bpfloader: add ability to disable btfloader

BTF support was added to bpfloader during Android T dev cycle.

As it causes bpfloader boot time process to shell out to a new
btfloader subprocess for every bpf.o file with BTF debugging
information compiled in, I'm worried this might have unforeseen
consequences - things like crashes or boot time bpfloader cpu
regressions.

However, BTF is exceedingly useful for debugging,
and it would be a huge pity if we were forced to disable it
and keep it disabled in mainline tethering module just to support
Android T devices for the next 5+ years.

It would also be a pity if the bpf.o files in prebuilt mainline
tethering apex in dev branches (like tm-dev) did not include
BTF debug information simply due to compatibility with older
kernels or OSes, since this would require rebuilding the module
everytime BTF information could be of use.

One of the things functioning BTF enables is 'cat /sys/fs/bpf/map_*'
as root on a userdebug build.  Among other things this can be
used to verify that in kernel bpf map state matches that dumped
by the mainline module's pretty dump code.

Even if there's issues wrt. BTF in Android T (or on older kernels),
we can always fix them in Android U or later (for example build
btfloader into bpfloader to avoid exec overhead, etc...).

Bug: 218408035
Bug: 230585250
Bug: 235559605
Test: TreeHugger, cuttlefish devices boots, and:
  adb root && adb shell cat /sys/fs/bpf/map_time_in_state_cpu_last_pid_map
  continues to show information which is available due to BTF debug provided
  in /system/etc/bpf/time_in_state.o file (due to "btf: true" in Android.bp)
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I553e90e0414453f8f3aaca8cf05d5decc8b911a2

1 files changed, 6 insertions, 3 deletions

diff --git a/libbpf_android/Loader.cpp b/libbpf_android/Loader.cpp
index 47cf4c9..1c462dd 100644
--- a/libbpf_android/Loader.cpp
+++ b/libbpf_android/Loader.cpp
@@ -724,13 +724,16 @@ static int createMaps(const char* elfPath, ifstream& elfFile, vector<unique_fd>&
     ret = getSectionSymNames(elfFile, "maps", mapNames);
     if (ret) return ret;
 
+    unsigned btfMinBpfLoaderVer = readSectionUint("btf_min_bpfloader_ver", elfFile, 0);
+    unsigned btfMinKernelVer = readSectionUint("btf_min_kernel_ver", elfFile, 0);
+    unsigned kvers = kernelVersion();
+
     std::optional<unique_fd> btfFd;
-    if (!readSectionByName(".BTF", elfFile, btfData)) {
+    if ((BPFLOADER_VERSION >= btfMinBpfLoaderVer) && (kvers >= btfMinKernelVer) &&
+        (!readSectionByName(".BTF", elfFile, btfData))) {
         btfFd = getMapBtfInfo(elfPath, btfTypeIdMap);
     }
 
-    unsigned kvers = kernelVersion();
-
     for (int i = 0; i < (int)mapNames.size(); i++) {
         if (BPFLOADER_VERSION < md[i].bpfloader_min_ver) {
             ALOGI("skipping map %s which requires bpfloader min ver 0x%05x", mapNames[i].c_str(),