[PATCH v5 03/10] powerpc/signal64: Move non-inline functions out of setup_sigcontext()

[Christopher M. Riedl]

There are non-inline functions which get called in setup_sigcontext() to
save register state to the thread struct. Move these functions into a
separate prepare_setup_sigcontext() function so that
setup_sigcontext() can be refactored later into an "unsafe" version
which assumes an open uaccess window. Non-inline functions should be
avoided when uaccess is open.

The majority of setup_sigcontext() can be refactored to execute in an
"unsafe" context (uaccess window is opened) except for some non-inline
functions. Move these out into a separate prepare_setup_sigcontext()
function which must be called first and before opening up a uaccess
window. A follow-up commit converts setup_sigcontext() to be "unsafe".

Signed-off-by: Christopher M. Riedl 
---
 arch/powerpc/kernel/signal_64.c | 32 +---
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index f9e4a1ac440f..b211a8ea4f6e 100644
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -79,6 +79,24 @@ static elf_vrreg_t __user *sigcontext_vmx_regs(struct 
sigcontext __user *sc)
 }
 #endif
 
+static void prepare_setup_sigcontext(struct task_struct *tsk, int 
ctx_has_vsx_region)
+{
+#ifdef CONFIG_ALTIVEC
+   /* save altivec registers */
+   if (tsk->thread.used_vr)
+   flush_altivec_to_thread(tsk);
+   if (cpu_has_feature(CPU_FTR_ALTIVEC))
+   tsk->thread.vrsave = mfspr(SPRN_VRSAVE);
+#endif /* CONFIG_ALTIVEC */
+
+   flush_fp_to_thread(tsk);
+
+#ifdef CONFIG_VSX
+   if (tsk->thread.used_vsr && ctx_has_vsx_region)
+   flush_vsx_to_thread(tsk);
+#endif /* CONFIG_VSX */
+}
+
 /*
  * Set up the sigcontext for the signal frame.
  */
@@ -97,7 +115,6 @@ static long setup_sigcontext(struct sigcontext __user *sc,
 */
 #ifdef CONFIG_ALTIVEC
elf_vrreg_t __user *v_regs = sigcontext_vmx_regs(sc);
-   unsigned long vrsave;
 #endif
struct pt_regs *regs = tsk->thread.regs;
unsigned long msr = regs->msr;
@@ -112,7 +129,6 @@ static long setup_sigcontext(struct sigcontext __user *sc,
 
/* save altivec registers */
if (tsk->thread.used_vr) {
-   flush_altivec_to_thread(tsk);
/* Copy 33 vec registers (vr0..31 and vscr) to the stack */
err |= __copy_to_user(v_regs, &tsk->thread.vr_state,
  33 * sizeof(vector128));
@@ -124,17 +140,10 @@ static long setup_sigcontext(struct sigcontext __user *sc,
/* We always copy to/from vrsave, it's 0 if we don't have or don't
 * use altivec.
 */
-   vrsave = 0;
-   if (cpu_has_feature(CPU_FTR_ALTIVEC)) {
-   vrsave = mfspr(SPRN_VRSAVE);
-   tsk->thread.vrsave = vrsave;
-   }
-
-   err |= __put_user(vrsave, (u32 __user *)&v_regs[33]);
+   err |= __put_user(tsk->thread.vrsave, (u32 __user *)&v_regs[33]);
 #else /* CONFIG_ALTIVEC */
err |= __put_user(0, &sc->v_regs);
 #endif /* CONFIG_ALTIVEC */
-   flush_fp_to_thread(tsk);
/* copy fpr regs and fpscr */
err |= copy_fpr_to_user(&sc->fp_regs, tsk);
 
@@ -150,7 +159,6 @@ static long setup_sigcontext(struct sigcontext __user *sc,
 * VMX data.
 */
if (tsk->thread.used_vsr && ctx_has_vsx_region) {
-   flush_vsx_to_thread(tsk);
v_regs += ELF_NVRREG;
err |= copy_vsx_to_user(v_regs, tsk);
/* set MSR_VSX in the MSR value in the frame to
@@ -655,6 +663,7 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, 
old_ctx,
ctx_has_vsx_region = 1;
 
if (old_ctx != NULL) {
+   prepare_setup_sigcontext(current, ctx_has_vsx_region);
if (!access_ok(old_ctx, ctx_size)
|| setup_sigcontext(&old_ctx->uc_mcontext, current, 0, 
NULL, 0,
ctx_has_vsx_region)
@@ -842,6 +851,7 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
 #endif
{
err |= __put_user(0, &frame->uc.uc_link);
+   prepare_setup_sigcontext(tsk, 1);
err |= setup_sigcontext(&frame->uc.uc_mcontext, tsk, ksig->sig,
NULL, (unsigned 
long)ksig->ka.sa.sa_handler,
1);
-- 
2.26.1

