no-pad: add toplevel -p option to disable frame padding

CMakeLists.txt

@@ -96,6 +96,7 @@ add_executable(ef-tests
     test/ef-tests.cxx
     test/test-ef-parse-bytes.cxx
     test/ifh-ignore.cxx
+    test/test-padding.cxx
 )
 
 target_link_libraries(ef-tests libef)

src/ef-args.c

@@ -90,6 +90,8 @@ void print_help() {
     po("Options:\n");
     po("  -v                    Print version.\n");
     po("  -h                    Top level help message.\n");
+    po("  -p                    No pad. Skip padding frames to 60 bytes,\n");
+    po("     allowing runt frames to be sent or matched as-is.\n");
     po("  -t <timeout-in-ms>    When listening on an interface (rx),\n");
     po("     When listening on an interface (rx), the tool will always\n");
     po("     listen during the entire timeout period. This is needed,\n");
@@ -320,13 +322,18 @@ int argc_cmds(int argc, const char *argv[]) {
     return res;
 }
 
+int NO_PAD = 0;
 int TIME_OUT_MS = 100;
 
 int main_(int argc, const char *argv[]) {
     int opt;
 
-    while ((opt = getopt(argc, (char * const*)argv, "vht:c:")) != -1) {
+    while ((opt = getopt(argc, (char * const*)argv, "pvht:c:")) != -1) {
         switch (opt) {
+            case 'p':
+                NO_PAD = 1;
+                break;
+
             case 'v':
                 print_version();
                 return 0;

src/ef.c

@@ -477,7 +477,7 @@ buf_t *frame_to_buf(frame_t *f) {
         frame_size += f->stack[i]->size;
     }
 
-    if (frame_size < 60)
+    if (frame_size < 60 && !NO_PAD)
         frame_size = 60;
 
     buf = balloc(frame_size);
@@ -503,7 +503,7 @@ buf_t *frame_mask_to_buf(frame_t *f) {
     }
 
     frame_size_no_padding = frame_size;
-    if (frame_size < 60)
+    if (frame_size < 60 && !NO_PAD)
         frame_size = 60;
 
     buf = balloc(frame_size);

src/ef.h

@@ -16,6 +16,7 @@ extern "C" {
 #define DIV_ROUND(a, b) (1 + ((a - 1) / b))
 #define BIT_TO_BYTE(x) (DIV_ROUND(x, 8))
 
+extern int NO_PAD;
 extern int TIME_OUT_MS;
 
 ///////////////////////////////////////////////////////////////////////////////

test/test-padding.cxx

@@ -0,0 +1,98 @@
+#include "ef.h"
+#include "ef-test.h"
+#include "catch_single_include.hxx"
+
+#include <cstring>
+
+// RAII guard to set NO_PAD for the duration of a scope
+struct NoPadGuard {
+    NoPadGuard()  { NO_PAD = 1; }
+    ~NoPadGuard() { NO_PAD = 0; }
+};
+
+// Build a frame and its expected buf/mask from a frame spec
+static void build_frame(std::vector<const char *> spec,
+                        buf_t **out_buf, buf_t **out_mask,
+                        frame_t **out_frame)
+{
+    frame_t *f = parse_frame_wrap(spec);
+    REQUIRE(f != NULL);
+    *out_buf = frame_to_buf(f);
+    REQUIRE(*out_buf != NULL);
+    *out_mask = f->has_mask ? frame_mask_to_buf(f) : NULL;
+    *out_frame = f;
+}
+
+TEST_CASE("no-pad: eth-only frame is 14 bytes", "[nopad]") {
+    NoPadGuard g;
+    buf_t *buf, *mask;
+    frame_t *f;
+    build_frame({"eth", "dmac", "::1", "smac", "::2"}, &buf, &mask, &f);
+
+    // eth header = 14 bytes, no padding to 60
+    CHECK(buf->size == 14);
+
+    bfree(buf);
+    frame_free(f);
+}
+
+TEST_CASE("no-pad: default pads to 60 bytes", "[nopad]") {
+    CHECK(NO_PAD == 0);
+    buf_t *buf, *mask;
+    frame_t *f;
+    build_frame({"eth", "dmac", "::1", "smac", "::2"}, &buf, &mask, &f);
+
+    CHECK(buf->size == 60);
+
+    bfree(buf);
+    frame_free(f);
+}
+
+TEST_CASE("no-pad: mask buf also skips padding", "[nopad]") {
+    NoPadGuard g;
+
+    // Use 'ign' on smac to force mask generation
+    frame_t *f = parse_frame_wrap({"eth", "dmac", "::1", "smac", "ign"});
+    REQUIRE(f != NULL);
+    buf_t *buf = frame_to_buf(f);
+    buf_t *mask = frame_mask_to_buf(f);
+    REQUIRE(buf != NULL);
+    REQUIRE(mask != NULL);
+
+    CHECK(buf->size == 14);
+    CHECK(mask->size == 14);
+
+    bfree(buf);
+    bfree(mask);
+    frame_free(f);
+}
+
+TEST_CASE("no-pad: frame with payload stays exact size", "[nopad]") {
+    NoPadGuard g;
+    buf_t *buf, *mask;
+    frame_t *f;
+
+    // eth(14) + data pattern cnt 4 = 18 bytes, well under 60
+    build_frame({"eth", "dmac", "::1", "smac", "::2",
+                 "data", "pattern", "cnt", "4"}, &buf, &mask, &f);
+
+    CHECK(buf->size == 18);
+
+    bfree(buf);
+    frame_free(f);
+}
+
+TEST_CASE("no-pad: large frame unaffected", "[nopad]") {
+    NoPadGuard g;
+    buf_t *buf, *mask;
+    frame_t *f;
+
+    // eth(14) + data pattern cnt 100 = 114 bytes, already > 60
+    build_frame({"eth", "dmac", "::1", "smac", "::2",
+                 "data", "pattern", "cnt", "100"}, &buf, &mask, &f);
+
+    CHECK(buf->size == 114);
+
+    bfree(buf);
+    frame_free(f);
+}