[PATCH 3/4] openrisc: Support floating point user api

[PATCH 3/4] openrisc: Support floating point user api

[Stafford Horne]

Add support for handling floating point exceptions and forwarding the
SIGFPE signal to processes.  Also, add fpu state to sigcontext.

Signed-off-by: Stafford Horne <shorne@gmail.com>
---
 arch/openrisc/include/uapi/asm/elf.h        |  3 +--
 arch/openrisc/include/uapi/asm/ptrace.h     |  4 ++++
 arch/openrisc/include/uapi/asm/sigcontext.h |  1 +
 arch/openrisc/kernel/entry.S                | 11 +++++++++--
 arch/openrisc/kernel/head.S                 |  4 ++--
 arch/openrisc/kernel/signal.c               |  2 ++
 arch/openrisc/kernel/traps.c                | 22 +++++++++++++++++++++
 7 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/arch/openrisc/include/uapi/asm/elf.h b/arch/openrisc/include/uapi/asm/elf.h
index e892d5061685..6868f81c281e 100644
--- a/arch/openrisc/include/uapi/asm/elf.h
+++ b/arch/openrisc/include/uapi/asm/elf.h
@@ -53,8 +53,7 @@ typedef unsigned long elf_greg_t;
 #define ELF_NGREG (sizeof(struct user_regs_struct) / sizeof(elf_greg_t))
 typedef elf_greg_t elf_gregset_t[ELF_NGREG];
 
-/* A placeholder; OR32 does not have fp support yes, so no fp regs for now.  */
-typedef unsigned long elf_fpregset_t;
+typedef struct __or1k_fpu_state elf_fpregset_t;
 
 /* EM_OPENRISC is defined in linux/elf-em.h */
 #define EM_OR32         0x8472
diff --git a/arch/openrisc/include/uapi/asm/ptrace.h b/arch/openrisc/include/uapi/asm/ptrace.h
index d4fab268f6aa..a77cc9915ca8 100644
--- a/arch/openrisc/include/uapi/asm/ptrace.h
+++ b/arch/openrisc/include/uapi/asm/ptrace.h
@@ -30,6 +30,10 @@ struct user_regs_struct {
 	unsigned long pc;
 	unsigned long sr;
 };
+
+struct __or1k_fpu_state {
+	unsigned long fpcsr;
+};
 #endif
 
 
diff --git a/arch/openrisc/include/uapi/asm/sigcontext.h b/arch/openrisc/include/uapi/asm/sigcontext.h
index 8ab775fc3450..ca585e4af6b8 100644
--- a/arch/openrisc/include/uapi/asm/sigcontext.h
+++ b/arch/openrisc/include/uapi/asm/sigcontext.h
@@ -28,6 +28,7 @@
 
 struct sigcontext {
 	struct user_regs_struct regs;  /* needs to be first */
+	struct __or1k_fpu_state fpu;
 	unsigned long oldmask;
 };
 
diff --git a/arch/openrisc/kernel/entry.S b/arch/openrisc/kernel/entry.S
index c7b47e571220..c9f48e750b72 100644
--- a/arch/openrisc/kernel/entry.S
+++ b/arch/openrisc/kernel/entry.S
@@ -848,9 +848,16 @@ _syscall_badsys:
 
 /******* END SYSCALL HANDLING *******/
 
-/* ---[ 0xd00: Trap exception ]------------------------------------------ */
+/* ---[ 0xd00: Floating Point exception ]-------------------------------- */
 
-UNHANDLED_EXCEPTION(_vector_0xd00,0xd00)
+EXCEPTION_ENTRY(_fpe_trap_handler)
+	CLEAR_LWA_FLAG(r3)
+	/* r4: EA of fault (set by EXCEPTION_HANDLE) */
+	l.jal   do_fpe_trap
+	 l.addi  r3,r1,0 /* pt_regs */
+
+	l.j     _ret_from_exception
+	 l.nop
 