Re: [PATCH v5 03/10] powerpc/signal64: Move non-inline functions out of setup_sigcontext()

[Daniel Axtens]

Hi Chris,

These two paragraphs are a little confusing and they seem slightly
repetitive. But I get the general idea. Two specific comments below:

> There are non-inline functions which get called in setup_sigcontext() to
> save register state to the thread struct. Move these functions into a
> separate prepare_setup_sigcontext() function so that
> setup_sigcontext() can be refactored later into an "unsafe" version
> which assumes an open uaccess window. Non-inline functions should be
> avoided when uaccess is open.

Why do we want to avoid non-inline functions? We came up with:

 - we want KUAP protection for as much of the kernel as possible: each
   extra bit of code run with the window open is another piece of attack
   surface.
   
 - non-inline functions default to traceable, which means we could end
   up ftracing while uaccess is enabled. That's a pretty big hole in the
   defences that KUAP provides.

I think we've also had problems with the window being opened or closed
unexpectedly by various bits of code? So the less code runs in uaccess
context the less likely that is to occur.
 
> The majority of setup_sigcontext() can be refactored to execute in an
> "unsafe" context (uaccess window is opened) except for some non-inline
> functions. Move these out into a separate prepare_setup_sigcontext()
> function which must be called first and before opening up a uaccess
> window. A follow-up commit converts setup_sigcontext() to be "unsafe".

This was a bit confusing until we realise that you're moving the _calls_
to the non-inline functions out, not the non-inline functions themselves.

> Signed-off-by: Christopher M. Riedl 
> ---
>  arch/powerpc/kernel/signal_64.c | 32 +---
>  1 file changed, 21 insertions(+), 11 deletions(-)
>
> diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
> index f9e4a1ac440f..b211a8ea4f6e 100644
> --- a/arch/powerpc/kernel/signal_64.c
> +++ b/arch/powerpc/kernel/signal_64.c
> @@ -79,6 +79,24 @@ static elf_vrreg_t __user *sigcontext_vmx_regs(struct 
> sigcontext __user *sc)
>  }
>  #endif
>  
> +static void prepare_setup_sigcontext(struct task_struct *tsk, int 
> ctx_has_vsx_region)

ctx_has_vsx_region should probably be a bool? Although setup_sigcontext
also has it as an int so I guess that's arguable, and maybe it's better
to stick with this for constency.

> +{
> +#ifdef CONFIG_ALTIVEC
> + /* save altivec registers */
> + if (tsk->thread.used_vr)
> + flush_altivec_to_thread(tsk);
> + if (cpu_has_feature(CPU_FTR_ALTIVEC))
> + tsk->thread.vrsave = mfspr(SPRN_VRSAVE);
> +#endif /* CONFIG_ALTIVEC */
> +
> + flush_fp_to_thread(tsk);
> +
> +#ifdef CONFIG_VSX
> + if (tsk->thread.used_vsr && ctx_has_vsx_region)
> + flush_vsx_to_thread(tsk);
> +#endif /* CONFIG_VSX */

Alternatively, given that this is the only use of ctx_has_vsx_region,
mpe suggested that perhaps we could drop it entirely and always
flush_vsx if used_vsr. The function is only ever called with either
`current` or wth ctx_has_vsx_region set to 1, so in either case I think
that's safe? I'm not sure if it would have performance implications.

Should we move this and the altivec ifdef to IS_ENABLED(CONFIG_VSX) etc?
I'm not sure if that runs into any problems with things like 'used_vsr'
only being defined if CONFIG_VSX is set, but I thought I'd ask.


