[RFC PATCH] powerpc/64/signal: balance return predictor stack in signal trampoline
Nicholas Piggin
npiggin at gmail.com
Fri Apr 17 19:17:47 AEST 2020
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 at ha
addi 2,2,.TOC.-.LCF0 at 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.
I don't know much about dwarf, gdb still seems to recognize the signal
frame and unwind properly if I break inside a signal handler.
qtrace-tools link_stack checker confirms the entire user/kernel/vdso
cycle is balanced after this patch, whereas it's not upstream
Performance seems to be in the noise on my old POWER9, but I don't
quite know where it's at with spectre mitigations.
---
arch/powerpc/include/asm/ppc-opcode.h | 1 +
arch/powerpc/kernel/signal_64.c | 20 +++++++++++---------
arch/powerpc/kernel/vdso64/sigtramp.S | 13 +++++--------
3 files changed, 17 insertions(+), 17 deletions(-)
diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h
index c1df75edde44..747b37f1ce09 100644
--- a/arch/powerpc/include/asm/ppc-opcode.h
+++ b/arch/powerpc/include/asm/ppc-opcode.h
@@ -329,6 +329,7 @@
#define PPC_INST_BLR 0x4e800020
#define PPC_INST_BLRL 0x4e800021
#define PPC_INST_BCTR 0x4e800420
+#define PPC_INST_BCTRL 0x4e800421
#define PPC_INST_MULLD 0x7c0001d2
#define PPC_INST_MULLW 0x7c0001d6
#define PPC_INST_MULHWU 0x7c000016
diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index adfde59cf4ba..6c17e2456ccc 100644
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -41,7 +41,7 @@
#define FP_REGS_SIZE sizeof(elf_fpregset_t)
#define TRAMP_TRACEBACK 3
-#define TRAMP_SIZE 6
+#define TRAMP_SIZE 7
/*
* When we have signals to deliver, we set up on the user stack,
@@ -603,13 +603,15 @@ 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++)
@@ -867,12 +869,12 @@ 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. */
@@ -881,8 +883,8 @@ 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
@@ -892,7 +894,7 @@ 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);
}
diff --git a/arch/powerpc/kernel/vdso64/sigtramp.S b/arch/powerpc/kernel/vdso64/sigtramp.S
index a8cc0409d7d2..bbf68cd01088 100644
--- a/arch/powerpc/kernel/vdso64/sigtramp.S
+++ b/arch/powerpc/kernel/vdso64/sigtramp.S
@@ -6,6 +6,7 @@
* Copyright (C) 2004 Benjamin Herrenschmuidt (benh at kernel.crashing.org), IBM Corp.
* Copyright (C) 2004 Alan Modra (amodra at au.ibm.com)), IBM Corp.
*/
+#include <asm/cache.h> /* IFETCH_ALIGN_BYTES */
#include <asm/processor.h>
#include <asm/ppc_asm.h>
#include <asm/unistd.h>
@@ -14,21 +15,17 @@
.text
-/* The nop here is a hack. The dwarf2 unwind routines subtract 1 from
- the return address to get an address in the middle of the presumed
- call instruction. Since we don't have a call here, we artificially
- extend the range covered by the unwind info by padding before the
- real start. */
- nop
.balign 8
+ .balign IFETCH_ALIGN_BYTES
V_FUNCTION_BEGIN(__kernel_sigtramp_rt64)
-.Lsigrt_start = . - 4
+.Lsigrt_start:
+ bctrl /* call the handler */
addi r1, r1, __SIGNAL_FRAMESIZE
li r0,__NR_rt_sigreturn
sc
.Lsigrt_end:
V_FUNCTION_END(__kernel_sigtramp_rt64)
-/* The ".balign 8" above and the following zeros mimic the old stack
+/* The .balign 8 above and the following zeros mimic the old stack
trampoline layout. The last magic value is the ucontext pointer,
chosen in such a way that older libgcc unwind code returns a zero
for a sigcontext pointer. */
--
2.23.0
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