From 16ab7e49d4070c4f68e88836b123dbe6da8bb015 Mon Sep 17 00:00:00 2001
From: Derek Parker <parkerderek86@gmail.com>
Date: Thu, 17 Jun 2021 20:22:40 +0000
Subject: [PATCH] [release-branch.go1.16] runtime: fix crash during VDSO calls on PowerPC
This patch reinstates a fix for PowerPC with regard to making VDSO calls
while receiving a signal, and subsequently crashing. The crash happens
because certain VDSO calls can modify the r30 register, which is where g
is stored. This change was reverted for PowerPC because r30 is supposed
to be a non-volatile register. This is true, but that only makes a
guarantee across function calls, but not "within" a function call. This
patch was seemingly fine before because the Linux kernel still had hand
rolled assembly VDSO function calls, however with a recent change to C
function calls it seems the compiler used can generate instructions
which temporarily clobber r30. This means that when we receive a signal
during one of these calls the value of r30 will not be the g as the
runtime expects, causing a segfault.
You can see from this assembly dump how the register is clobbered during
the call:
(the following is from a 5.13rc2 kernel)
```
Dump of assembler code for function __cvdso_clock_gettime_data:
0x00007ffff7ff0700 <+0>: cmplwi r4,15
0x00007ffff7ff0704 <+4>: bgt 0x7ffff7ff07f0 <__cvdso_clock_gettime_data+240>
0x00007ffff7ff0708 <+8>: li r9,1
0x00007ffff7ff070c <+12>: slw r9,r9,r4
0x00007ffff7ff0710 <+16>: andi. r10,r9,2179
0x00007ffff7ff0714 <+20>: beq 0x7ffff7ff0810 <__cvdso_clock_gettime_data+272>
0x00007ffff7ff0718 <+24>: rldicr r10,r4,4,59
0x00007ffff7ff071c <+28>: lis r9,32767
0x00007ffff7ff0720 <+32>: std r30,-16(r1)
0x00007ffff7ff0724 <+36>: std r31,-8(r1)
0x00007ffff7ff0728 <+40>: add r6,r3,r10
0x00007ffff7ff072c <+44>: ori r4,r9,65535
0x00007ffff7ff0730 <+48>: lwz r8,0(r3)
0x00007ffff7ff0734 <+52>: andi. r9,r8,1
0x00007ffff7ff0738 <+56>: bne 0x7ffff7ff07d0 <__cvdso_clock_gettime_data+208>
0x00007ffff7ff073c <+60>: lwsync
0x00007ffff7ff0740 <+64>: mftb r30 <---- RIGHT HERE
=> 0x00007ffff7ff0744 <+68>: ld r12,40(r6)
```
What I believe is happening is that the kernel changed the PowerPC VDSO
calls to use standard C calls instead of using hand rolled assembly. The
hand rolled assembly calls never touched r30, so this change was safe to
roll back. That does not seem to be the case anymore as on the 5.13rc2
kernel the compiler *is* generating assembly which modifies r30, making
this change again unsafe and causing a crash when the program receives a
signal during these calls (which will happen often due to async
preempt). This change happened here:
https://lwn.net/ml/linux-kernel/235e5571959cfa89ced081d7e838ed5ff38447d2.1601365870.git.christophe.leroy@csgroup.eu/.
I realize this was reverted due to unexplained hangs in PowerPC
builders, but I think we should reinstate this change and investigate
those issues separately:
https://github.com/golang/go/commit/f4ca3c1e0a2066ca4f7bd6203866d282ed34acf2
Fixes #46858
Change-Id: Ib18d7bbfc80a1a9cb558f0098878d41081324b52
GitHub-Last-Rev: c3002bcfca3ef58b27485e31328e6297b7a9dfe7
GitHub-Pull-Request: golang/go#46767
Reviewed-on: https://go-review.googlesource.com/c/go/+/328110
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>
(cherry picked from commit 16e82be454cbf41299e6a055d54d489ca4612ee0)
Reviewed-on: https://go-review.googlesource.com/c/go/+/334410
Run-TryBot: Cherry Mui <cherryyz@google.com>
---
diff --git a/src/runtime/signal_unix.go b/src/runtime/signal_unix.go
index 3f70707..89f936e 100644
--- a/src/runtime/signal_unix.go
+++ b/src/runtime/signal_unix.go
@@ -381,7 +381,7 @@
//go:nosplit
func sigFetchG(c *sigctxt) *g {
switch GOARCH {
- case "arm", "arm64":
+ case "arm", "arm64", "ppc64", "ppc64le":
if !iscgo && inVDSOPage(c.sigpc()) {
// When using cgo, we save the g on TLS and load it from there
// in sigtramp. Just use that.
