hw/s390/ccw.c: Don't take address of packed members

Message ID 20181210135803.20208-1-peter.maydell@linaro.org
State New
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Series
  • hw/s390/ccw.c: Don't take address of packed members
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Commit Message

Peter Maydell Dec. 10, 2018, 1:58 p.m.
Taking the address of a field in a packed struct is a bad idea, because
it might not be actually aligned enough for that pointer type (and
thus cause a crash on dereference on some host architectures). Newer
versions of clang warn about this.

Avoid the problem by using local copies of the PMCW and SCSW
struct fields in copy_schib_from_guest() and copy_schib_to_guest().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

---
This seemed like a not totally ugly and reasonably localised fix
that satisfies clang. Oddly, this makes the generated object file
15K smaller (421K vs 406K), so it might even be better code...

 hw/s390x/css.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

-- 
2.19.2

Comments

Cornelia Huck Dec. 10, 2018, 2:13 p.m. | #1
On Mon, 10 Dec 2018 13:58:03 +0000
Peter Maydell <peter.maydell@linaro.org> wrote:

> Taking the address of a field in a packed struct is a bad idea, because

> it might not be actually aligned enough for that pointer type (and

> thus cause a crash on dereference on some host architectures). Newer

> versions of clang warn about this.

> 

> Avoid the problem by using local copies of the PMCW and SCSW

> struct fields in copy_schib_from_guest() and copy_schib_to_guest().

> 

> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

> ---

> This seemed like a not totally ugly and reasonably localised fix

> that satisfies clang. Oddly, this makes the generated object file

> 15K smaller (421K vs 406K), so it might even be better code...


Nice :)

> 

>  hw/s390x/css.c | 20 ++++++++++++++++----

>  1 file changed, 16 insertions(+), 4 deletions(-)

> 

> diff --git a/hw/s390x/css.c b/hw/s390x/css.c

> index 04ec5cc9705..ef07691e36b 100644

> --- a/hw/s390x/css.c

> +++ b/hw/s390x/css.c

> @@ -1290,9 +1290,15 @@ void copy_scsw_to_guest(SCSW *dest, const SCSW *src)

>  static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)

>  {

>      int i;

> +    PMCW srcpmcw, destpmcw;

> +    SCSW srcscsw, destscsw;


<bikeshed>
I would find src_pmcw etc. easier to read. Other opinions?
</bikeshed>

>  

> -    copy_pmcw_to_guest(&dest->pmcw, &src->pmcw);

> -    copy_scsw_to_guest(&dest->scsw, &src->scsw);

> +    srcpmcw = src->pmcw;

> +    copy_pmcw_to_guest(&destpmcw, &srcpmcw);

> +    dest->pmcw = destpmcw;

> +    srcscsw = src->scsw;

> +    copy_scsw_to_guest(&destscsw, &srcscsw);

> +    dest->scsw = destscsw;

>      dest->mba = cpu_to_be64(src->mba);

>      for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {

>          dest->mda[i] = src->mda[i];

> @@ -1339,9 +1345,15 @@ static void copy_scsw_from_guest(SCSW *dest, const SCSW *src)

>  static void copy_schib_from_guest(SCHIB *dest, const SCHIB *src)

>  {

>      int i;

> +    PMCW srcpmcw, destpmcw;

> +    SCSW srcscsw, destscsw;

>  

> -    copy_pmcw_from_guest(&dest->pmcw, &src->pmcw);

> -    copy_scsw_from_guest(&dest->scsw, &src->scsw);

> +    srcpmcw = src->pmcw;

> +    copy_pmcw_from_guest(&destpmcw, &srcpmcw);

> +    dest->pmcw = destpmcw;

> +    srcscsw = src->scsw;

> +    copy_scsw_from_guest(&destscsw, &srcscsw);

> +    dest->scsw = destscsw;

>      dest->mba = be64_to_cpu(src->mba);

>      for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {

>          dest->mda[i] = src->mda[i];
Peter Maydell Dec. 10, 2018, 2:23 p.m. | #2
On Mon, 10 Dec 2018 at 14:13, Cornelia Huck <cohuck@redhat.com> wrote:
>

> On Mon, 10 Dec 2018 13:58:03 +0000

> Peter Maydell <peter.maydell@linaro.org> wrote:

>

> > Taking the address of a field in a packed struct is a bad idea, because

> > it might not be actually aligned enough for that pointer type (and

> > thus cause a crash on dereference on some host architectures). Newer

> > versions of clang warn about this.

