Re: [PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode()
On 3/19/24 11:12 AM, Roman Smirnov wrote:
[...]
On 3/5/24 14:51, Roman Smirnov wrote:
> The expression htotal * vtotal can have a zero value on
> overflow.
I'm not sure if those always results in zero in kernel on overflow.
Might be architecture-depended too, but let's assume it
can become zero,
> It is necessary to prevent division by zero like in
> fb_var_to_videomode().
>
> Found by Linux Verification Center (linuxtesting.org) with Svace.
>
> Signed-off-by: Roman Smirnov
> Reviewed-by: Sergey Shtylyov
> ---
>V1 -> V2: Replaced the code of the first version with a check.
>
>drivers/video/fbdev/core/fbmon.c | 2 +-
>1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/video/fbdev/core/fbmon.c
> b/drivers/video/fbdev/core/fbmon.c
> index 79e5bfbdd34c..b137590386da 100644
> --- a/drivers/video/fbdev/core/fbmon.c
> +++ b/drivers/video/fbdev/core/fbmon.c
> @@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct
> videomode *vm,
> vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
> vm->vsync_len;
> /* prevent division by zero */
> - if (htotal && vtotal) {
> + if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {
why don't you then simply check for
if .. ((htotal * vtotal) == 0) ...
instead?
Helge
>>>
>>> Thomas Zimmermann from the previous discussion said:
>>> On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote:
Maybe use
if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))
for the test. That rules out overflowing multiplication and sets
refresh to 0 in such cases.
>>>
>>> This prevents overflow, which is also a problematic case.
>>
>> I don't like adding another division here and I doubt we have
>> a problem with possible overflow.
>> So, I suggest to keep it simple, something like:
>>...
>>total = htotal * vtotal;
>>if (total)
>>fbmode->refresh = vm->pixelclock / total;
>>else...
>
> Okay, I'll prepare a third version with that change:
>
> if (htotal && vtotal && (htotal * vtotal))
I think the 1st 2 checks here are now redundant. Also, the inner
parens are not necessary...
> I think that will be enough.
More than enough. :-)
MBR, Sergey
Re: [PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode()
On Mon, 18 Mar 2024 20:15:55 +0100 Helge Deller wrote:
> On 3/18/24 09:11, Roman Smirnov wrote:
> > On Fri, 15 Mar 2024 09:44:08 +0100 Helge Deller wrote:
> > > On 3/5/24 14:51, Roman Smirnov wrote:
> > > > The expression htotal * vtotal can have a zero value on
> > > > overflow.
> > >
> > > I'm not sure if those always results in zero in kernel on overflow.
> > > Might be architecture-depended too, but let's assume it
> > > can become zero,
> > >
> > > > It is necessary to prevent division by zero like in
> > > > fb_var_to_videomode().
> > > >
> > > > Found by Linux Verification Center (linuxtesting.org) with Svace.
> > > >
> > > > Signed-off-by: Roman Smirnov
> > > > Reviewed-by: Sergey Shtylyov
> > > > ---
> > > >V1 -> V2: Replaced the code of the first version with a check.
> > > >
> > > >drivers/video/fbdev/core/fbmon.c | 2 +-
> > > >1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/video/fbdev/core/fbmon.c
> > > > b/drivers/video/fbdev/core/fbmon.c
> > > > index 79e5bfbdd34c..b137590386da 100644
> > > > --- a/drivers/video/fbdev/core/fbmon.c
> > > > +++ b/drivers/video/fbdev/core/fbmon.c
> > > > @@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct
> > > > videomode *vm,
> > > > vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
> > > > vm->vsync_len;
> > > > /* prevent division by zero */
> > > > - if (htotal && vtotal) {
> > > > + if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {
> > >
> > > why don't you then simply check for
> > > if .. ((htotal * vtotal) == 0) ...
> > > instead?
> > >
> > > Helge
> >
> > Thomas Zimmermann from the previous discussion said:
> > On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote:
> > > Maybe use
> > >
> > > if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))
> > >
> > > for the test. That rules out overflowing multiplication and sets
> > > refresh to 0 in such cases.
> >
> > This prevents overflow, which is also a problematic case.
>
> I don't like adding another division here and I doubt we have
> a problem with possible overflow.
> So, I suggest to keep it simple, something like:
>...
>total = htotal * vtotal;
>if (total)
>fbmode->refresh = vm->pixelclock / total;
>else...
Okay, I'll prepare a third version with that change:
if (htotal && vtotal && (htotal * vtotal))
I think that will be enough.
