backends/ebpf: Output an error if a header is not byte-aligned (p4lan…

…g#4176).

Although P4 allows headers to be an arbitrary number of bits, the
eBPF backend does not support it properly. Specifically, the code
in compileExtractField() assumes that the header starts at a byte
boundary. In any case, there is no realistic use case for headers
which aren't a whole number of bytes. Output an error rather than
fail silently. The BMv2 backend has a similar limitation already.

backends/ebpf/ebpfParser.cpp

@@ -407,7 +407,14 @@ void StateTranslationVisitor::compileExtract(const IR::Expression *destination)
         return;
     }
 
+    // We expect all headers to start on a byte boundary.
     unsigned width = ht->width_bits();
+    if ((width % 8) != 0) {
+        ::error(ErrorType::ERR_UNSUPPORTED_ON_TARGET,
+                "Header %1% size %2% is not a multiple of 8 bits.", destination, width);
+        return;
+    }
+
     auto program = state->parser->program;
 
     cstring offsetStr =

testdata/p4_16_ebpf_errors/header_unaligned.p4

@@ -0,0 +1,28 @@
+#include <ebpf_model.p4>
+#include <core.p4>
+
+header ThreeBit_h {
+    bit<3> x;
+}
+
+struct Headers_t {
+    ThreeBit_h foo;
+    ThreeBit_h bar;
+}
+
+parser prs(packet_in p, out Headers_t headers) {
+    state start {
+        p.extract(headers.foo);
+        p.extract(headers.bar);
+        transition accept;
+    }
+}
+
+control pipe(inout Headers_t headers, out bool pass) {
+
+    apply {
+        pass = true;
+    }
+}
+
+ebpfFilter(prs(), pipe()) main;

testdata/p4_16_ebpf_errors_outputs/header_unaligned.p4-stderr

@@ -0,0 +1,6 @@
+header_unaligned.p4(15): [--Werror=target-error] error: Header headers.foo size 3 is not a multiple of 8 bits.
+        p.extract(headers.foo);
+                  ^^^^^^^^^^^
+header_unaligned.p4(16): [--Werror=target-error] error: Header headers.bar size 3 is not a multiple of 8 bits.
+        p.extract(headers.bar);
+                  ^^^^^^^^^^^