 /* ---[ 0xe00: Trap exception ]------------------------------------------ */
 
diff --git a/arch/openrisc/kernel/head.S b/arch/openrisc/kernel/head.S
index e11699f3d6bd..439e00f81e5d 100644
--- a/arch/openrisc/kernel/head.S
+++ b/arch/openrisc/kernel/head.S
@@ -424,9 +424,9 @@ _dispatch_do_ipage_fault:
     .org 0xc00
 	EXCEPTION_HANDLE(_sys_call_handler)
 
-/* ---[ 0xd00: Trap exception ]------------------------------------------ */
+/* ---[ 0xd00: Floating point exception ]--------------------------------- */
     .org 0xd00
-	UNHANDLED_EXCEPTION(_vector_0xd00)
+	EXCEPTION_HANDLE(_fpe_trap_handler)
 
 /* ---[ 0xe00: Trap exception ]------------------------------------------ */
     .org 0xe00
diff --git a/arch/openrisc/kernel/signal.c b/arch/openrisc/kernel/signal.c
index 80f69740c731..4664a18f0787 100644
--- a/arch/openrisc/kernel/signal.c
+++ b/arch/openrisc/kernel/signal.c
@@ -50,6 +50,7 @@ static int restore_sigcontext(struct pt_regs *regs,
 	err |= __copy_from_user(regs, sc->regs.gpr, 32 * sizeof(unsigned long));
 	err |= __copy_from_user(&regs->pc, &sc->regs.pc, sizeof(unsigned long));
 	err |= __copy_from_user(&regs->sr, &sc->regs.sr, sizeof(unsigned long));
+	err |= __copy_from_user(&regs->fpcsr, &sc->fpu.fpcsr, sizeof(unsigned long));
 
 	/* make sure the SM-bit is cleared so user-mode cannot fool us */
 	regs->sr &= ~SPR_SR_SM;
@@ -112,6 +113,7 @@ static int setup_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
 	err |= __copy_to_user(sc->regs.gpr, regs, 32 * sizeof(unsigned long));
 	err |= __copy_to_user(&sc->regs.pc, &regs->pc, sizeof(unsigned long));
 	err |= __copy_to_user(&sc->regs.sr, &regs->sr, sizeof(unsigned long));
+	err |= __copy_to_user(&sc->fpu.fpcsr, &regs->fpcsr, sizeof(unsigned long));
 
 	return err;
 }
diff --git a/arch/openrisc/kernel/traps.c b/arch/openrisc/kernel/traps.c
index f5bbe6b55849..0aa6b07efda1 100644
--- a/arch/openrisc/kernel/traps.c
+++ b/arch/openrisc/kernel/traps.c
@@ -243,6 +243,28 @@ asmlinkage void unhandled_exception(struct pt_regs *regs, int ea, int vector)
 	die("Oops", regs, 9);
 }
 
+asmlinkage void do_fpe_trap(struct pt_regs *regs, unsigned long address)
+{
+	int code = FPE_FLTUNK;
+	unsigned long fpcsr = regs->fpcsr;
+
+	if (fpcsr & SPR_FPCSR_IVF)
+		code = FPE_FLTINV;
+	else if (fpcsr & SPR_FPCSR_OVF)
+		code = FPE_FLTOVF;
+	else if (fpcsr & SPR_FPCSR_UNF)
+		code = FPE_FLTUND;
+	else if (fpcsr & SPR_FPCSR_DZF)
+		code = FPE_FLTDIV;
+	else if (fpcsr & SPR_FPCSR_IXF)
+		code = FPE_FLTRES;
+
+	/* Clear all flags */
+	regs->fpcsr &= ~SPR_FPCSR_ALLF;
+
+	force_sig_fault(SIGFPE, code, (void __user *)regs->pc);
+}
+
 asmlinkage void do_trap(struct pt_regs *regs, unsigned long address)
 {
 	force_sig_fault(SIGTRAP, TRAP_BRKPT, (void __user *)regs->pc);
-- 
2.39.1

Re: [PATCH 3/4] openrisc: Support floating point user api

[Szabolcs Nagy]