> >

> > Avoid the problem by using local copies of the PMCW and SCSW

> > struct fields in copy_schib_from_guest() and copy_schib_to_guest().

> >

> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

> > ---

> > This seemed like a not totally ugly and reasonably localised fix

> > that satisfies clang. Oddly, this makes the generated object file

> > 15K smaller (421K vs 406K), so it might even be better code...

>

> Nice :)

>

> >

> >  hw/s390x/css.c | 20 ++++++++++++++++----

> >  1 file changed, 16 insertions(+), 4 deletions(-)

> >

> > diff --git a/hw/s390x/css.c b/hw/s390x/css.c

> > index 04ec5cc9705..ef07691e36b 100644

> > --- a/hw/s390x/css.c

> > +++ b/hw/s390x/css.c

> > @@ -1290,9 +1290,15 @@ void copy_scsw_to_guest(SCSW *dest, const SCSW *src)

> >  static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)

> >  {

> >      int i;

> > +    PMCW srcpmcw, destpmcw;

> > +    SCSW srcscsw, destscsw;

>

> <bikeshed>

> I would find src_pmcw etc. easier to read. Other opinions?

> </bikeshed>


CODING_STYLE's "Naming" section agrees with you...

thanks
-- PMM
Farhan Ali Dec. 10, 2018, 2:58 p.m. | #3
On 12/10/2018 08:58 AM, Peter Maydell wrote:
> Taking the address of a field in a packed struct is a bad idea, because

> it might not be actually aligned enough for that pointer type (and

> thus cause a crash on dereference on some host architectures). Newer

> versions of clang warn about this.

> 

> Avoid the problem by using local copies of the PMCW and SCSW

> struct fields in copy_schib_from_guest() and copy_schib_to_guest().

> 

> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

> ---

> This seemed like a not totally ugly and reasonably localised fix

> that satisfies clang. Oddly, this makes the generated object file

> 15K smaller (421K vs 406K), so it might even be better code...

> 

>   hw/s390x/css.c | 20 ++++++++++++++++----

>   1 file changed, 16 insertions(+), 4 deletions(-)

> 

> diff --git a/hw/s390x/css.c b/hw/s390x/css.c

> index 04ec5cc9705..ef07691e36b 100644

> --- a/hw/s390x/css.c

> +++ b/hw/s390x/css.c

> @@ -1290,9 +1290,15 @@ void copy_scsw_to_guest(SCSW *dest, const SCSW *src)

>   static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)

>   {

>       int i;

> +    PMCW srcpmcw, destpmcw;

> +    SCSW srcscsw, destscsw;

> 

> -    copy_pmcw_to_guest(&dest->pmcw, &src->pmcw);

> -    copy_scsw_to_guest(&dest->scsw, &src->scsw);

> +    srcpmcw = src->pmcw;

> +    copy_pmcw_to_guest(&destpmcw, &srcpmcw);

> +    dest->pmcw = destpmcw;

> +    srcscsw = src->scsw;

> +    copy_scsw_to_guest(&destscsw, &srcscsw);

> +    dest->scsw = destscsw;

>       dest->mba = cpu_to_be64(src->mba);

>       for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {

>           dest->mda[i] = src->mda[i];

> @@ -1339,9 +1345,15 @@ static void copy_scsw_from_guest(SCSW *dest, const SCSW *src)

>   static void copy_schib_from_guest(SCHIB *dest, const SCHIB *src)

>   {

>       int i;

> +    PMCW srcpmcw, destpmcw;

> +    SCSW srcscsw, destscsw;

> 

> -    copy_pmcw_from_guest(&dest->pmcw, &src->pmcw);

> -    copy_scsw_from_guest(&dest->scsw, &src->scsw);

> +    srcpmcw = src->pmcw;

> +    copy_pmcw_from_guest(&destpmcw, &srcpmcw);

> +    dest->pmcw = destpmcw;

> +    srcscsw = src->scsw;

> +    copy_scsw_from_guest(&destscsw, &srcscsw);

