From f3af51145be318f914f5a54eff1f6a8593181e16 Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <campbell+golang@mumble.net>
Date: Fri, 3 Dec 2021 19:53:33 +0000
Subject: [PATCH] runtime: Sanity-check that syscalls preserve _lwp_self().

---
 src/runtime/proc.go     | 31 +++++++++++++++++++++++++++++++
 src/runtime/runtime2.go |  1 +
 2 files changed, 32 insertions(+)

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index a238ea77f3..9517485c45 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -3764,6 +3764,8 @@ func reentersyscall(pc, sp uintptr) {
 	// but can have inconsistent g->sched, do not let GC observe it.
 	_g_.m.locks++
 
+	_g_.lwpid = lwp_self()
+
 	// Entersyscall must not call any function that might split/grow the stack.
 	// (See details in comment above.)
 	// Catch calls that might, by replacing the stack guard with something that
@@ -3776,6 +3778,15 @@ func reentersyscall(pc, sp uintptr) {
 	_g_.syscallsp = sp
 	_g_.syscallpc = pc
 	casgstatus(_g_, _Grunning, _Gsyscall)
+	if _g_.lwpid != lwp_self() {
+		gu := uintptr(unsafe.Pointer(_g_))
+		lwpid := _g_.lwpid
+		lwpidself := lwp_self()
+		systemstack(func() {
+			print("entersyscall lwpself inconsistent g=", hex(gu), "(", hex(uintptr(unsafe.Pointer(_g_))), ") lwpid=", lwpid, "(", _g_.lwpid, ", ", lwpidself, ", ", lwp_self(), ")\n")
+			throw("entersyscall lwpself")
+		})
+	}
 	if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
 		systemstack(func() {
 			print("entersyscall inconsistent ", hex(_g_.syscallsp), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
@@ -3860,6 +3871,7 @@ func entersyscallblock() {
 	_g_ := getg()
 
 	_g_.m.locks++ // see comment in entersyscall
+	_g_.lwpid = lwp_self()
 	_g_.throwsplit = true
 	_g_.stackguard0 = stackPreempt // see comment in entersyscall
 	_g_.m.syscalltick = _g_.m.p.ptr().syscalltick
@@ -3882,6 +3894,15 @@ func entersyscallblock() {
 		})
 	}
 	casgstatus(_g_, _Grunning, _Gsyscall)
+	if _g_.lwpid != lwp_self() {
+		gu := uintptr(unsafe.Pointer(_g_))
+		lwpid := _g_.lwpid
+		lwpidself := lwp_self()
+		systemstack(func() {
+			print("entersyscall lwpself inconsistent g=", hex(gu), "(", hex(uintptr(unsafe.Pointer(_g_))), ") lwpid=", lwpid, "(", _g_.lwpid, ", ", lwpidself, ", ", lwp_self(), ")\n")
+			throw("entersyscall lwpself")
+		})
+	}
 	if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
 		systemstack(func() {
 			print("entersyscallblock inconsistent ", hex(sp), " ", hex(_g_.sched.sp), " ", hex(_g_.syscallsp), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
@@ -3924,6 +3945,16 @@ func exitsyscall() {
 	if getcallersp() > _g_.syscallsp {
 		throw("exitsyscall: syscall frame is no longer valid")
 	}
+	if _g_.lwpid != lwp_self() {
+		gu := uintptr(unsafe.Pointer(_g_))
+		lwpid := _g_.lwpid
+		lwpidself := lwp_self()
+		systemstack(func() {
+			print("exitsyscall lwpself inconsistent g=", hex(gu), "(", hex(uintptr(unsafe.Pointer(_g_))), ") lwpid=", lwpid, "(", _g_.lwpid, ", ", lwpidself, ", ", lwp_self(), ")\n")
+			throw("exitsyscall lwpself")
+		})
+	}
+	_g_.lwpid = 0
 
 	_g_.waitsince = 0
 	oldp := _g_.m.oldp.ptr()
diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index bfd857e8d5..39a8e80c3e 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -416,6 +416,7 @@ type g struct {
 	_defer    *_defer // innermost defer
 	m         *m      // current m; offset known to arm liblink
 	sched     gobuf
+	lwpid     int32
 	syscallsp uintptr // if status==Gsyscall, syscallsp = sched.sp to use during gc
 	syscallpc uintptr // if status==Gsyscall, syscallpc = sched.pc to use during gc
 	stktopsp  uintptr // expected sp at top of stack, to check in traceback