powerpc/64/signal: Balance return predictor stack in signal trampoline

Returning from an interrupt or syscall to a signal handler currently
begins execution directly at the handler's entry point, with LR set to
the address of the sigreturn trampoline. When the signal handler
function returns, it runs the trampoline. It looks like this:

    # interrupt at user address xyz
    # kernel stuff... signal is raised
    rfid
    # void handler(int sig)
    addis 2,12,.TOC.-.LCF0@ha
    addi 2,2,.TOC.-.LCF0@l
    mflr 0
    std 0,16(1)
    stdu 1,-96(1)
    # handler stuff
    ld 0,16(1)
    mtlr 0
    blr
    # __kernel_sigtramp_rt64
    addi    r1,r1,__SIGNAL_FRAMESIZE
    li      r0,__NR_rt_sigreturn
    sc
    # kernel executes rt_sigreturn
    rfid
    # back to user address xyz

Note the blr with no matching bl. This can corrupt the return
predictor.

Solve this by instead resuming execution at the signal trampoline
which then calls the signal handler. qtrace-tools link_stack checker
confirms the entire user/kernel/vdso cycle is balanced after this
patch, whereas it's not upstream.

Alan confirms the dwarf unwind info still looks good. gdb still
recognises the signal frame and can step into parent frames if it
break inside a signal handler.

Performance is pretty noisy, not a very significant change on a POWER9
here, but branch misses are consistently a lot lower on a
microbenchmark:

 Performance counter stats for './signal':

       13,085.72 msec task-clock                #    1.000 CPUs utilized
  45,024,760,101      cycles                    #    3.441 GHz
  65,102,895,542      instructions              #    1.45  insn per cycle
  11,271,673,787      branches                  #  861.372 M/sec
      59,468,979      branch-misses             #    0.53% of all branches

       12,989.09 msec task-clock                #    1.000 CPUs utilized
  44,692,719,559      cycles                    #    3.441 GHz
  65,109,984,964      instructions              #    1.46  insn per cycle
  11,282,136,057      branches                  #  868.585 M/sec
      39,786,942      branch-misses             #    0.35% of all branches

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200511101952.1463138-1-npiggin@gmail.com

1 files changed, 12 insertions, 10 deletions

diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index 55e5f76554da..97729642f955 100644
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -39,8 +39,8 @@
 #define GP_REGS_SIZE	min(sizeof(elf_gregset_t), sizeof(struct pt_regs))
 #define FP_REGS_SIZE	sizeof(elf_fpregset_t)
 
-#define TRAMP_TRACEBACK	3
-#define TRAMP_SIZE	6
+#define TRAMP_TRACEBACK	4
+#define TRAMP_SIZE	7
 
 /*
  * When we have signals to deliver, we set up on the user stack,
@@ -600,13 +600,15 @@ static long setup_trampoline(unsigned int syscall, unsigned int __user *tramp)
 	int i;
 	long err = 0;
 
+	/* bctrl # call the handler */
+	err |= __put_user(PPC_INST_BCTRL, &tramp[0]);
 	/* addi r1, r1, __SIGNAL_FRAMESIZE  # Pop the dummy stackframe */
 	err |= __put_user(PPC_INST_ADDI | __PPC_RT(R1) | __PPC_RA(R1) |
-			  (__SIGNAL_FRAMESIZE & 0xffff), &tramp[0]);
+			  (__SIGNAL_FRAMESIZE & 0xffff), &tramp[1]);
 	/* li r0, __NR_[rt_]sigreturn| */
-	err |= __put_user(PPC_INST_ADDI | (syscall & 0xffff), &tramp[1]);
+	err |= __put_user(PPC_INST_ADDI | (syscall & 0xffff), &tramp[2]);
 	/* sc */
-	err |= __put_user(PPC_INST_SC, &tramp[2]);
+	err |= __put_user(PPC_INST_SC, &tramp[3]);
 
 	/* Minimal traceback info */
 	for (i=TRAMP_TRACEBACK; i < TRAMP_SIZE ;i++)
@@ -864,12 +866,12 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
 
 	/* Set up to return from userspace. */
 	if (vdso64_rt_sigtramp && tsk->mm->context.vdso_base) {
-		regs->link = tsk->mm->context.vdso_base + vdso64_rt_sigtramp;
+		regs->nip = tsk->mm->context.vdso_base + vdso64_rt_sigtramp;
 	} else {
 		err |= setup_trampoline(__NR_rt_sigreturn, &frame->tramp[0]);
 		if (err)
 			goto badframe;
-		regs->link = (unsigned long) &frame->tramp[0];
+		regs->nip = (unsigned long) &frame->tramp[0];
 	}
 
 	/* Allocate a dummy caller frame for the signal handler. */
@@ -878,8 +880,8 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
 
 	/* Set up "regs" so we "return" to the signal handler. */
 	if (is_elf2_task()) {
-		regs->nip = (unsigned long) ksig->ka.sa.sa_handler;
-		regs->gpr[12] = regs->nip;
+		regs->ctr = (unsigned long) ksig->ka.sa.sa_handler;
+		regs->gpr[12] = regs->ctr;
 	} else {
 		/* Handler is *really* a pointer to the function descriptor for
 		 * the signal routine.  The first entry in the function
@@ -889,7 +891,7 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
 		func_descr_t __user *funct_desc_ptr =
 			(func_descr_t __user *) ksig->ka.sa.sa_handler;
 
-		err |= get_user(regs->nip, &funct_desc_ptr->entry);
+		err |= get_user(regs->ctr, &funct_desc_ptr->entry);
 		err |= get_user(regs->gpr[2], &funct_desc_ptr->toc);
 	}