> +}
> +
>  /*
>   * Set up the sigcontext for the signal frame.
>   */
> @@ -97,7 +115,6 @@ static long setup_sigcontext(struct sigcontext __user *sc,
>*/
>  #ifdef CONFIG_ALTIVEC
>   elf_vrreg_t __user *v_regs = sigcontext_vmx_regs(sc);
> - unsigned long vrsave;
>  #endif
>   struct pt_regs *regs = tsk->thread.regs;
>   unsigned long msr = regs->msr;
> @@ -112,7 +129,6 @@ static long setup_sigcontext(struct sigcontext __user *sc,
>  
>   /* save altivec registers */
>   if (tsk->thread.used_vr) {
> - flush_altivec_to_thread(tsk);
>   /* Copy 33 vec registers (vr0..31 and vscr) to the stack */
>   err |= __copy_to_user(v_regs, &tsk->thread.vr_state,
> 33 * sizeof(vector128));
> @@ -124,17 +140,10 @@ static long setup_sigcontext(struct sigcontext __user 
> *sc,
>   /* We always copy to/from vrsave, it's 0 if we don't have or don't
>* use altivec.
>*/
> - vrsave = 0;
> - if (cpu_has_feature(CPU_FTR_ALTIVEC)) {
> - vrsave = mfspr(SPRN_VRSAVE);
> - tsk->thread.vrsave = vrsave;
> - }
> -
> - err |= __put_user(vrsave, (u32 __user *)&v_regs[33]);
> + err |= __put_user(tsk->thread.vrsave, (u32 __user *)&v_regs[33]);

Previously, if !cpu_has_feature(ALTIVEC), v_regs[33] had vrsave stored,
which was set to 0 explicitly. Now we store thread.vrsave instead of the
local vrsave. That should be safe - it is initalised to 0 elsewhere.

So you don't have to do anything here, this is just letting you know
that we c

Re: [PATCH v5 03/10] powerpc/signal64: Move non-inline functions out of setup_sigcontext()

[Christopher M. Riedl]

On Sun Feb 7, 2021 at 10:44 PM CST, Daniel Axtens wrote:
> Hi Chris,
>
> These two paragraphs are a little confusing and they seem slightly
> repetitive. But I get the general idea. Two specific comments below:

Umm... yeah only one of those was supposed to be sent. I will reword
this for the next spin and address the comment below about how it is
not entirely clear that the inline functions are being moved out.

>
> > There are non-inline functions which get called in setup_sigcontext() to
> > save register state to the thread struct. Move these functions into a
> > separate prepare_setup_sigcontext() function so that
> > setup_sigcontext() can be refactored later into an "unsafe" version
> > which assumes an open uaccess window. Non-inline functions should be
> > avoided when uaccess is open.
>
> Why do we want to avoid non-inline functions? We came up with:
>
> - we want KUAP protection for as much of the kernel as possible: each
> extra bit of code run with the window open is another piece of attack
> surface.
>
> - non-inline functions default to traceable, which means we could end
> up ftracing while uaccess is enabled. That's a pretty big hole in the
> defences that KUAP provides.
>
> I think we've also had problems with the window being opened or closed
> unexpectedly by various bits of code? So the less code runs in uaccess
> context the less likely that is to occur.

That is my understanding as well.

>  
> > The majority of setup_sigcontext() can be refactored to execute in an
> > "unsafe" context (uaccess window is opened) except for some non-inline
> > functions. Move these out into a separate prepare_setup_sigcontext()
> > function which must be called first and before opening up a uaccess
> > window. A follow-up commit converts setup_sigcontext() to be "unsafe".
>
> This was a bit confusing until we realise that you're moving the _calls_
> to the non-inline functions out, not the non-inline functions
> themselves.
>
> > Signed-off-by: Christopher M. Riedl 
> > ---
> >  arch/powerpc/kernel/signal_64.c | 32 +---
> >  1 file changed, 21 insertions(+), 11 deletions(-)
> >
> > diff --git a/arch/powerpc/kernel/signal_64.c 
> > b/arch/powerpc/kernel/signal_64.c
> > index f9e4a1ac440f..b211a8ea4f6e 100644
> > --- a/arch/powerpc/kernel/signal_64.c
> > +++ b/arch/powerpc/kernel/signal_64.c
> > @@ -79,6 +79,24 @@ static elf_vrreg_t __user *sigcontext_vmx_regs(struct 
> > sigcontext __user *sc)
> >  }
> >  #endif
> >  
> > +static void prepare_setup_sigcontext(struct task_struct *tsk, int 
> > ctx_has_vsx_region)
>
> ctx_has_vsx_region should probably be a bool? Although setup_sigcontext
> also has it as an int so I guess that's arguable, and maybe it's better
> to stick with this for constency.

I've been told not to introduce unrelated changes in my patches before
so chose to keep this as an int for consistency.