> +    dest->scsw = destscsw;

>       dest->mba = be64_to_cpu(src->mba);

>       for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {

>           dest->mda[i] = src->mda[i];

> 


Reviewed-by: Farhan Ali <alifm@linux.ibm.com>
Cornelia Huck Dec. 12, 2018, 5:34 p.m. | #4
On Mon, 10 Dec 2018 14:23:15 +0000
Peter Maydell <peter.maydell@linaro.org> wrote:

> On Mon, 10 Dec 2018 at 14:13, Cornelia Huck <cohuck@redhat.com> wrote:

> >

> > On Mon, 10 Dec 2018 13:58:03 +0000

> > Peter Maydell <peter.maydell@linaro.org> wrote:

> >  

> > > Taking the address of a field in a packed struct is a bad idea, because

> > > it might not be actually aligned enough for that pointer type (and

> > > thus cause a crash on dereference on some host architectures). Newer

> > > versions of clang warn about this.

> > >

> > > Avoid the problem by using local copies of the PMCW and SCSW

> > > struct fields in copy_schib_from_guest() and copy_schib_to_guest().

> > >

> > > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

> > > ---

> > > This seemed like a not totally ugly and reasonably localised fix

> > > that satisfies clang. Oddly, this makes the generated object file

> > > 15K smaller (421K vs 406K), so it might even be better code...  

> >

> > Nice :)

> >  

> > >

> > >  hw/s390x/css.c | 20 ++++++++++++++++----

> > >  1 file changed, 16 insertions(+), 4 deletions(-)

> > >

> > > diff --git a/hw/s390x/css.c b/hw/s390x/css.c

> > > index 04ec5cc9705..ef07691e36b 100644

> > > --- a/hw/s390x/css.c

> > > +++ b/hw/s390x/css.c

> > > @@ -1290,9 +1290,15 @@ void copy_scsw_to_guest(SCSW *dest, const SCSW *src)

> > >  static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)

> > >  {

> > >      int i;

> > > +    PMCW srcpmcw, destpmcw;

> > > +    SCSW srcscsw, destscsw;  

> >

> > <bikeshed>

> > I would find src_pmcw etc. easier to read. Other opinions?

> > </bikeshed>  

> 

> CODING_STYLE's "Naming" section agrees with you...


Do you plan to send a v2, or should I just rename and apply?
Peter Maydell Dec. 12, 2018, 9:15 p.m. | #5
On Wed, 12 Dec 2018 at 17:34, Cornelia Huck <cohuck@redhat.com> wrote:
>

> On Mon, 10 Dec 2018 14:23:15 +0000

> Peter Maydell <peter.maydell@linaro.org> wrote:

>

> > On Mon, 10 Dec 2018 at 14:13, Cornelia Huck <cohuck@redhat.com> wrote:

> > >

> > > On Mon, 10 Dec 2018 13:58:03 +0000

> > > Peter Maydell <peter.maydell@linaro.org> wrote:

> > >

> > > > Taking the address of a field in a packed struct is a bad idea, because

> > > > it might not be actually aligned enough for that pointer type (and

> > > > thus cause a crash on dereference on some host architectures). Newer

> > > > versions of clang warn about this.

> > > >

> > > > Avoid the problem by using local copies of the PMCW and SCSW

> > > > struct fields in copy_schib_from_guest() and copy_schib_to_guest().

> > > >

> > > > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

> > > > ---

> > > > This seemed like a not totally ugly and reasonably localised fix

> > > > that satisfies clang. Oddly, this makes the generated object file

> > > > 15K smaller (421K vs 406K), so it might even be better code...

> > >

> > > Nice :)

> > >

> > > >

> > > >  hw/s390x/css.c | 20 ++++++++++++++++----

> > > >  1 file changed, 16 insertions(+), 4 deletions(-)

> > > >

> > > > diff --git a/hw/s390x/css.c b/hw/s390x/css.c

> > > > index 04ec5cc9705..ef07691e36b 100644

> > > > --- a/hw/s390x/css.c

> > > > +++ b/hw/s390x/css.c

> > > > @@ -1290,9 +1290,15 @@ void copy_scsw_to_guest(SCSW *dest, const SCSW *src)

> > > >  static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)

> > > >  {

> > > >      int i;

> > > > +    PMCW srcpmcw, destpmcw;

> > > > +    SCSW srcscsw, destscsw;

> > >

> > > <bikeshed>

> > > I would find src_pmcw etc. easier to read. Other opinions?

