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, 16 insertions, 0 deletions

diff --git a/src/runtime/cgocall.go b/src/runtime/cgocall.go
index 0b69ff3233..7ab42a0ed0 100644
--- a/src/runtime/cgocall.go
+++ b/src/runtime/cgocall.go
@@ -93,6 +93,22 @@ import (
 // Length must match arg.Max in x_cgo_callers in runtime/cgo/gcc_traceback.c.
 type cgoCallers [32]uintptr
 
+// argset matches runtime/cgo/linux_syscall.c:argset_t
+type argset struct {
+	args   unsafe.Pointer
+	retval uintptr
+}
+
+// wrapper for syscall package to call cgocall for libc (cgo) calls.
+//go:linkname syscall_cgocaller syscall.cgocaller
+//go:nosplit
+//go:uintptrescapes
+func syscall_cgocaller(fn unsafe.Pointer, args ...uintptr) uintptr {
+	as := argset{args: unsafe.Pointer(&args[0])}
+	cgocall(fn, unsafe.Pointer(&as))
+	return as.retval
+}
+
 // Call from Go to C.
 //
 // This must be nosplit because it's used for syscalls on some