>
> > +{
> > +#ifdef CONFIG_ALTIVEC
> > +   /* save altivec registers */
> > +   if (tsk->thread.used_vr)
> > +   flush_altivec_to_thread(tsk);
> > +   if (cpu_has_feature(CPU_FTR_ALTIVEC))
> > +   tsk->thread.vrsave = mfspr(SPRN_VRSAVE);
> > +#endif /* CONFIG_ALTIVEC */
> > +
> > +   flush_fp_to_thread(tsk);
> > +
> > +#ifdef CONFIG_VSX
> > +   if (tsk->thread.used_vsr && ctx_has_vsx_region)
> > +   flush_vsx_to_thread(tsk);
> > +#endif /* CONFIG_VSX */
>
> Alternatively, given that this is the only use of ctx_has_vsx_region,
> mpe suggested that perhaps we could drop it entirely and always
> flush_vsx if used_vsr. The function is only ever called with either
> `current` or wth ctx_has_vsx_region set to 1, so in either case I think
> that's safe? I'm not sure if it would have performance implications.

I think that could work as long as we can guarantee that the context
passed to swapcontext will always be sufficiently sized if used_vsr,
which I think *has* to be the case?

>
> Should we move this and the altivec ifdef to IS_ENABLED(CONFIG_VSX) etc?
> I'm not sure if that runs into any problems with things like 'used_vsr'
> only being defined if CONFIG_VSX is set, but I thought I'd ask.

That's why I didn't use IS_ENABLED(CONFIG_...) here - all of these
field (used_vr, vrsave, used_vsr) declarations are guarded by #ifdefs :/

>
>
> > +}
> > +
> >  /*
> >   * Set up the sigcontext for the signal frame.
> >   */
> > @@ -97,7 +115,6 @@ static long setup_sigcontext(struct sigcontext __user 
> > *sc,
> >  */
> >  #ifdef CONFIG_ALTIVEC
> > elf_vrreg_t __user *v_regs = sigcontext_vmx_regs(sc);
> > -   unsigned long vrsave;
> >  #endif
> > struct pt_regs *regs = tsk->thread.regs;
> > unsigned long msr = regs->msr;
> > @@ -112,7 +129,6 @@ static long setup_sigcontext(struct sigcontext __user 
> > *sc,
> >  
> > /* save altivec registers */
> > if (tsk->thread.used_vr) {
> > -   flush_altivec_to_thread(tsk);
> > /* Copy 33 vec registers (vr0..31 and vscr) to the stack */
> > err |= __copy_to

Re: [PATCH v5 03/10] powerpc/signal64: Move non-inline functions out of setup_sigcontext()

[Daniel Axtens]

"Christopher M. Riedl"  writes:

> On Sun Feb 7, 2021 at 10:44 PM CST, Daniel Axtens wrote:
>> Hi Chris,
>>
>> These two paragraphs are a little confusing and they seem slightly
>> repetitive. But I get the general idea. Two specific comments below:
>
> Umm... yeah only one of those was supposed to be sent. I will reword
> this for the next spin and address the comment below about how it is
> not entirely clear that the inline functions are being moved out.
>
>>
>> > There are non-inline functions which get called in setup_sigcontext() to
>> > save register state to the thread struct. Move these functions into a
>> > separate prepare_setup_sigcontext() function so that
>> > setup_sigcontext() can be refactored later into an "unsafe" version
>> > which assumes an open uaccess window. Non-inline functions should be
>> > avoided when uaccess is open.
>>
>> Why do we want to avoid non-inline functions? We came up with:
>>
>> - we want KUAP protection for as much of the kernel as possible: each
>> extra bit of code run with the window open is another piece of attack
>> surface.
>>
>> - non-inline functions default to traceable, which means we could end
>> up ftracing while uaccess is enabled. That's a pretty big hole in the
>> defences that KUAP provides.
>>
>> I think we've also had problems with the window being opened or closed
>> unexpectedly by various bits of code? So the less code runs in uaccess
>> context the less likely that is to occur.
>
> That is my understanding as well.
>
>>  
>> > The majority of setup_sigcontext() can be refactored to execute in an
>> > "unsafe" context (uaccess window is opened) except for some non-inline
>> > functions. Move these out into a separate prepare_setup_sigcontext()
>> > function which must be called first and before opening up a uaccess
>> > window. A follow-up commit converts setup_sigcontext() to be "unsafe".
>>
>> This was a bit confusing until we realise that you're moving the _calls_
>> to the non-inline functions out, not the non-inline functions
>> themselves.
>>
>> > Signed-off-by: Christopher M. Riedl 
>> > ---
>> >  arch/powerpc/kernel/signal_64.c | 32 +---
>> >  1 file changed, 21 insertions(+), 11 deletions(-)
>> >
>> > diff --git a/arch/powerpc/kernel/signal_64.c 
>> > b/arch/powerpc/kernel/signal_64.c
>> > index f9e4a1ac440f..b211a8ea4f6e 100644
>> > --- a/arch/powerpc/kernel/signal_64.c
>> > +++ b/arch/powerpc/kernel/signal_64.c
>> > @@ -79,6 +79,24 @@ static elf_vrreg_t __user *sigcontext_vmx_regs(struct 
>> > sigcontext __user *sc)
>> >  }
>> >  #endif
>> >  
>> > +static void prepare_setup_sigcontext(struct task_struct *tsk, int 
>> > ctx_has_vsx_region)
>>
>> ctx_has_vsx_region should probably be a bool? Although setup_sigcontext
>> also has it as an int so I guess that's arguable, and maybe it's better
>> to stick with this for constency.
>
> I've been told not to introduce unrelated changes in my patches before
> so chose to keep this as an int for consistency.

