTY_MINORS is
> > an invariant in that subexpression.
> > 
> > And the other accesses to serial_table[] in get_free_serial() are also only
> > done when the index is < SERIAL_TTY_MINORS.
> 
> guess i was too quick on that one, sorry. Here is the coverity
> report for completeness.
> 
> Event assignment: Assigning "1" to "j"
> Also see events: [overrun-local]
> At conditional (11): "j <= (num_ports - 1)" taking true path
> At conditional (16): "j <= (num_ports - 1)" taking true path
> 
> 85   			for (j = 1; j <= num_ports-1; ++j)
> 
> Event overrun-local: Overrun of static array "serial_table" of size 255
> at position 255 with index variable "(i + j)"
> Also see events: [assignment]
> At conditional (12): "(i + j) >= 255" taking true path
> At conditional (17): "(i + j) >= 255" taking false path
> 
> 86   				if ((i+j >= SERIAL_TTY_MINORS) ||
> (serial_table[i+j])) {
> 87   					good_spot = 0;
> 88   					i += j;
> 89   					break;
> 90   				}

So, what does this mean?  That coverity is broken, yet again?

I'm getting very tired of these false positives from them, it is getting
so that I can't trust the output of the tool at all :(

thanks,

greg k-h

Re: [Patch] Off by one in drivers/usb/serial/usb-serial.c

[Eric Sesterhenn / Snakebyte]

> So, what does this mean?  That coverity is broken, yet again?

right, it means that this was a false report.

> I'm getting very tired of these false positives from them, it is getting
> so that I can't trust the output of the tool at all :(

you shouldnt trust it anyways. At the moment ~11% of the
stuff we checked is marked as bug, ~24% as false or ignore,
5% pending, 4% resolved and 54% uninspected. If we count
pending and resolved to the bugs, that would mean a 50/50
split between bugs and false positives. So we should not trust
it blind and i try my best to avoid such mistakes.
But I wouldnt say coverity is that bad, and it already helped
us fixing several bugs.

Greetings, Eric

[Patch] Off by one in drivers/usb/serial/usb-serial.c

[Eric Sesterhenn]

hi,

this fixes coverity id #554. since serial table
is defines as serial_table[SERIAL_TTY_MINORS] we
should make sure we dont acess with an index
of SERIAL_TTY_MINORS.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>

--- linux-2.6.17-git2/drivers/usb/serial/usb-serial.c.orig	2006-06-21 23:24:07.000000000 +0200
+++ linux-2.6.17-git2/drivers/usb/serial/usb-serial.c	2006-06-21 23:25:12.000000000 +0200
@@ -83,7 +83,7 @@ static struct usb_serial *get_free_seria
 
 		good_spot = 1;
 		for (j = 1; j <= num_ports-1; ++j)
-			if ((i+j >= SERIAL_TTY_MINORS) || (serial_table[i+j])) {
+			if ((i+j >= SERIAL_TTY_MINORS-1)||(serial_table[i+j])) {
 				good_spot = 0;
 				i += j;
 				break;