Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches
On Thu, Jan 24, 2019 at 8:18 AM Matthew Wilcox wrote:
>
> On Wed, Jan 23, 2019 at 04:17:30PM +0200, Jani Nikula wrote:
> > Can't have:
> >
> > switch (i) {
> > int j;
> > case 0:
> > /* ... */
> > }
> >
> > because it can't be turned into:
> >
> > switch (i) {
> > int j = 0; /* not valid C */
> > case 0:
> > /* ... */
> > }
> >
> > but can have e.g.:
> >
> > switch (i) {
> > case 0:
> > {
> > int j = 0;
> > /* ... */
> > }
> > }
> >
> > I think Kees' approach of moving such variable declarations to the
> > enclosing block scope is better than adding another nesting block.
>
> Another nesting level would be bad, but I think this is OK:
>
> switch (i) {
> case 0: {
> int j = 0;
> /* ... */
> }
> case 1: {
> void *p = q;
> /* ... */
> }
> }
>
> I can imagine Kees' patch might have a bad effect on stack consumption,
> unless GCC can be relied on to be smart enough to notice the
> non-overlapping liveness of the vriables and use the same stack slots
> for both.
GCC is reasonable at this. The main issue, though, was most of these
places were using the variables in multiple case statements, so they
couldn't be limited to a single block (or they'd need to be manually
repeated in each block, which is even more ugly, IMO).
Whatever the consensus, I'm happy to tweak the patch.
Thanks!
--
Kees Cook
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Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches
On Thu, Jan 24, 2019 at 4:44 AM Jani Nikula wrote:
>
> On Wed, 23 Jan 2019, Edwin Zimmerman wrote:
> > On Wed, 23 Jan 2019, Jani Nikula wrote:
> >> On Wed, 23 Jan 2019, Greg KH wrote:
> >> > On Wed, Jan 23, 2019 at 03:03:47AM -0800, Kees Cook wrote:
> >> >> Variables declared in a switch statement before any case statements
> >> >> cannot be initialized, so move all instances out of the switches.
> >> >> After this, future always-initialized stack variables will work
> >> >> and not throw warnings like this:
> >> >>
> >> >> fs/fcntl.c: In function ‘send_sigio_to_task’:
> >> >> fs/fcntl.c:738:13: warning: statement will never be executed
> >> >> [-Wswitch-unreachable]
> >> >>siginfo_t si;
> >> >> ^~
> >> >
> >> > That's a pain, so this means we can't have any new variables in { }
> >> > scope except for at the top of a function?
Just in case this wasn't clear: no, it's just the switch statement
before the first "case". I cannot imagine how bad it would be if we
couldn't have block-scoped variables! Heh. :)
> >> >
> >> > That's going to be a hard thing to keep from happening over time, as
> >> > this is valid C :(
> >>
> >> Not all valid C is meant to be used! ;)
> >
> > Very true. The other thing to keep in mind is the burden of enforcing
> > a prohibition on a valid C construct like this. It seems to me that
> > patch reviewers and maintainers have enough to do without forcing them
> > to watch for variable declarations in switch statements. Automating
> > this prohibition, should it be accepted, seems like a good idea to me.
>
> Considering that the treewide diffstat to fix this is:
>
> 18 files changed, 45 insertions(+), 46 deletions(-)
>
> and using the gcc plugin in question will trigger the switch-unreachable
> warning, I think we're good. There'll probably be the occasional
> declarations that pass through, and will get fixed afterwards.
Yeah, that was my thinking as well: it's a rare use, and we get a
warning when it comes up.
Thanks!
--
Kees Cook
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Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches
On Wed, Jan 23, 2019 at 04:17:30PM +0200, Jani Nikula wrote:
> Can't have:
>
> switch (i) {
> int j;
> case 0:
> /* ... */
> }
>
> because it can't be turned into:
>
> switch (i) {
> int j = 0; /* not valid C */
> case 0:
> /* ... */
> }
>
> but can have e.g.:
>
> switch (i) {
> case 0:
> {
> int j = 0;
> /* ... */
> }
> }
>
> I think Kees' approach of moving such variable declarations to the
> enclosing block scope is better than adding another nesting block.