Seems reasonable.

>
>>
>> > +{
>> > +#ifdef CONFIG_ALTIVEC
>> > +  /* save altivec registers */
>> > +  if (tsk->thread.used_vr)
>> > +  flush_altivec_to_thread(tsk);
>> > +  if (cpu_has_feature(CPU_FTR_ALTIVEC))
>> > +  tsk->thread.vrsave = mfspr(SPRN_VRSAVE);
>> > +#endif /* CONFIG_ALTIVEC */
>> > +
>> > +  flush_fp_to_thread(tsk);
>> > +
>> > +#ifdef CONFIG_VSX
>> > +  if (tsk->thread.used_vsr && ctx_has_vsx_region)
>> > +  flush_vsx_to_thread(tsk);
>> > +#endif /* CONFIG_VSX */
>>
>> Alternatively, given that this is the only use of ctx_has_vsx_region,
>> mpe suggested that perhaps we could drop it entirely and always
>> flush_vsx if used_vsr. The function is only ever called with either
>> `current` or wth ctx_has_vsx_region set to 1, so in either case I think
>> that's safe? I'm not sure if it would have performance implications.
>
> I think that could work as long as we can guarantee that the context
> passed to swapcontext will always be sufficiently sized if used_vsr,
> which I think *has* to be the case?

I think you're always guaranteed that you'll have a big enough one
in your kernel thread, which is what you end up writing to, iiuc?

>>
>> Should we move this and the altivec ifdef to IS_ENABLED(CONFIG_VSX) etc?
>> I'm not sure if that runs into any problems with things like 'used_vsr'
>> only being defined if CONFIG_VSX is set, but I thought I'd ask.
>
> That's why I didn't use IS_ENABLED(CONFIG_...) here - all of these
> field (used_vr, vrsave, used_vsr) declarations are guarded by #ifdefs :/

Dang. Oh well.
>
>>
>>
>> > +}
>> > +
>> >  /*
>> >   * Set up the sigcontext for the signal frame.
>> >   */
>> > @@ -97,7 +115,6 @@ static long setup_sigcontext(struct sigcontext __user 
>> > *sc,
>> > */
>> >  #ifdef CONFIG_ALTIVEC
>> >elf_vrreg_t __user *v_regs = sigcontext_vmx_regs(sc);
>> > -  unsigned long vrsave;
>> >  #endif
>> >struct pt_regs *regs = tsk->thread.regs;
>> >unsigned long msr = regs->msr

Re: [PATCH v5 03/10] powerpc/signal64: Move non-inline functions out of setup_sigcontext()

[Christopher M. Riedl]

