runtime: msanread segfaults when called without a g

[bcmills]

When building with -buildmode=c-shared, runtime.rt_sigaction ends up calling msanread on its arguments during libpreinit (before the runtime has set up a g).

Unfortunately, msanread assumes that it is always called with a valid g, leading to a segfault.

src/msanshared/msanshared.go:

package main

import "C"

func main() {}

src/msanshared/main.c:

int main() {
  return 0;
}

$ go version
go version devel +c1730ae424 Wed Jan 18 15:40:33 2017 +0000 linux/amd64
$ go env CC
clang-3.8
$ go env GOGCCFLAGS
-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build181672777=/tmp/go-build -gno-record-gcc-switches
$ go build -msan -buildmode=c-shared -o libmsanshared.so src/msanshared/msanshared.go
$ $(go env CC) $(go env GOGCCFLAGS) -fsanitize=memory -o msanshared src/msanshared/main.c libmsanshared.so
$ LD_LIBRARY_PATH=. ./msanshared
Segmentation fault (core dumped)

[bradfitz]

No mention of the priority of whether this is a regression from Go 1.7, so I'm going to assume Go 1.9 for now, since Go 1.8rc2 is probably tomorrow.

[bradfitz]

/cc @ianlancetaylor

[bcmills]

It is a regression. Tests which were (at worst) mildly flaky on 1.7 are failing consistently at startup on the 1.8 RC.

[bcmills]

Not sure why gobot didn't update the comment, but https://go-review.googlesource.com/#/c/35452/ contains a test and a fix.