> > > </bikeshed>

> >

> > CODING_STYLE's "Naming" section agrees with you...

>

> Do you plan to send a v2, or should I just rename and apply?


If you want to rename and apply that would be great; I can
send a v2 if that's easier for you.

thanks
-- PMM
Thomas Huth Dec. 13, 2018, 6:32 a.m. | #6
On 2018-12-10 14:58, Peter Maydell wrote:
> Taking the address of a field in a packed struct is a bad idea, because

> it might not be actually aligned enough for that pointer type (and

> thus cause a crash on dereference on some host architectures). Newer

> versions of clang warn about this.

> 

> Avoid the problem by using local copies of the PMCW and SCSW

> struct fields in copy_schib_from_guest() and copy_schib_to_guest().

> 

> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

> ---

> This seemed like a not totally ugly and reasonably localised fix

> that satisfies clang. Oddly, this makes the generated object file

> 15K smaller (421K vs 406K), so it might even be better code...

> 

>  hw/s390x/css.c | 20 ++++++++++++++++----

>  1 file changed, 16 insertions(+), 4 deletions(-)

> 

> diff --git a/hw/s390x/css.c b/hw/s390x/css.c

> index 04ec5cc9705..ef07691e36b 100644

> --- a/hw/s390x/css.c

> +++ b/hw/s390x/css.c

> @@ -1290,9 +1290,15 @@ void copy_scsw_to_guest(SCSW *dest, const SCSW *src)

>  static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)

>  {

>      int i;

> +    PMCW srcpmcw, destpmcw;

> +    SCSW srcscsw, destscsw;

>  

> -    copy_pmcw_to_guest(&dest->pmcw, &src->pmcw);

> -    copy_scsw_to_guest(&dest->scsw, &src->scsw);

> +    srcpmcw = src->pmcw;

> +    copy_pmcw_to_guest(&destpmcw, &srcpmcw);

> +    dest->pmcw = destpmcw;

> +    srcscsw = src->scsw;

> +    copy_scsw_to_guest(&destscsw, &srcscsw);

> +    dest->scsw = destscsw;

>      dest->mba = cpu_to_be64(src->mba);

>      for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {

>          dest->mda[i] = src->mda[i];

> @@ -1339,9 +1345,15 @@ static void copy_scsw_from_guest(SCSW *dest, const SCSW *src)

>  static void copy_schib_from_guest(SCHIB *dest, const SCHIB *src)

>  {

>      int i;

> +    PMCW srcpmcw, destpmcw;

> +    SCSW srcscsw, destscsw;

>  

> -    copy_pmcw_from_guest(&dest->pmcw, &src->pmcw);

> -    copy_scsw_from_guest(&dest->scsw, &src->scsw);

> +    srcpmcw = src->pmcw;

> +    copy_pmcw_from_guest(&destpmcw, &srcpmcw);

> +    dest->pmcw = destpmcw;

> +    srcscsw = src->scsw;

> +    copy_scsw_from_guest(&destscsw, &srcscsw);

> +    dest->scsw = destscsw;

>      dest->mba = be64_to_cpu(src->mba);

>      for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {

>          dest->mda[i] = src->mda[i];

> 


May I suggest to add a comment to the code here a la:

  /* Use a local copy to avoid unaligned access to packed structs */

or something similar? Otherwise, I'm pretty sure somebody will revert
this in a couple of years because they thinks the local copy is not
really necessary here...

 Thomas
Cornelia Huck Dec. 13, 2018, 8:21 a.m. | #7
On Wed, 12 Dec 2018 21:15:29 +0000
Peter Maydell <peter.maydell@linaro.org> wrote:

> On Wed, 12 Dec 2018 at 17:34, Cornelia Huck <cohuck@redhat.com> wrote:

> >

> > On Mon, 10 Dec 2018 14:23:15 +0000

> > Peter Maydell <peter.maydell@linaro.org> wrote:

> >  

> > > On Mon, 10 Dec 2018 at 14:13, Cornelia Huck <cohuck@redhat.com> wrote:  

> > > >

> > > > On Mon, 10 Dec 2018 13:58:03 +0000

> > > > Peter Maydell <peter.maydell@linaro.org> wrote:

> > > >  

> > > > > Taking the address of a field in a packed struct is a bad idea, because

> > > > > it might not be actually aligned enough for that pointer type (and

> > > > > thus cause a crash on dereference on some host architectures). Newer

> > > > > versions of clang warn about this.

