Add support for OCI seccomp filters in the sandbox.

OCI configuration includes support for specifying seccomp filters. In runc,
these filter configurations are converted into seccomp BPF programs and loaded
into the kernel via libseccomp. runsc needs to be a static binary so, for
runsc, we cannot rely on a C library and need to implement the functionality
in Go.

The generator added here implements basic support for taking OCI seccomp
configuration and converting it into a seccomp BPF program with the same
behavior as a program generated by libseccomp.

- New conditional operations were added to pkg/seccomp to support operations
  available in OCI.
- AllowAny and AllowValue were renamed to MatchAny and EqualTo to better reflect
  that syscalls matching the conditionals result in the provided action not
  simply SCMP_RET_ALLOW.
- BuildProgram in pkg/seccomp no longer panics if provided an empty list of
  rules. It now builds a program with the architecture sanity check only.
- ProgramBuilder now allows adding labels that are unused. However, backwards
  jumps are still not permitted.

Fixes #510

PiperOrigin-RevId: 331938697

1 files changed, 18 insertions, 5 deletions

diff --git a/pkg/abi/linux/seccomp.go b/pkg/abi/linux/seccomp.go
index d0607e256..b07cafe12 100644
--- a/pkg/abi/linux/seccomp.go
+++ b/pkg/abi/linux/seccomp.go
@@ -34,11 +34,11 @@ type BPFAction uint32
 
 const (
 	SECCOMP_RET_KILL_PROCESS BPFAction = 0x80000000
-	SECCOMP_RET_KILL_THREAD            = 0x00000000
-	SECCOMP_RET_TRAP                   = 0x00030000
-	SECCOMP_RET_ERRNO                  = 0x00050000
-	SECCOMP_RET_TRACE                  = 0x7ff00000
-	SECCOMP_RET_ALLOW                  = 0x7fff0000
+	SECCOMP_RET_KILL_THREAD  BPFAction = 0x00000000
+	SECCOMP_RET_TRAP         BPFAction = 0x00030000
+	SECCOMP_RET_ERRNO        BPFAction = 0x00050000
+	SECCOMP_RET_TRACE        BPFAction = 0x7ff00000
+	SECCOMP_RET_ALLOW        BPFAction = 0x7fff0000
 )
 
 func (a BPFAction) String() string {
@@ -64,6 +64,19 @@ func (a BPFAction) Data() uint16 {
 	return uint16(a & SECCOMP_RET_DATA)
 }
 
+// WithReturnCode sets the lower 16 bits of the SECCOMP_RET_ERRNO or
+// SECCOMP_RET_TRACE actions to the provided return code, overwriting the previous
+// action, and returns a new BPFAction. If not SECCOMP_RET_ERRNO or
+// SECCOMP_RET_TRACE then this panics.
+func (a BPFAction) WithReturnCode(code uint16) BPFAction {
+	// mask out the previous return value
+	baseAction := a & SECCOMP_RET_ACTION_FULL
+	if baseAction == SECCOMP_RET_ERRNO || baseAction == SECCOMP_RET_TRACE {
+		return BPFAction(uint32(baseAction) | uint32(code))
+	}
+	panic("WithReturnCode only valid for SECCOMP_RET_ERRNO and SECCOMP_RET_TRACE")
+}
+
 // SockFprog is sock_fprog taken from <linux/filter.h>.
 type SockFprog struct {
 	Len    uint16