powerpc: ELF2 binaries signal handling

For the ELFv2 ABI, the hander is the entry point, not a function descriptor.
We also need to set up r12, and fortunately the fast_exception_return
exit path restores r12 for us so nothing else is required.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/signal_64.c

@@ -701,12 +701,6 @@ int sys_rt_sigreturn(unsigned long r3, unsigned long r4, unsigned long r5,
 int handle_rt_signal64(int signr, struct k_sigaction *ka, siginfo_t *info,
 		sigset_t *set, struct pt_regs *regs)
 {
-	/* Handler is *really* a pointer to the function descriptor for
-	 * the signal routine.  The first entry in the function
-	 * descriptor is the entry address of signal and the second
-	 * entry is the TOC value we need to use.
-	 */
-	func_descr_t __user *funct_desc_ptr;
 	struct rt_sigframe __user *frame;
 	unsigned long newsp = 0;
 	long err = 0;
@@ -766,19 +760,32 @@ int handle_rt_signal64(int signr, struct k_sigaction *ka, siginfo_t *info,
 			goto badframe;
 		regs->link = (unsigned long) &frame->tramp[0];
 	}
-	funct_desc_ptr = (func_descr_t __user *) ka->sa.sa_handler;
 
 	/* Allocate a dummy caller frame for the signal handler. */
 	newsp = ((unsigned long)frame) - __SIGNAL_FRAMESIZE;
 	err |= put_user(regs->gpr[1], (unsigned long __user *)newsp);
 
 	/* Set up "regs" so we "return" to the signal handler. */
-	err |= get_user(regs->nip, &funct_desc_ptr->entry);
+	if (is_elf2_task()) {
+		regs->nip = (unsigned long) ka->sa.sa_handler;
+		regs->gpr[12] = regs->nip;
+	} else {
+		/* Handler is *really* a pointer to the function descriptor for
+		 * the signal routine.  The first entry in the function
+		 * descriptor is the entry address of signal and the second
+		 * entry is the TOC value we need to use.
+		 */
+		func_descr_t __user *funct_desc_ptr =
+			(func_descr_t __user *) ka->sa.sa_handler;
+
+		err |= get_user(regs->nip, &funct_desc_ptr->entry);
+		err |= get_user(regs->gpr[2], &funct_desc_ptr->toc);
+	}
+
 	/* enter the signal handler in native-endian mode */
 	regs->msr &= ~MSR_LE;
 	regs->msr |= (MSR_KERNEL & MSR_LE);
 	regs->gpr[1] = newsp;
-	err |= get_user(regs->gpr[2], &funct_desc_ptr->toc);
 	regs->gpr[3] = signr;
 	regs->result = 0;
 	if (ka->sa.sa_flags & SA_SIGINFO) {