On Wed Feb 10, 2021 at 3:06 PM CST, Daniel Axtens wrote:
> "Christopher M. Riedl"  writes:
>
> > On Sun Feb 7, 2021 at 10:44 PM CST, Daniel Axtens wrote:
> >> Hi Chris,
> >>
> >> These two paragraphs are a little confusing and they seem slightly
> >> repetitive. But I get the general idea. Two specific comments below:
> >
> > Umm... yeah only one of those was supposed to be sent. I will reword
> > this for the next spin and address the comment below about how it is
> > not entirely clear that the inline functions are being moved out.
> >
> >>
> >> > There are non-inline functions which get called in setup_sigcontext() to
> >> > save register state to the thread struct. Move these functions into a
> >> > separate prepare_setup_sigcontext() function so that
> >> > setup_sigcontext() can be refactored later into an "unsafe" version
> >> > which assumes an open uaccess window. Non-inline functions should be
> >> > avoided when uaccess is open.
> >>
> >> Why do we want to avoid non-inline functions? We came up with:
> >>
> >> - we want KUAP protection for as much of the kernel as possible: each
> >> extra bit of code run with the window open is another piece of attack
> >> surface.
> >>
> >> - non-inline functions default to traceable, which means we could end
> >> up ftracing while uaccess is enabled. That's a pretty big hole in the
> >> defences that KUAP provides.
> >>
> >> I think we've also had problems with the window being opened or closed
> >> unexpectedly by various bits of code? So the less code runs in uaccess
> >> context the less likely that is to occur.
> >
> > That is my understanding as well.
> >
> >>  
> >> > The majority of setup_sigcontext() can be refactored to execute in an
> >> > "unsafe" context (uaccess window is opened) except for some non-inline
> >> > functions. Move these out into a separate prepare_setup_sigcontext()
> >> > function which must be called first and before opening up a uaccess
> >> > window. A follow-up commit converts setup_sigcontext() to be "unsafe".
> >>
> >> This was a bit confusing until we realise that you're moving the _calls_
> >> to the non-inline functions out, not the non-inline functions
> >> themselves.
> >>
> >> > Signed-off-by: Christopher M. Riedl 
> >> > ---
> >> >  arch/powerpc/kernel/signal_64.c | 32 +---
> >> >  1 file changed, 21 insertions(+), 11 deletions(-)
> >> >
> >> > diff --git a/arch/powerpc/kernel/signal_64.c 
> >> > b/arch/powerpc/kernel/signal_64.c
> >> > index f9e4a1ac440f..b211a8ea4f6e 100644
> >> > --- a/arch/powerpc/kernel/signal_64.c
> >> > +++ b/arch/powerpc/kernel/signal_64.c
> >> > @@ -79,6 +79,24 @@ static elf_vrreg_t __user *sigcontext_vmx_regs(struct 
> >> > sigcontext __user *sc)
> >> >  }
> >> >  #endif
> >> >  
> >> > +static void prepare_setup_sigcontext(struct task_struct *tsk, int 
> >> > ctx_has_vsx_region)
> >>
> >> ctx_has_vsx_region should probably be a bool? Although setup_sigcontext
> >> also has it as an int so I guess that's arguable, and maybe it's better
> >> to stick with this for constency.
> >
> > I've been told not to introduce unrelated changes in my patches before
> > so chose to keep this as an int for consistency.
>
> Seems reasonable.
>
> >
> >>
> >> > +{
> >> > +#ifdef CONFIG_ALTIVEC
> >> > +/* save altivec registers */
> >> > +if (tsk->thread.used_vr)
> >> > +flush_altivec_to_thread(tsk);
> >> > +if (cpu_has_feature(CPU_FTR_ALTIVEC))
> >> > +tsk->thread.vrsave = mfspr(SPRN_VRSAVE);
> >> > +#endif /* CONFIG_ALTIVEC */
> >> > +
> >> > +flush_fp_to_thread(tsk);
> >> > +
> >> > +#ifdef CONFIG_VSX
> >> > +if (tsk->thread.used_vsr && ctx_has_vsx_region)
> >> > +flush_vsx_to_thread(tsk);
> >> > +#endif /* CONFIG_VSX */
> >>
> >> Alternatively, given that this is the only use of ctx_has_vsx_region,
> >> mpe suggested that perhaps we could drop it entirely and always
> >> flush_vsx if used_vsr. The function is only ever called with either
> >> `current` or wth ctx_has_vsx_region set to 1, so in either case I think
> >> that's safe? I'm not sure if it would have performance implications.
> >
> > I think that could work as long as we can guarantee that the context
> > passed to swapcontext will always be sufficiently sized if used_vsr,
> > which I think *has* to be the case?
>
> I think you're always guaranteed that you'll have a big enough one
> in your kernel thread, which is what you end up writing to, iiuc?

Ah yup you are correct. I confused myself with the comment in
swapcontext about the ctx_size. We call prepare_setup_sigcontext() with
current which will always have space. The ctx_size only matters on the
next call to setup_sigcontext() which ends up potentially copying the
VSX region to userspace (v_regs).

TL;DR - yes, I'll remove the ctx_has_vsx_region argument to
prepare_setup_sigcontext() with the next version. Thanks!

>
> >>
> >> Should we move this and the altivec ifdef to IS_