* Stafford Horne <shorne@gmail.com> [2023-04-18 17:58:12 +0100]:
> Add support for handling floating point exceptions and forwarding the
> SIGFPE signal to processes.  Also, add fpu state to sigcontext.
> 
> Signed-off-by: Stafford Horne <shorne@gmail.com>
> ---
...
> --- a/arch/openrisc/include/uapi/asm/sigcontext.h
> +++ b/arch/openrisc/include/uapi/asm/sigcontext.h
> @@ -28,6 +28,7 @@
>  
>  struct sigcontext {
>  	struct user_regs_struct regs;  /* needs to be first */
> +	struct __or1k_fpu_state fpu;
>  	unsigned long oldmask;
>  };

this seems to break userspace abi.
glibc and musl have or1k abi without this field.

either this is a new abi where binaries opt-in with some marking
and then the base sigcontext should be unmodified,

or the fp state needs to be added to the signal frame in a way that
does not break existing abi (e.g. end of the struct ?) and also
advertise the new thing via a hwcap, otherwise userspace cannot
make use of it.

unless i'm missing something.

Re: [PATCH 3/4] openrisc: Support floating point user api

[Stafford Horne]

On Mon, Jun 26, 2023 at 11:38:40PM +0200, Szabolcs Nagy wrote:
> * Stafford Horne <shorne@gmail.com> [2023-04-18 17:58:12 +0100]:
> > Add support for handling floating point exceptions and forwarding the
> > SIGFPE signal to processes.  Also, add fpu state to sigcontext.
> > 
> > Signed-off-by: Stafford Horne <shorne@gmail.com>
> > ---
> ...
> > --- a/arch/openrisc/include/uapi/asm/sigcontext.h
> > +++ b/arch/openrisc/include/uapi/asm/sigcontext.h
> > @@ -28,6 +28,7 @@
> >  
> >  struct sigcontext {
> >  	struct user_regs_struct regs;  /* needs to be first */
> > +	struct __or1k_fpu_state fpu;
> >  	unsigned long oldmask;
> >  };
> 
> this seems to break userspace abi.
> glibc and musl have or1k abi without this field.
> 
> either this is a new abi where binaries opt-in with some marking
> and then the base sigcontext should be unmodified,
> 
> or the fp state needs to be added to the signal frame in a way that
> does not break existing abi (e.g. end of the struct ?) and also
> advertise the new thing via a hwcap, otherwise userspace cannot
> make use of it.
> 
> unless i'm missing something.

I think you are right, I meant to look into this but it must have slipped
though.  Is this something causing you issues or did you just notice it?

I didn't run into issues when running the glibc test suite, but I may have
missed it.

Just moving this to the end of the sigcontext may be all that is needed.

-Stafford

Re: [PATCH 3/4] openrisc: Support floating point user api

[Szabolcs Nagy]

* Stafford Horne <shorne@gmail.com> [2023-06-27 17:41:03 +0100]:
> On Mon, Jun 26, 2023 at 11:38:40PM +0200, Szabolcs Nagy wrote:
> > * Stafford Horne <shorne@gmail.com> [2023-04-18 17:58:12 +0100]:
> > > Add support for handling floating point exceptions and forwarding the
> > > SIGFPE signal to processes.  Also, add fpu state to sigcontext.
> > > 
> > > Signed-off-by: Stafford Horne <shorne@gmail.com>
> > > ---
> > ...
> > > --- a/arch/openrisc/include/uapi/asm/sigcontext.h
> > > +++ b/arch/openrisc/include/uapi/asm/sigcontext.h
> > > @@ -28,6 +28,7 @@
> > >  
> > >  struct sigcontext {
> > >  	struct user_regs_struct regs;  /* needs to be first */
> > > +	struct __or1k_fpu_state fpu;
> > >  	unsigned long oldmask;
> > >  };
> > 
> > this seems to break userspace abi.
> > glibc and musl have or1k abi without this field.
> > 
> > either this is a new abi where binaries opt-in with some marking
> > and then the base sigcontext should be unmodified,
> > 
> > or the fp state needs to be added to the signal frame in a way that
> > does not break existing abi (e.g. end of the struct ?) and also
> > advertise the new thing via a hwcap, otherwise userspace cannot
> > make use of it.
> > 
> > unless i'm missing something.
> 
> I think you are right, I meant to look into this but it must have slipped
> though.  Is this something causing you issues or did you just notice it?

i noticed it while trying to update musl headers to linux 6.4 uapi.