Re: [PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode()
On 3/18/24 09:11, Roman Smirnov wrote:
On Fri, 15 Mar 2024 09:44:08 +0100 Helge Deller wrote:
On 3/5/24 14:51, Roman Smirnov wrote:
The expression htotal * vtotal can have a zero value on
overflow.
I'm not sure if those always results in zero in kernel on overflow.
Might be architecture-depended too, but let's assume it
can become zero,
It is necessary to prevent division by zero like in
fb_var_to_videomode().
Found by Linux Verification Center (linuxtesting.org) with Svace.
Signed-off-by: Roman Smirnov
Reviewed-by: Sergey Shtylyov
---
V1 -> V2: Replaced the code of the first version with a check.
drivers/video/fbdev/core/fbmon.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
index 79e5bfbdd34c..b137590386da 100644
--- a/drivers/video/fbdev/core/fbmon.c
+++ b/drivers/video/fbdev/core/fbmon.c
@@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct videomode
*vm,
vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
vm->vsync_len;
/* prevent division by zero */
- if (htotal && vtotal) {
+ if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {
why don't you then simply check for
if .. ((htotal * vtotal) == 0) ...
instead?
Helge
Thomas Zimmermann from the previous discussion said:
On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote:
Maybe use
if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))
for the test. That rules out overflowing multiplication and sets
refresh to 0 in such cases.
This prevents overflow, which is also a problematic case.
I don't like adding another division here and I doubt we have
a problem with possible overflow.
So, I suggest to keep it simple, something like:
...
total = htotal * vtotal;
if (total)
fbmode->refresh = vm->pixelclock / total;
else...
Helge
Re: [PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode()
On Fri, 15 Mar 2024 09:44:08 +0100 Helge Deller wrote:
> On 3/5/24 14:51, Roman Smirnov wrote:
> > The expression htotal * vtotal can have a zero value on
> > overflow.
>
> I'm not sure if thos always results in zero in kernel on overflow.
> Might be architecture-depended too, but let's assume it
> can become zero,
>
> > It is necessary to prevent division by zero like in
> > fb_var_to_videomode().
> >
> > Found by Linux Verification Center (linuxtesting.org) with Svace.
> >
> > Signed-off-by: Roman Smirnov
> > Reviewed-by: Sergey Shtylyov
> > ---
> > V1 -> V2: Replaced the code of the first version with a check.
> >
> > drivers/video/fbdev/core/fbmon.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/video/fbdev/core/fbmon.c
> > b/drivers/video/fbdev/core/fbmon.c
> > index 79e5bfbdd34c..b137590386da 100644
> > --- a/drivers/video/fbdev/core/fbmon.c
> > +++ b/drivers/video/fbdev/core/fbmon.c
> > @@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct
> > videomode *vm,
> >vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
> > vm->vsync_len;
> >/* prevent division by zero */
> > - if (htotal && vtotal) {
> > + if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {
>
> why don't you then simply check for
> if .. ((htotal * vtotal) == 0) ...
> instead?
>
> Helge
Thomas Zimmermann from the previous discussion said:
On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote:
> Maybe use
>
>if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))
>
> for the test. That rules out overflowing multiplication and sets
> refresh to 0 in such cases.
This prevents overflow, which is also a problematic case.
Re: [PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode()
On 3/5/24 14:51, Roman Smirnov wrote:
The expression htotal * vtotal can have a zero value on
overflow.
I'm not sure if thos always results in zero in kernel on overflow.
Might be architecture-depended too, but let's assume it
can become zero,
It is necessary to prevent division by zero like in
fb_var_to_videomode().
Found by Linux Verification Center (linuxtesting.org) with Svace.
Signed-off-by: Roman Smirnov
Reviewed-by: Sergey Shtylyov
---
V1 -> V2: Replaced the code of the first version with a check.
drivers/video/fbdev/core/fbmon.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
index 79e5bfbdd34c..b137590386da 100644
--- a/drivers/video/fbdev/core/fbmon.c
+++ b/drivers/video/fbdev/core/fbmon.c
@@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct videomode
*vm,
vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
vm->vsync_len;
/* prevent division by zero */
- if (htotal && vtotal) {
+ if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {
why don't you then simply check for
if .. ((htotal * vtotal) == 0) ...
instead?
Helge
fbmode->refresh = vm->pixelclock / (htotal * vtotal);
/* a mode must have htotal and vtotal != 0 or it is invalid */
} else {
