[v1] hw/s390/ccw.c: Don't take address of packed members

[Qemu-devel] [PATCH] hw/s390/ccw.c: Don't take address of packed members

Posted by Peter Maydell 5 years, 4 months ago

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];
-- 
2.19.2

Re: [Qemu-devel] [PATCH] hw/s390/ccw.c: Don't take address of packed members

Posted by Cornelia Huck 5 years, 4 months ago

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];

Re: [Qemu-devel] [PATCH] hw/s390/ccw.c: Don't take address of packed members

Posted by Peter Maydell 5 years, 4 months ago

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

Re: [Qemu-devel] [PATCH] hw/s390/ccw.c: Don't take address of packed members

Posted by Cornelia Huck 5 years, 4 months ago

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?

Re: [Qemu-devel] [PATCH] hw/s390/ccw.c: Don't take address of packed members

Posted by Peter Maydell 5 years, 4 months ago

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

Re: [Qemu-devel] [PATCH] hw/s390/ccw.c: Don't take address of packed members

Posted by Cornelia Huck 5 years, 4 months ago

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.

Re: [Qemu-devel] [PATCH] hw/s390/ccw.c: Don't take address of packed members

Posted by Farhan Ali 5 years, 4 months ago


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>

Re: [Qemu-devel] [qemu-s390x] [PATCH] hw/s390/ccw.c: Don't take address of packed members

Posted by Thomas Huth 5 years, 4 months ago

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