syscall: support POSIX semantics for Linux syscalls

This change adds two new methods for invoking system calls
under Linux: syscall.AllThreadsSyscall() and
syscall.AllThreadsSyscall6().

These system call wrappers ensure that all OSThreads mirror
a common system call. The wrappers serialize execution of the
runtime to ensure no race conditions where any Go code observes
a non-atomic OS state change. As such, the syscalls have
higher runtime overhead than regular system calls, and only
need to be used where such thread (or 'm' in the parlance
of the runtime sources) consistency is required.

The new support is used to enable these functions under Linux:

  syscall.Setegid(), syscall.Seteuid(), syscall.Setgroups(),
  syscall.Setgid(), syscall.Setregid(), syscall.Setreuid(),
  syscall.Setresgid(), syscall.Setresuid() and syscall.Setuid().

They work identically to their glibc counterparts.

Extensive discussion of the background issue addressed in this
patch can be found here:

   https://github.com/golang/go/issues/1435

In the case where cgo is used, the C runtime can launch pthreads that
are not managed by the Go runtime. As such, the added
syscall.AllThreadsSyscall*() return ENOTSUP when cgo is enabled.
However, for the 9 syscall.Set*() functions listed above, when cgo is
active, these functions redirect to invoke their C.set*() equivalents
in glibc, which wraps the raw system calls with a nptl:setxid fixup
mechanism. This achieves POSIX semantics for these functions in the
combined Go and C runtime.

As a side note, the glibc/nptl:setxid support (2019-11-30) does not
extend to all security related system calls under Linux so using
native Go (CGO_ENABLED=0) and these AllThreadsSyscall*()s, where
needed, will yield more well defined/consistent behavior over all
threads of a Go program. That is, using the
syscall.AllThreadsSyscall*() wrappers for things like setting state
through SYS_PRCTL and SYS_CAPSET etc.

Fixes #1435

Change-Id: Ib1a3e16b9180f64223196a32fc0f9dce14d9105c
Reviewed-on: https://go-review.googlesource.com/c/go/+/210639
Trust: Emmanuel Odeke <emm.odeke@gmail.com>
Trust: Ian Lance Taylor <iant@golang.org>
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 0758a35e01..21dd7b3949 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -528,6 +528,7 @@ type m struct {
 	ncgo          int32       // number of cgo calls currently in progress
 	cgoCallersUse uint32      // if non-zero, cgoCallers in use temporarily
 	cgoCallers    *cgoCallers // cgo traceback if crashing in cgo call
+	doesPark      bool        // non-P running threads: sysmon and newmHandoff never use .park
 	park          note
 	alllink       *m // on allm
 	schedlink     muintptr
@@ -544,6 +545,13 @@ type m struct {
 	syscalltick   uint32
 	freelink      *m // on sched.freem
 
+	// mFixup is used to synchronize OS related m state (credentials etc)
+	// use mutex to access.
+	mFixup struct {
+		lock mutex
+		fn   func(bool) bool
+	}
+
 	// these are here because they are too large to be on the stack
 	// of low-level NOSPLIT functions.
 	libcall   libcall
@@ -768,6 +776,10 @@ type schedt struct {
 	sysmonwait uint32
 	sysmonnote note
 
+	// While true, sysmon not ready for mFixup calls.
+	// Accessed atomically.
+	sysmonStarting uint32
+
 	// safepointFn should be called on each P at the next GC
 	// safepoint if p.runSafePointFn is set.
 	safePointFn   func(*p)