[RFC PATCH v2] openrisc: Union fpcsr and oldmask in sigcontext to unbreak userspace ABI

[Stafford Horne]

With commit 27267655c531 ("openrisc: Support floating point user api") I
inproperly added an entry to the struct sigcontext which caused an
unwanted to chagne to the userspace ABI.

Since oldmask was never used in OpenRISC we now use it's space for the
floating point fpcsr state.  We do this with a union to restore the ABI
back to the pre kernel v6.4 ABI and keep API compatibility.

This does mean if there is some code somewhere that is setting oldmask
in a userspace sighandler it would end up setting the floating point
register status, but I think it's unlikely as oldmask was never used
before.

Fixes: 27267655c531 ("openrisc: Support floating point user api")
Reported-by: Szabolcs Nagy <nsz@xxxxxxxxxx>
Closes: https://lore.kernel.org/openrisc/20230626213840.GA1236108@xxxxxxxxxx/
Signed-off-by: Stafford Horne <shorne@xxxxxxxxx>
---
Changes since v1:
 - Rather than revert the change, just use the oldmask slot for fpu state.

 arch/openrisc/include/uapi/asm/sigcontext.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/openrisc/include/uapi/asm/sigcontext.h b/arch/openrisc/include/uapi/asm/sigcontext.h
index ca585e4af6b8..93817d7051aa 100644
--- a/arch/openrisc/include/uapi/asm/sigcontext.h
+++ b/arch/openrisc/include/uapi/asm/sigcontext.h
@@ -28,8 +28,10 @@
 
 struct sigcontext {
 	struct user_regs_struct regs;  /* needs to be first */
-	struct __or1k_fpu_state fpu;
-	unsigned long oldmask;
+	union {
+		struct __or1k_fpu_state fpu;
+		unsigned long oldmask;	/* unused */
+	};
 };
 
 #endif /* __ASM_OPENRISC_SIGCONTEXT_H */
-- 
2.39.1