diff --git a/src/runtime/sys_linux_ppc64x.s b/src/runtime/sys_linux_ppc64x.s
index fd69ee7..7be8c4c 100644
--- a/src/runtime/sys_linux_ppc64x.s
+++ b/src/runtime/sys_linux_ppc64x.s
@@ -215,15 +215,45 @@
MOVD (g_sched+gobuf_sp)(R7), R1 // Set SP to g0 stack
noswitch:
- SUB $16, R1 // Space for results
- RLDICR $0, R1, $59, R1 // Align for C code
+ SUB $16, R1 // Space for results
+ RLDICR $0, R1, $59, R1 // Align for C code
MOVD R12, CTR
MOVD R1, R4
- BL (CTR) // Call from VDSO
- MOVD $0, R0 // Restore R0
- MOVD 0(R1), R3 // sec
- MOVD 8(R1), R5 // nsec
- MOVD R15, R1 // Restore SP
+
+ // Store g on gsignal's stack, so if we receive a signal
+ // during VDSO code we can find the g.
+ // If we don't have a signal stack, we won't receive signal,
+ // so don't bother saving g.
+ // When using cgo, we already saved g on TLS, also don't save
+ // g here.
+ // Also don't save g if we are already on the signal stack.
+ // We won't get a nested signal.
+ MOVBZ runtime·iscgo(SB), R22
+ CMP R22, $0
+ BNE nosaveg
+ MOVD m_gsignal(R21), R22 // g.m.gsignal
+ CMP R22, $0
+ BEQ nosaveg
+
+ CMP g, R22
+ BEQ nosaveg
+ MOVD (g_stack+stack_lo)(R22), R22 // g.m.gsignal.stack.lo
+ MOVD g, (R22)
+
+ BL (CTR) // Call from VDSO
+
+ MOVD $0, (R22) // clear g slot, R22 is unchanged by C code
+
+ JMP finish
+
+nosaveg:
+ BL (CTR) // Call from VDSO
+
+finish:
+ MOVD $0, R0 // Restore R0
+ MOVD 0(R1), R3 // sec
+ MOVD 8(R1), R5 // nsec
+ MOVD R15, R1 // Restore SP
// Restore vdsoPC, vdsoSP
// We don't worry about being signaled between the two stores.
@@ -235,7 +265,7 @@
MOVD 32(R1), R6
MOVD R6, m_vdsoPC(R21)
-finish:
+return:
MOVD R3, sec+0(FP)
MOVW R5, nsec+8(FP)
RET
@@ -246,7 +276,7 @@
SYSCALL $SYS_clock_gettime
MOVD 32(R1), R3
MOVD 40(R1), R5
- JMP finish
+ JMP return
TEXT runtime·nanotime1(SB),NOSPLIT,$16-8
MOVD $1, R3 // CLOCK_MONOTONIC
@@ -282,7 +312,37 @@
RLDICR $0, R1, $59, R1 // Align for C code
MOVD R12, CTR
MOVD R1, R4
- BL (CTR) // Call from VDSO
+
+ // Store g on gsignal's stack, so if we receive a signal
+ // during VDSO code we can find the g.
+ // If we don't have a signal stack, we won't receive signal,
+ // so don't bother saving g.
+ // When using cgo, we already saved g on TLS, also don't save
+ // g here.
+ // Also don't save g if we are already on the signal stack.
+ // We won't get a nested signal.
+ MOVBZ runtime·iscgo(SB), R22
+ CMP R22, $0
+ BNE nosaveg
+ MOVD m_gsignal(R21), R22 // g.m.gsignal
+ CMP R22, $0
+ BEQ nosaveg
+
+ CMP g, R22
+ BEQ nosaveg
+ MOVD (g_stack+stack_lo)(R22), R22 // g.m.gsignal.stack.lo
+ MOVD g, (R22)
+
+ BL (CTR) // Call from VDSO
+
+ MOVD $0, (R22) // clear g slot, R22 is unchanged by C code
+
+ JMP finish
+
+nosaveg:
+ BL (CTR) // Call from VDSO
+
+finish:
MOVD $0, R0 // Restore R0
MOVD 0(R1), R3 // sec
MOVD 8(R1), R5 // nsec
@@ -298,7 +358,7 @@
MOVD 32(R1), R6
MOVD R6, m_vdsoPC(R21)
-finish:
+return:
// sec is in R3, nsec in R5
// return nsec in R3
MOVD $1000000000, R4
@@ -313,7 +373,7 @@
SYSCALL $SYS_clock_gettime
MOVD 32(R1), R3
MOVD 40(R1), R5
- JMP finish
+ JMP return
TEXT runtime·rtsigprocmask(SB),NOSPLIT|NOFRAME,$0-28
MOVW how+0(FP), R3
@@ -366,7 +426,7 @@
// this might be called in external code context,
// where g is not set.
MOVBZ runtime·iscgo(SB), R6
- CMP R6, $0
+ CMP R6, $0
BEQ 2(PC)
BL runtime·load_g(SB)