> I didn't run into issues when running the glibc test suite, but I may have
> missed it.

i would only expect issues when accessing ucontext entries
after uc_mcontext.regs in a signal handler registered with
SA_SIGINFO.

in particular uc_sigmask is after uc_mcontext on or1k and e.g.
musl thread cancellation uses this entry to affect the mask on
signal return which will not work on a 6.4 kernel (not tested).

i don't think glibc has tests for the ucontext signal abi.

> Just moving this to the end of the sigcontext may be all that is needed.

that won't help since uc_sigmask comes after sigcontext in ucontext.
it has to go to the end of ucontext or outside of ucontext then.

one way to have fpu in sigcontext is

struct sigcontext {
	struct user_regs_struct regs;
	unsigned long oldmask;
	char padding[sizeof(__userspace_sigset_t)];
	struct __or1k_fpu_state fpu;
};

but the kernel still has to interpret the padding in a bwcompat
way. (and if libc wants to expose fpu in its ucontext then it
needs a flag day abi break as the ucontext size is abi.)

(part of the userspace uc_sigmask is unused because sigset_t is
larger than necessary so may be that can be reused but this is
a hack as that's libc owned.)

not sure how important this fpu field is, arm does not seem to
have fpu state in ucontext and armhf works.

there may be other ways, i'm adding Rich (musl maintainer) on cc
in case he has an opinion.

Re: [PATCH 3/4] openrisc: Support floating point user api

[Rich Felker]

On Tue, Jun 27, 2023 at 07:56:38PM +0200, Szabolcs Nagy wrote:
> * Stafford Horne <shorne@gmail.com> [2023-06-27 17:41:03 +0100]:
> > On Mon, Jun 26, 2023 at 11:38:40PM +0200, Szabolcs Nagy wrote:
> > > * Stafford Horne <shorne@gmail.com> [2023-04-18 17:58:12 +0100]:
> > > > Add support for handling floating point exceptions and forwarding the
> > > > SIGFPE signal to processes.  Also, add fpu state to sigcontext.
> > > > 
> > > > Signed-off-by: Stafford Horne <shorne@gmail.com>
> > > > ---
> > > ...
> > > > --- a/arch/openrisc/include/uapi/asm/sigcontext.h
> > > > +++ b/arch/openrisc/include/uapi/asm/sigcontext.h
> > > > @@ -28,6 +28,7 @@
> > > >  
> > > >  struct sigcontext {
> > > >  	struct user_regs_struct regs;  /* needs to be first */
> > > > +	struct __or1k_fpu_state fpu;
> > > >  	unsigned long oldmask;
> > > >  };
> > > 
> > > this seems to break userspace abi.
> > > glibc and musl have or1k abi without this field.
> > > 
> > > either this is a new abi where binaries opt-in with some marking
> > > and then the base sigcontext should be unmodified,
> > > 
> > > or the fp state needs to be added to the signal frame in a way that
> > > does not break existing abi (e.g. end of the struct ?) and also
> > > advertise the new thing via a hwcap, otherwise userspace cannot
> > > make use of it.
> > > 
> > > unless i'm missing something.
> > 
> > I think you are right, I meant to look into this but it must have slipped
> > though.  Is this something causing you issues or did you just notice it?
> 
> i noticed it while trying to update musl headers to linux 6.4 uapi.
> 
> > I didn't run into issues when running the glibc test suite, but I may have
> > missed it.
> 
> i would only expect issues when accessing ucontext entries
> after uc_mcontext.regs in a signal handler registered with
> SA_SIGINFO.
> 
> in particular uc_sigmask is after uc_mcontext on or1k and e.g.
> musl thread cancellation uses this entry to affect the mask on
> signal return which will not work on a 6.4 kernel (not tested).
> 
> i don't think glibc has tests for the ucontext signal abi.
> 
> > Just moving this to the end of the sigcontext may be all that is needed.
> 
> that won't help since uc_sigmask comes after sigcontext in ucontext.
> it has to go to the end of ucontext or outside of ucontext then.
> 
> one way to have fpu in sigcontext is
> 
> struct sigcontext {
> 	struct user_regs_struct regs;
> 	unsigned long oldmask;
> 	char padding[sizeof(__userspace_sigset_t)];
> 	struct __or1k_fpu_state fpu;
> };
> 
> but the kernel still has to interpret the padding in a bwcompat
> way. (and if libc wants to expose fpu in its ucontext then it
> needs a flag day abi break as the ucontext size is abi.)
> 
> (part of the userspace uc_sigmask is unused because sigset_t is
> larger than necessary so may be that can be reused but this is
> a hack as that's libc owned.)
> 
> not sure how important this fpu field is, arm does not seem to
> have fpu state in ucontext and armhf works.
> 
> there may be other ways, i'm adding Rich (musl maintainer) on cc
> in case he has an opinion.

