[PATCH v4] openrisc: Union fpcsr and oldmask in sigcontext to unbreak userspace ABI

[Stafford Horne]

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

To fix this we use the previously unused oldmask field 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 an OpenRISC specific userspace sighandler it would end up setting the
floating point register status, but I think it's unlikely as oldmask was
never functional 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 v3:
 - Fix typos in commit message
 - Add back change log
Changes since v2:
 - Don't use a struct in sigcontext as hinted by Rich Felker
Changes since v1:
 - Rather than revert the change, just use the oldmask slot for fpu state.

 arch/openrisc/include/uapi/asm/sigcontext.h | 6 ++++--
 arch/openrisc/kernel/signal.c               | 4 ++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/openrisc/include/uapi/asm/sigcontext.h b/arch/openrisc/include/uapi/asm/sigcontext.h
index ca585e4af6b8..e7ffb58ff58f 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 {
+		unsigned long fpcsr;
+		unsigned long oldmask;	/* unused */
+	};
 };
 
 #endif /* __ASM_OPENRISC_SIGCONTEXT_H */
diff --git a/arch/openrisc/kernel/signal.c b/arch/openrisc/kernel/signal.c
index 4664a18f0787..2e7257a433ff 100644
--- a/arch/openrisc/kernel/signal.c
+++ b/arch/openrisc/kernel/signal.c
@@ -50,7 +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));
+	err |= __copy_from_user(&regs->fpcsr, &sc->fpcsr, sizeof(unsigned long));
 
 	/* make sure the SM-bit is cleared so user-mode cannot fool us */
 	regs->sr &= ~SPR_SR_SM;
@@ -113,7 +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));
+	err |= __copy_to_user(&sc->fpcsr, &regs->fpcsr, sizeof(unsigned long));
 
 	return err;
 }
-- 
2.39.1