implement support for functions which may optionally fail to load

This is useful for critical functions with fallbacks, but
may even be useful for non-critical functions, where a function
in the middle of the file may fail to load, but you still want
other (later) functions to be attempted.

Critical applies to the entire .c file (or to be more correct to
the entire resulting .o).  Optional applies to a specific section
of that .o (ie. a specific individual function).

This new optional attribute is necessary to be able to declare
a .c/.o file critical even if *some* of the individual functions
might fail to load due to missing kernel patches.

(Note: we currently have no way to specify a map as optional)

Critical guarantees that all non-optional programs, and all maps,
have been created, pinned, chowned, and chmoded successfully
(or that they already existed).

For an example of use see:
  system/netd/bpf_progs/offload.c

(while at it also add retrieveProgram() and mapRetrieve{RW,RO,WO}()
 helpers to BpfUtils.h)

Test: builds, atest, see paired netd change for extra details
Bug: 150040815
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I50b292c061b05fc8f4b4b8574f128345c45c78db

1 files changed, 12 insertions, 5 deletions

diff --git a/libbpf_android/Loader.cpp b/libbpf_android/Loader.cpp
index 8f5318d..5a65c1b 100644
--- a/libbpf_android/Loader.cpp
+++ b/libbpf_android/Loader.cpp
@@ -564,8 +564,9 @@ static int loadCodeSections(const char* elfPath, vector<codeSection>& cs, const
         progPinLoc += '_';
         progPinLoc += name;
         if (access(progPinLoc.c_str(), F_OK) == 0) {
-            fd = bpf_obj_get(progPinLoc.c_str());
-            ALOGD("New bpf prog load reusing prog %s, ret: %d\n", progPinLoc.c_str(), fd);
+            fd = retrieveProgram(progPinLoc.c_str());
+            ALOGD("New bpf prog load reusing prog %s, ret: %d (%s)\n", progPinLoc.c_str(), fd,
+                  (fd < 0 ? std::strerror(errno) : "no error"));
             reuse = true;
         } else {
             vector<char> log_buf(BPF_LOAD_LOG_SZ, 0);
@@ -579,9 +580,15 @@ static int loadCodeSections(const char* elfPath, vector<codeSection>& cs, const
             if (fd < 0) {
                 std::vector<std::string> lines = android::base::Split(log_buf.data(), "\n");
 
-                ALOGE("bpf_prog_load - BEGIN log_buf contents:");
-                for (const auto& line : lines) ALOGE("%s", line.c_str());
-                ALOGE("bpf_prog_load - END log_buf contents.");
+                ALOGW("bpf_prog_load - BEGIN log_buf contents:");
+                for (const auto& line : lines) ALOGW("%s", line.c_str());
+                ALOGW("bpf_prog_load - END log_buf contents.");
+
+                if (cs[i].prog_def->optional) {
+                    ALOGW("failed program is marked optional - continuing...");
+                    continue;
+                }
+                ALOGE("non-optional program failed to load.");
             }
         }