document how to debug bpfloader failures

Test: N/A
Bug: 204131517
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ia6a8ca026f3ab403443e130dceb2aca0ce0f2343

bpfloader/bpfloader.rc

@@ -57,6 +57,30 @@ service bpfloader /system/bin/bpfloader
     #
     rlimit memlock 1073741824 1073741824
     oneshot
+    #
+    # How to debug bootloops caused by 'bpfloader-failed'.
+    #
+    # 1. On some lower RAM devices (like wembley) you may need to first enable developer mode
+    #    (from the Settings app UI), and change the developer option "Logger buffer sizes"
+    #    from the default (wembley: 64kB) to the maximum (1M) per log buffer.
+    #    Otherwise buffer will overflow before you manage to dump it and you'll get useless logs.
+    #
+    # 2. comment out 'reboot_on_failure reboot,bpfloader-failed' below
+    # 3. rebuild/reflash/reboot
+    # 4. as the device is booting up capture bpfloader logs via:
+    #    adb logcat -s 'bpfloader:*' 'LibBpfLoader:*'
+    #
+    # something like:
+    #   $ adb reboot; sleep 1; adb wait-for-device; adb root; sleep 1; adb wait-for-device; adb logcat -s 'bpfloader:*' 'LibBpfLoader:*'
+    # will take care of capturing logs as early as possible
+    #
+    # 5. look through the logs from the kernel's bpf verifier that bpfloader dumps out,
+    #    it usually makes sense to search back from the end and find the particular
+    #    bpf verifier failure that caused bpfloader to terminate early with an error code.
+    #    This will probably be something along the lines of 'too many jumps' or
+    #    'cannot prove return value is 0 or 1' or 'unsupported / unknown operation / helper',
+    #    'invalid bpf_context access', etc.
+    #
     reboot_on_failure reboot,bpfloader-failed
     # we're not really updatable, but want to be able to load bpf programs shipped in apexes
     updatable