> > > > >

> > > > > Avoid the problem by using local copies of the PMCW and SCSW

> > > > > struct fields in copy_schib_from_guest() and copy_schib_to_guest().

> > > > >

> > > > > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

> > > > > ---

> > > > > This seemed like a not totally ugly and reasonably localised fix

> > > > > that satisfies clang. Oddly, this makes the generated object file

> > > > > 15K smaller (421K vs 406K), so it might even be better code...  

> > > >

> > > > Nice :)

> > > >  

> > > > >

> > > > >  hw/s390x/css.c | 20 ++++++++++++++++----

> > > > >  1 file changed, 16 insertions(+), 4 deletions(-)

> > > > >

> > > > > diff --git a/hw/s390x/css.c b/hw/s390x/css.c

> > > > > index 04ec5cc9705..ef07691e36b 100644

> > > > > --- a/hw/s390x/css.c

> > > > > +++ b/hw/s390x/css.c

> > > > > @@ -1290,9 +1290,15 @@ void copy_scsw_to_guest(SCSW *dest, const SCSW *src)

> > > > >  static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)

> > > > >  {

> > > > >      int i;

> > > > > +    PMCW srcpmcw, destpmcw;

> > > > > +    SCSW srcscsw, destscsw;  

> > > >

> > > > <bikeshed>

> > > > I would find src_pmcw etc. easier to read. Other opinions?

> > > > </bikeshed>  

> > >

> > > CODING_STYLE's "Naming" section agrees with you...  

> >

> > Do you plan to send a v2, or should I just rename and apply?  

> 

> If you want to rename and apply that would be great; I can

> send a v2 if that's easier for you.

> 

> thanks

> -- PMM


Given that Thomas also wanted a comment added, a v2 would be easier for
me.

Patch

diff --git a/hw/s390x/css.c b/hw/s390x/css.c
index 04ec5cc9705..ef07691e36b 100644
--- a/hw/s390x/css.c
+++ b/hw/s390x/css.c
@@ -1290,9 +1290,15 @@  void copy_scsw_to_guest(SCSW *dest, const SCSW *src)
 static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)
 {
     int i;
+    PMCW srcpmcw, destpmcw;
+    SCSW srcscsw, destscsw;
 
-    copy_pmcw_to_guest(&dest->pmcw, &src->pmcw);
-    copy_scsw_to_guest(&dest->scsw, &src->scsw);
+    srcpmcw = src->pmcw;
+    copy_pmcw_to_guest(&destpmcw, &srcpmcw);
+    dest->pmcw = destpmcw;
+    srcscsw = src->scsw;
+    copy_scsw_to_guest(&destscsw, &srcscsw);
+    dest->scsw = destscsw;
     dest->mba = cpu_to_be64(src->mba);
     for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {
         dest->mda[i] = src->mda[i];
@@ -1339,9 +1345,15 @@  static void copy_scsw_from_guest(SCSW *dest, const SCSW *src)
 static void copy_schib_from_guest(SCHIB *dest, const SCHIB *src)
 {
     int i;
+    PMCW srcpmcw, destpmcw;
+    SCSW srcscsw, destscsw;
 
-    copy_pmcw_from_guest(&dest->pmcw, &src->pmcw);
-    copy_scsw_from_guest(&dest->scsw, &src->scsw);
+    srcpmcw = src->pmcw;
+    copy_pmcw_from_guest(&destpmcw, &srcpmcw);
+    dest->pmcw = destpmcw;
+    srcscsw = src->scsw;
+    copy_scsw_from_guest(&destscsw, &srcscsw);
+    dest->scsw = destscsw;
     dest->mba = be64_to_cpu(src->mba);
     for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {
         dest->mda[i] = src->mda[i];