Another nesting level would be bad, but I think this is OK:
switch (i) {
case 0: {
int j = 0;
/* ... */
}
case 1: {
void *p = q;
/* ... */
}
}
I can imagine Kees' patch might have a bad effect on stack consumption,
unless GCC can be relied on to be smart enough to notice the
non-overlapping liveness of the vriables and use the same stack slots
for both.
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RE: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches
On Wed, 23 Jan 2019, Jani Nikula wrote:
> On Wed, 23 Jan 2019, Greg KH wrote:
> > On Wed, Jan 23, 2019 at 03:03:47AM -0800, Kees Cook wrote:
> >> Variables declared in a switch statement before any case statements
> >> cannot be initialized, so move all instances out of the switches.
> >> After this, future always-initialized stack variables will work
> >> and not throw warnings like this:
> >>
> >> fs/fcntl.c: In function ‘send_sigio_to_task’:
> >> fs/fcntl.c:738:13: warning: statement will never be executed
> >> [-Wswitch-unreachable]
> >>siginfo_t si;
> >> ^~
> >
> > That's a pain, so this means we can't have any new variables in { }
> > scope except for at the top of a function?
> >
> > That's going to be a hard thing to keep from happening over time, as
> > this is valid C :(
>
> Not all valid C is meant to be used! ;)
Very true. The other thing to keep in mind is the burden of enforcing a
prohibition on a valid C construct like this.
It seems to me that patch reviewers and maintainers have enough to do without
forcing them to watch for variable
declarations in switch statements. Automating this prohibition, should it be
accepted, seems like a good idea to me.
-Edwin Zimmerman
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Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches
On Thu, Jan 24, 2019 at 07:55:51AM +1300, Kees Cook wrote:
> On Thu, Jan 24, 2019 at 4:44 AM Jani Nikula
> wrote:
> >
> > On Wed, 23 Jan 2019, Edwin Zimmerman wrote:
> > > On Wed, 23 Jan 2019, Jani Nikula wrote:
> > >> On Wed, 23 Jan 2019, Greg KH wrote:
> > >> > On Wed, Jan 23, 2019 at 03:03:47AM -0800, Kees Cook wrote:
> > >> >> Variables declared in a switch statement before any case statements
> > >> >> cannot be initialized, so move all instances out of the switches.
> > >> >> After this, future always-initialized stack variables will work
> > >> >> and not throw warnings like this:
> > >> >>
> > >> >> fs/fcntl.c: In function ‘send_sigio_to_task’:
> > >> >> fs/fcntl.c:738:13: warning: statement will never be executed
> > >> >> [-Wswitch-unreachable]
> > >> >>siginfo_t si;
> > >> >> ^~
> > >> >
> > >> > That's a pain, so this means we can't have any new variables in { }
> > >> > scope except for at the top of a function?
>
> Just in case this wasn't clear: no, it's just the switch statement
> before the first "case". I cannot imagine how bad it would be if we
> couldn't have block-scoped variables! Heh. :)
Sorry, it was not clear at first glance. So no more objection from me
for this change.
greg k-h
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RE: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches
On Wed, 23 Jan 2019, Edwin Zimmerman wrote:
> On Wed, 23 Jan 2019, Jani Nikula wrote:
>> On Wed, 23 Jan 2019, Greg KH wrote:
>> > On Wed, Jan 23, 2019 at 03:03:47AM -0800, Kees Cook wrote:
>> >> Variables declared in a switch statement before any case statements
>> >> cannot be initialized, so move all instances out of the switches.
>> >> After this, future always-initialized stack variables will work
>> >> and not throw warnings like this:
>> >>
>> >> fs/fcntl.c: In function ‘send_sigio_to_task’:
>> >> fs/fcntl.c:738:13: warning: statement will never be executed
>> >> [-Wswitch-unreachable]
>> >>siginfo_t si;
>> >> ^~
>> >
>> > That's a pain, so this means we can't have any new variables in { }
>> > scope except for at the top of a function?