Indeed, mcontext_t cannot be modified because uc_sigmask follows it in
ucontext_t. The only clean solution here is probably to store the
additional data at offsets past

	sizeof(struct sigcontext) + sizeof(sigset_t)

and not expose this at all in the uapi types. Some hwcap flag can
inform userspace that this additional space is present and accessible
if that's needed.

Optionally you could consider exposing this in the uapi headers'
ucontext_t structure; whether it's an API breakage depends on whether
userspace is relying on being able to allocate its own ucontext_t etc.
This would leave the actual userspace headers (provided by libc) free
to decide whether to modify their type or not according to an
assessment of whether it's a breaking change to application linkage.

What's not workable though is the ABI break that shipped in 6.4. It's
a serious violation of "don't break userspace" and makes existing
application binaries just *not work* (cancellation breaks and possibly
corrupts program state). This needs to be reverted.

Rich

Re: [PATCH 3/4] openrisc: Support floating point user api

[Stafford Horne]

On Tue, Jun 27, 2023 at 03:27:19PM -0400, Rich Felker wrote:
> On Tue, Jun 27, 2023 at 07:56:38PM +0200, Szabolcs Nagy wrote:
> > * Stafford Horne <shorne@gmail.com> [2023-06-27 17:41:03 +0100]:
> > > On Mon, Jun 26, 2023 at 11:38:40PM +0200, Szabolcs Nagy wrote:
> > > > * Stafford Horne <shorne@gmail.com> [2023-04-18 17:58:12 +0100]:
> > > > > Add support for handling floating point exceptions and forwarding the
> > > > > SIGFPE signal to processes.  Also, add fpu state to sigcontext.
> > > > > 
> > > > > Signed-off-by: Stafford Horne <shorne@gmail.com>
> > > > > ---
> > > > ...
> > > > > --- a/arch/openrisc/include/uapi/asm/sigcontext.h
> > > > > +++ b/arch/openrisc/include/uapi/asm/sigcontext.h
> > > > > @@ -28,6 +28,7 @@
> > > > >  
> > > > >  struct sigcontext {
> > > > >  	struct user_regs_struct regs;  /* needs to be first */
> > > > > +	struct __or1k_fpu_state fpu;
> > > > >  	unsigned long oldmask;
> > > > >  };
> > > > 
> > > > this seems to break userspace abi.
> > > > glibc and musl have or1k abi without this field.
> > > > 
> > > > either this is a new abi where binaries opt-in with some marking
> > > > and then the base sigcontext should be unmodified,
> > > > 
> > > > or the fp state needs to be added to the signal frame in a way that
> > > > does not break existing abi (e.g. end of the struct ?) and also
> > > > advertise the new thing via a hwcap, otherwise userspace cannot
> > > > make use of it.
> > > > 
> > > > unless i'm missing something.
> > > 
> > > I think you are right, I meant to look into this but it must have slipped
> > > though.  Is this something causing you issues or did you just notice it?
> > 
> > i noticed it while trying to update musl headers to linux 6.4 uapi.
> > 
> > > I didn't run into issues when running the glibc test suite, but I may have
> > > missed it.
> > 
> > i would only expect issues when accessing ucontext entries
