util/kmod: call unix.Capget directly, without kernel.org/pub/linux/libs/security/libcap/cap

The kernel.org/pub/linux/libs/security/libcap/cap or its dependencies somehow
broke the Android build, bringing in cgo stuff or something.

It was quicker to remove the dependency than debug.

Change-Id: Ib9bf6f81ce199460ed1ac920afc542a1e3549138
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -165,8 +165,6 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         inet.af/netaddr                                              from inet.af/wf+
         inet.af/peercred                                             from tailscale.com/ipn/ipnserver
    W 💣 inet.af/wf                                                   from tailscale.com/wf
-   L 💣 kernel.org/pub/linux/libs/security/libcap/cap                from tailscale.com/util/kmod
-   L    kernel.org/pub/linux/libs/security/libcap/psx                from kernel.org/pub/linux/libs/security/libcap/cap
    L    nhooyr.io/websocket                                          from tailscale.com/derp/derphttp+
    L    nhooyr.io/websocket/internal/errd                            from nhooyr.io/websocket
    L    nhooyr.io/websocket/internal/xsync                           from nhooyr.io/websocket

go.mod

@@ -63,7 +63,6 @@ require (
 	inet.af/netaddr v0.0.0-20211027220019-c74959edd3b6
 	inet.af/peercred v0.0.0-20210906144145-0893ea02156a
 	inet.af/wf v0.0.0-20211204062712-86aaea0a7310
-	kernel.org/pub/linux/libs/security/libcap/cap v1.2.64
 	nhooyr.io/websocket v1.8.7
 	pault.ag/go/modprobe v0.1.2
 )
@@ -262,7 +261,6 @@ require (
 	gopkg.in/yaml.v2 v2.4.0 // indirect
 	gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b // indirect
 	howett.net/plist v1.0.0 // indirect
-	kernel.org/pub/linux/libs/security/libcap/psx v1.2.64 // indirect
 	mvdan.cc/gofumpt v0.2.0 // indirect
 	mvdan.cc/interfacer v0.0.0-20180901003855-c20040233aed // indirect
 	mvdan.cc/lint v0.0.0-20170908181259-adc824a0674b // indirect

go.sum

@@ -1828,10 +1828,6 @@ inet.af/peercred v0.0.0-20210906144145-0893ea02156a h1:qdkS8Q5/i10xU2ArJMKYhVa1D
 inet.af/peercred v0.0.0-20210906144145-0893ea02156a/go.mod h1:FjawnflS/udxX+SvpsMgZfdqx2aykOlkISeAsADi5IU=
 inet.af/wf v0.0.0-20211204062712-86aaea0a7310 h1:0jKHTf+W75kYRyg5bto1UT+r18QmAz2u/5pAs/fx4zo=
 inet.af/wf v0.0.0-20211204062712-86aaea0a7310/go.mod h1:ViGMZRA6+RA318D7GCncrjv5gHUrPYrNDejjU12tikA=
-kernel.org/pub/linux/libs/security/libcap/cap v1.2.64 h1:E1U4GNGSXEdzQUT+mop0iYawCNXDUU46Y8nfodb+ZY0=
-kernel.org/pub/linux/libs/security/libcap/cap v1.2.64/go.mod h1:gtBlgvjXflnxHng9/3bXyXG3XmBYKDt35zu+lNmB+IA=
-kernel.org/pub/linux/libs/security/libcap/psx v1.2.64 h1:zlw/KoDjEObyddpFcvLiuu8frEvyEwVNc62WZQBp68w=
-kernel.org/pub/linux/libs/security/libcap/psx v1.2.64/go.mod h1:+l6Ee2F59XiJ2I6WR5ObpC1utCQJZ/VLsEbQCD8RG24=
 mvdan.cc/gofumpt v0.0.0-20200802201014-ab5a8192947d/go.mod h1:bzrjFmaD6+xqohD3KYP0H2FEuxknnBmyyOxdhLdaIws=
 mvdan.cc/gofumpt v0.0.0-20201129102820-5c11c50e9475/go.mod h1:E4LOcu9JQEtnYXtB1Y51drqh2Qr2Ngk9J3YrRCwcbd0=
 mvdan.cc/gofumpt v0.1.1/go.mod h1:yXG1r1WqZVKWbVRtBWKWX9+CxGYfA51nSomhM0woR48=

util/kmod/kmod.go

@@ -26,7 +26,6 @@ import (
 
 	"go4.org/mem"
 	"golang.org/x/sys/unix"
-	"kernel.org/pub/linux/libs/security/libcap/cap"
 	"pault.ag/go/modprobe"
 	"tailscale.com/util/lineread"
 	"tailscale.com/util/multierr"
@@ -65,16 +64,12 @@ func hasKernelModule(name string) (bool, error) {
 // result of true implies that it may be worth trying to install a module, not
 // that doing so will work.
 func canInstallModule() (bool, error) {
-	caps, err := cap.GetPID(0) // 0 = current process
-	if err == nil {
-		// errors from GetFlag are either due to the receiver being
-		// uninitialized, or the kernel gave junk results, both of which aren't
-		// very meaningful out of context to a user, so this error is mostly
-		// ignored.
-		b, err := caps.GetFlag(cap.Effective, cap.SYS_MODULE)
-		if err == nil {
-			return b, nil
-		}
+	var capData unix.CapUserData
+	if unix.Capget(&unix.CapUserHeader{
+		Version: 0x20080522, // V3 added in Linux 2.6.26
+		Pid:     0,          // current
+	}, &capData) == nil {
+		return capData.Effective&unix.CAP_SYS_MODULE != 0, nil
 	}
 
 	// could not determine a well known result from capabilities, make an