>> >
>> > That's going to be a hard thing to keep from happening over time, as
>> > this is valid C :(
>>
>> Not all valid C is meant to be used! ;)
>
> Very true. The other thing to keep in mind is the burden of enforcing
> a prohibition on a valid C construct like this. It seems to me that
> patch reviewers and maintainers have enough to do without forcing them
> to watch for variable declarations in switch statements. Automating
> this prohibition, should it be accepted, seems like a good idea to me.
Considering that the treewide diffstat to fix this is:
18 files changed, 45 insertions(+), 46 deletions(-)
and using the gcc plugin in question will trigger the switch-unreachable
warning, I think we're good. There'll probably be the occasional
declarations that pass through, and will get fixed afterwards.
BR,
Jani.
--
Jani Nikula, Intel Open Source Graphics Center
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Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches
On Wed, 23 Jan 2019, Jani Nikula wrote:
> On Wed, 23 Jan 2019, Greg KH wrote:
>> On Wed, Jan 23, 2019 at 03:03:47AM -0800, Kees Cook wrote:
>>> Variables declared in a switch statement before any case statements
>>> cannot be initialized, so move all instances out of the switches.
>>> After this, future always-initialized stack variables will work
>>> and not throw warnings like this:
>>>
>>> fs/fcntl.c: In function ‘send_sigio_to_task’:
>>> fs/fcntl.c:738:13: warning: statement will never be executed
>>> [-Wswitch-unreachable]
>>>siginfo_t si;
>>> ^~
>>
>> That's a pain, so this means we can't have any new variables in { }
>> scope except for at the top of a function?
>>
>> That's going to be a hard thing to keep from happening over time, as
>> this is valid C :(
>
> Not all valid C is meant to be used! ;)
>
> Anyway, I think you're mistaking the limitation to arbitrary blocks
> while it's only about the switch block IIUC.
>
> Can't have:
>
> switch (i) {
> int j;
> case 0:
> /* ... */
> }
>
> because it can't be turned into:
>
> switch (i) {
> int j = 0; /* not valid C */
> case 0:
> /* ... */
> }
>
> but can have e.g.:
>
> switch (i) {
> case 0:
> {
> int j = 0;
> /* ... */
> }
> }
>
> I think Kees' approach of moving such variable declarations to the
> enclosing block scope is better than adding another nesting block.
PS. The patch is
Reviewed-by: Jani Nikula
and the drivers/gpu/drm/i915/* parts are
Acked-by: Jani Nikula
for merging via whichever tree is appropriate. (There'll be minor
conflicts with in-flight work in our -next tree, but no biggie.)
--
Jani Nikula, Intel Open Source Graphics Center
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Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches
On Wed, 23 Jan 2019, Greg KH wrote:
> On Wed, Jan 23, 2019 at 03:03:47AM -0800, Kees Cook wrote:
>> Variables declared in a switch statement before any case statements
>> cannot be initialized, so move all instances out of the switches.
>> After this, future always-initialized stack variables will work
>> and not throw warnings like this:
>>
>> fs/fcntl.c: In function ‘send_sigio_to_task’:
>> fs/fcntl.c:738:13: warning: statement will never be executed
>> [-Wswitch-unreachable]
>>siginfo_t si;
>> ^~
>
> That's a pain, so this means we can't have any new variables in { }
> scope except for at the top of a function?
>
> That's going to be a hard thing to keep from happening over time, as
> this is valid C :(
Not all valid C is meant to be used! ;)
Anyway, I think you're mistaking the limitation to arbitrary blocks
while it's only about the switch block IIUC.
Can't have:
switch (i) {
int j;
case 0:
/* ... */
}
because it can't be turned into:
switch (i) {
int j = 0; /* not valid C */
case 0:
/* ... */
}
but can have e.g.:
switch (i) {
case 0:
{
int j = 0;
/* ... */
}
}
I think Kees' approach of moving such variable declarations to the
enclosing block scope is better than adding another nesting block.
BR,
Jani.
--
Jani Nikula, Intel Open Source Graphics Center
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