> > after uc_mcontext.regs in a signal handler registered with
> > SA_SIGINFO.
> > 
> > in particular uc_sigmask is after uc_mcontext on or1k and e.g.
> > musl thread cancellation uses this entry to affect the mask on
> > signal return which will not work on a 6.4 kernel (not tested).
> > 
> > i don't think glibc has tests for the ucontext signal abi.
> > 
> > > Just moving this to the end of the sigcontext may be all that is needed.
> > 
> > that won't help since uc_sigmask comes after sigcontext in ucontext.
> > it has to go to the end of ucontext or outside of ucontext then.
> > 
> > one way to have fpu in sigcontext is
> > 
> > struct sigcontext {
> > 	struct user_regs_struct regs;
> > 	unsigned long oldmask;
> > 	char padding[sizeof(__userspace_sigset_t)];
> > 	struct __or1k_fpu_state fpu;
> > };
> > 
> > but the kernel still has to interpret the padding in a bwcompat
> > way. (and if libc wants to expose fpu in its ucontext then it
> > needs a flag day abi break as the ucontext size is abi.)
> > 
> > (part of the userspace uc_sigmask is unused because sigset_t is
> > larger than necessary so may be that can be reused but this is
> > a hack as that's libc owned.)
> > 
> > not sure how important this fpu field is, arm does not seem to
> > have fpu state in ucontext and armhf works.
> > 
> > there may be other ways, i'm adding Rich (musl maintainer) on cc
> > in case he has an opinion.
> 
> Indeed, mcontext_t cannot be modified because uc_sigmask follows it in
> ucontext_t. The only clean solution here is probably to store the
> additional data at offsets past
> 
> 	sizeof(struct sigcontext) + sizeof(sigset_t)
> 
> and not expose this at all in the uapi types. Some hwcap flag can
> inform userspace that this additional space is present and accessible
> if that's needed.
> 
> Optionally you could consider exposing this in the uapi headers'
> ucontext_t structure; whether it's an API breakage depends on whether
> userspace is relying on being able to allocate its own ucontext_t etc.
> This would leave the actual userspace headers (provided by libc) free
> to decide whether to modify their type or not according to an
> assessment of whether it's a breaking change to application linkage.
> 
> What's not workable though is the ABI break that shipped in 6.4. It's
> a serious violation of "don't break userspace" and makes existing
> application binaries just *not work* (cancellation breaks and possibly
> corrupts program state). This needs to be reverted.

Hi Szabolcs, Rich,

Let me work on reverting the bits that try to expose fpcsr in sigcontext.  I am
very aware of rules about not breaking userspace, but for some reason this was
completely missed.

I don't think we do have any need to expose this to userspace at the moment so I
prefer to just leave the fpu state out of sigcontext if that is usable.

The fix will take me about a day or two to get tested and sent.

-Stafford

Re: [PATCH 3/4] openrisc: Support floating point user api

[Stafford Horne]

