xdp: add breadcrumbs for logging

src/app/fdctl/Local.mk

@@ -5,7 +5,8 @@ ifdef FD_HAS_DOUBLE
 $(call make-lib,fd_fdctl)
 $(call add-objs,main config security utility run keygen monitor/monitor monitor/helper configure/configure configure/large_pages configure/sysctl configure/shmem configure/xdp configure/xdp_leftover configure/ethtool configure/workspace_leftover configure/workspace,fd_fdctl)
 $(call make-bin,fdctl,main1,fd_fdctl fd_frank fd_disco fd_ballet fd_tango fd_util fd_quic solana_validator_fd)
-$(OBJDIR)/bin/fdctl: src/app/fdctl/config/default.toml
+$(OBJDIR)/obj/app/fdctl/configure/xdp.o: src/tango/xdp/fd_xdp_redirect_prog.o
+$(OBJDIR)/obj/app/fdctl/config.o: src/app/fdctl/config/default.toml
 endif
 endif
 endif

src/tango/xdp/fd_xdp_license.h

@@ -0,0 +1 @@
+#define FD_LICENSE "Apache-2.0"

src/tango/xdp/fd_xdp_redirect_prog.c

@@ -19,6 +19,7 @@
 
 #include "../ebpf/fd_ebpf_base.h"
 #include "fd_xdp_redirect_prog.h"
+#include "fd_xdp_license.h"
 
 #include <linux/bpf.h>
 
@@ -30,9 +31,10 @@
 
 /* Metadata ***********************************************************/
 
-char __license[] __attribute__(( section("license") )) = "Apache-2.0";
+char __license[] __attribute__(( section("license") )) = FD_LICENSE;
 
 /* eBPF syscalls ******************************************************/
+/* https://github.com/torvalds/linux/blob/91aa6c412d7f85e48aead7b00a7d9e91f5cf5863/include/uapi/linux/bpf.h#L5577 */
 
 static void *
 (* bpf_map_lookup_elem)( void *       map,
@@ -45,6 +47,31 @@ static long
                       ulong  flags )
   = (void *)51U;
 
+#ifdef FD_XDP_LOGGING
+
+/* To do logging, you should enable this import and then call it like
+
+     char fmt[] = "hello %d";
+     bpf_trace_printk( fmt, sizeof(fmt), 5 );
+
+   Note that BPF logging only supports certain format specifiers and
+   can take at most three arguments. It is extremely fickle and the
+   program may just start randomly failing to load with a verifier
+   error. Move things around and try again.
+
+   Firedancer is not using a GPL compatible license, and BPF printk
+   is a GPL part of BPF. This means that we cannot use it, or ship
+   this code in the binary. The kernel checks this with the "license"
+   section exported above, which you should not change.
+*/
+static long
+(*bpf_trace_printk)( const char * fmt,
+                     uint         fmt_size,
+                     ... )
+  = (void *) 6;
+
+#endif
+
 /* eBPF maps **********************************************************/
 
 /* eBPF maps allows sharing information between the Linux userspace and

src/tango/xdp/fd_xdp_redirect_user.c

@@ -5,6 +5,7 @@
 #define _DEFAULT_SOURCE
 #include "fd_xdp_redirect_user.h"
 #include "fd_xdp_redirect_prog.h"
+#include "fd_xdp_license.h"
 #include "../../ballet/ebpf/fd_ebpf.h"
 #include "../../util/fd_util.h"
 
@@ -328,7 +329,7 @@ fd_xdp_hook_iface( char const * app_name,
     .prog_type = BPF_PROG_TYPE_XDP,
     .insn_cnt  = (uint) ( res->bpf_sz / 8UL ),
     .insns     = (ulong)( res->bpf ),
-    .license   = (ulong)"Apache-2.0",
+    .license   = (ulong)FD_LICENSE,
     /* Verifier logs */
     .log_level = 6,
     .log_size  = EBPF_KERN_LOG_BUFSZ,
@@ -338,9 +339,7 @@ fd_xdp_hook_iface( char const * app_name,
   if( FD_UNLIKELY( prog_fd<0 ) ) {
     FD_LOG_WARNING(( "bpf(BPF_PROG_LOAD, insns=%p, insn_cnt=%lu) failed (%d-%s)",
                      (void *)res->bpf, res->bpf_sz / 8UL, errno, strerror( errno ) ));
-    if( errno==EACCES ) {
-      FD_LOG_NOTICE(( "eBPF verifier log:\n%s", ebpf_kern_log ));
-    }
+    FD_LOG_NOTICE(( "eBPF verifier log:\n%s", ebpf_kern_log ));
     return -1;
   }