From: Austin Clements <austin@google.com>
Date: Thu, 23 Nov 2017 02:58:57 +0000 (-0500)
Subject: runtime: document sigtrampgo better
X-Git-Tag: go1.10beta1~152
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=294963fb7f8983c7f5560dc20ab75c869f30da14;p=gostls13.git

runtime: document sigtrampgo better

Add an explanation of why sigtrampgo is nosplit.

Updates #21314.

Change-Id: I3f5909d2b2c180f9fa74d53df13e501826fd4316
Reviewed-on: https://go-review.googlesource.com/79615
Reviewed-by: Ian Lance Taylor <iant@golang.org>
---

diff --git a/src/runtime/signal_unix.go b/src/runtime/signal_unix.go
index a616d46bac..9dae058f2c 100644
--- a/src/runtime/signal_unix.go
+++ b/src/runtime/signal_unix.go
@@ -275,6 +275,12 @@ func sigpipe() {
 // sigtrampgo is called from the signal handler function, sigtramp,
 // written in assembly code.
 // This is called by the signal handler, and the world may be stopped.
+//
+// It must be nosplit because getg() is still the G that was running
+// (if any) when the signal was delivered, but it's (usually) called
+// on the gsignal stack. Until this switches the G to gsignal, the
+// stack bounds check won't work.
+//
 //go:nosplit
 //go:nowritebarrierrec
 func sigtrampgo(sig uint32, info *siginfo, ctx unsafe.Pointer) {