On Tue, Jun 27, 2023 at 03:27:19PM -0400, Rich Felker wrote:
> On Tue, Jun 27, 2023 at 07:56:38PM +0200, Szabolcs Nagy wrote:
> > * Stafford Horne <shorne@gmail.com> [2023-06-27 17:41:03 +0100]:
> > > On Mon, Jun 26, 2023 at 11:38:40PM +0200, Szabolcs Nagy wrote:
> > > > * Stafford Horne <shorne@gmail.com> [2023-04-18 17:58:12 +0100]:
> > > > > Add support for handling floating point exceptions and forwarding the
> > > > > SIGFPE signal to processes.  Also, add fpu state to sigcontext.
> > > > > 
> > > > > Signed-off-by: Stafford Horne <shorne@gmail.com>
> > > > > ---
> > > > ...
> > > > > --- a/arch/openrisc/include/uapi/asm/sigcontext.h
> > > > > +++ b/arch/openrisc/include/uapi/asm/sigcontext.h
> > > > > @@ -28,6 +28,7 @@
> > > > >  
> > > > >  struct sigcontext {
> > > > >  	struct user_regs_struct regs;  /* needs to be first */
> > > > > +	struct __or1k_fpu_state fpu;
> > > > >  	unsigned long oldmask;
> > > > >  };
> > > > 
> > > > this seems to break userspace abi.
> > > > glibc and musl have or1k abi without this field.
> > > > 
> > > > either this is a new abi where binaries opt-in with some marking
> > > > and then the base sigcontext should be unmodified,
> > > > 
> > > > or the fp state needs to be added to the signal frame in a way that
> > > > does not break existing abi (e.g. end of the struct ?) and also
> > > > advertise the new thing via a hwcap, otherwise userspace cannot
> > > > make use of it.
> > > > 
> > > > unless i'm missing something.
> > > 
> > > I think you are right, I meant to look into this but it must have slipped
> > > though.  Is this something causing you issues or did you just notice it?
> > 
> > i noticed it while trying to update musl headers to linux 6.4 uapi.
> > 
> > > I didn't run into issues when running the glibc test suite, but I may have
> > > missed it.
> > 
> > i would only expect issues when accessing ucontext entries
> > after uc_mcontext.regs in a signal handler registered with
> > SA_SIGINFO.
> > 
> > in particular uc_sigmask is after uc_mcontext on or1k and e.g.
> > musl thread cancellation uses this entry to affect the mask on
> > signal return which will not work on a 6.4 kernel (not tested).
> > 
> > i don't think glibc has tests for the ucontext signal abi.
> > 
> > > Just moving this to the end of the sigcontext may be all that is needed.
> > 
> > that won't help since uc_sigmask comes after sigcontext in ucontext.
> > it has to go to the end of ucontext or outside of ucontext then.
> > 
> > one way to have fpu in sigcontext is
> > 
> > struct sigcontext {
> > 	struct user_regs_struct regs;
> > 	unsigned long oldmask;
> > 	char padding[sizeof(__userspace_sigset_t)];
> > 	struct __or1k_fpu_state fpu;
> > };
> > 
> > but the kernel still has to interpret the padding in a bwcompat
> > way. (and if libc wants to expose fpu in its ucontext then it
> > needs a flag day abi break as the ucontext size is abi.)
> > 
> > (part of the userspace uc_sigmask is unused because sigset_t is
> > larger than necessary so may be that can be reused but this is
> > a hack as that's libc owned.)
> > 
> > not sure how important this fpu field is, arm does not seem to
> > have fpu state in ucontext and armhf works.
> > 
> > there may be other ways, i'm adding Rich (musl maintainer) on cc
> > in case he has an opinion.
> 
> Indeed, mcontext_t cannot be modified because uc_sigmask follows it in
> ucontext_t. The only clean solution here is probably to store the
> additional data at offsets past
> 
> 	sizeof(struct sigcontext) + sizeof(sigset_t)
> 
> and not expose this at all in the uapi types. Some hwcap flag can
> inform userspace that this additional space is present and accessible
> if that's needed.
> 
> Optionally you could consider exposing this in the uapi headers'
> ucontext_t structure; whether it's an API breakage depends on whether
> userspace is relying on being able to allocate its own ucontext_t etc.
> This would leave the actual userspace headers (provided by libc) free
> to decide whether to modify their type or not according to an
> assessment of whether it's a breaking change to application linkage.
> 
> What's not workable though is the ABI break that shipped in 6.4. It's
> a serious violation of "don't break userspace" and makes existing
> application binaries just *not work* (cancellation breaks and possibly
> corrupts program state). This needs to be reverted.

Hi Szabolcs, Rich,

My fix for this has now made it into 6.4.5 stable release.  You should be able
to use this release to update musl.

The fix was to use some unused space in sigcontext, for the fpu state.

-Stafford