Add support to libcap for overriding system call functions.

Note, this override only supports the system calls that
libcap uses to change kernel state associated with the
current process. This is primarily intended to permit the
user to use libpsx to force all pthreads to mirror capability
and other security relevant state.

Use a weak function definition feature of libpsx share_psx_syscall()
to transparently arrange for libcap to so force itself to use the
psx_syscall() abstraction when linked against -lpsx. This has the
effect of using linker magic to make libcap transparently observe
POSIX semantics for security state setting operations. That is, when
linked as follows:

   gcc .... -lcap -lpsx -lpthread -Wl,-wrap,pthread_create

all pthreads maintain a common security state with respect to the
libcap API.

This also adds full capability setting support to the Go package
libcap/cap via a libcap/psx package which uses cgo+libpsx syscalls
that share capabilities over all pthreads including those of the
Go runtime.

Finally, if Go supports syscall.PosixSyscall() etc. then provide
a non-psx mechanism for libcap/cap to "just work" in all Go code.

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>

2 files changed, 61 insertions, 0 deletions

diff --git a/psx/psx.go b/psx/psx.go
new file mode 100644
index 0000000..e90297d
--- /dev/null
+++ b/psx/psx.go
@@ -0,0 +1,37 @@
+// Package psx provides Go wrappers for two system call functions that
+// work by calling the C libpsx functions of these names.
+package psx
+
+import (
+	"syscall"
+)
+
+// #cgo LDFLAGS: -lpsx -lpthread -Wl,-wrap,pthread_create
+//
+// #include <errno.h>
+// #include <sys/psx_syscall.h>
+//
+// long __errno_too() { return errno ; }
+import "C"
+
+// Syscall3 performs a 3 argument syscall using the libpsx C function
+// psx_syscall3().
+func Syscall3(syscallnr, arg1, arg2, arg3 uintptr) (uintptr, uintptr, syscall.Errno) {
+	v := C.psx_syscall3(C.long(syscallnr), C.long(arg1), C.long(arg2), C.long(arg3))
+	var errno syscall.Errno
+	if v < 0 {
+		errno = syscall.Errno(C.__errno_too())
+	}
+	return uintptr(v), uintptr(v), errno
+}
+
+// Syscall6 performs a 6 argument syscall using the libpsx C function
+// psx_syscall6()
+func Syscall6(syscallnr, arg1, arg2, arg3, arg4, arg5, arg6 uintptr) (uintptr, uintptr, syscall.Errno) {
+	v := C.psx_syscall6(C.long(syscallnr), C.long(arg1), C.long(arg2), C.long(arg3), C.long(arg4), C.long(arg5), C.long(arg6))
+	var errno syscall.Errno
+	if v < 0 {
+		errno = syscall.Errno(C.__errno_too())
+	}
+	return uintptr(v), uintptr(v), errno
+}
diff --git a/psx/psx_test.go b/psx/psx_test.go
new file mode 100644
index 0000000..8dd67ca
--- /dev/null
+++ b/psx/psx_test.go
@@ -0,0 +1,24 @@
+package psx
+
+import (
+	"syscall"
+	"testing"
+)
+
+func TestSyscall3(t *testing.T) {
+	want := syscall.Getpid()
+	if got, _, err := Syscall3(syscall.SYS_GETPID, 0, 0, 0); err != 0 {
+		t.Errorf("failed to get PID via libpsx: %v", err)
+	} else if int(got) != want {
+		t.Errorf("pid mismatch: got=%d want=%d", got, want)
+	}
+}
+
+func TestSyscall6(t *testing.T) {
+	want := syscall.Getpid()
+	if got, _, err := Syscall6(syscall.SYS_GETPID, 0, 0, 0, 0, 0, 0); err != 0 {
+		t.Errorf("failed to get PID via libpsx: %v", err)
+	} else if int(got) != want {
+		t.Errorf("pid mismatch: got=%d want=%d", got, want)
+	}
+}