Using kgdb requires at least some level of architecture-level initialization. If nothing else, it relies on the architecture to pass breakpoints / crashes onto kgdb. On some architectures this all works super early, specifically it starts working at some point in time before Linux parses early_params's. On other architectures it doesn't. A survey of a few platforms: a) x86: Presumably it all works early since "ekgdboc" is documented to work here. b) arm64: Catching crashes works; with a simple patch breakpoints can also be made to work. c) arm: Nothing in kgdb works until paging_init() -> devicemaps_init() -> early_trap_init() Let's be conservative and, by default, process "kgdbwait" (which tells the kernel to drop into the debugger ASAP at boot) a bit later at dbg_late_init() time. If an architecture has tested it and wants to re-enable super early debugging, they can select the ARCH_HAS_EARLY_DEBUG KConfig option. We'll do this for x86 to start. It should be noted that dbg_late_init() is still called quite early in the system. Note that this patch doesn't affect when kgdb runs its init. If kgdb is set to initialize early it will still initialize when parsing early_param's. This patch _only_ inhibits the initial breakpoint from "kgdbwait". This means: * Without any extra patches arm64 platforms will at least catch crashes after kgdb inits. * arm platforms will catch crashes (and could handle a hardcoded kgdb_breakpoint()) any time after early_trap_init() runs, even before dbg_late_init().
Showing the changes to kernel/debug/debug_core.c (1 of 3 files). Show all
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| @@ -948,6 +948,14 @@ void kgdb_panic(const char *msg) · 947 unmodified lines | ||
| 948 | 948 | kgdb_breakpoint(); |
| 949 | 949 | } |
| 950 | 950 | |
| 951 | +static void kgdb_initial_breakpoint(void) | |
| 952 | +{ | |
| 953 | + kgdb_break_asap = 0; | |
| 954 | + | |
| 955 | + pr_crit("Waiting for connection from remote gdb...\n"); | |
| 956 | + kgdb_breakpoint(); | |
| 957 | +} | |
| 958 | + | |
| 951 | 959 | void __weak kgdb_arch_late(void) |
| 952 | 960 | { |
| 953 | 961 | } |
| @@ -958,6 +966,9 @@ void __init dbg_late_init(void) · 4 unmodified lines | ||
| 958 | 966 | if (kgdb_io_module_registered) |
| 959 | 967 | kgdb_arch_late(); |
| 960 | 968 | kdb_init(KDB_INIT_FULL); |
| 969 | + | |
| 970 | + if (kgdb_io_module_registered && kgdb_break_asap) | |
| 971 | + kgdb_initial_breakpoint(); | |
| 961 | 972 | } |
| 962 | 973 | |
| 963 | 974 | static int |
| @@ -1053,14 +1064,6 @@ void kgdb_schedule_breakpoint(void) · 89 unmodified lines | ||
| 1053 | 1064 | } |
| 1054 | 1065 | EXPORT_SYMBOL_GPL(kgdb_schedule_breakpoint); |
| 1055 | 1066 | |
| 1056 | −static void kgdb_initial_breakpoint(void) | |
| 1057 | −{ | |
| 1058 | − kgdb_break_asap = 0; | |
| 1059 | − | |
| 1060 | − pr_crit("Waiting for connection from remote gdb...\n"); | |
| 1061 | − kgdb_breakpoint(); | |
| 1062 | −} | |
| 1063 | − | |
| 1064 | 1067 | /** |
| 1065 | 1068 | * kgdb_register_io_module - register KGDB IO module |
| 1066 | 1069 | * @new_dbg_io_ops: the io ops vector |
| @@ -1097,7 +1100,8 @@ int kgdb_register_io_module(struct kgdb_io *new_dbg_io_ops) · 30 unmodified lines | ||
| 1097 | 1100 | /* Arm KGDB now. */ |
| 1098 | 1101 | kgdb_register_callbacks(); |
| 1099 | 1102 | |
| 1100 | − if (kgdb_break_asap) | |
| 1103 | + if (kgdb_break_asap && | |
| 1104 | + (!dbg_is_early || IS_ENABLED(CONFIG_ARCH_HAS_EARLY_DEBUG))) | |
| 1101 | 1105 | kgdb_initial_breakpoint(); |
| 1102 | 1106 | |
| 1103 | 1107 | return 0; |
| @@ -1167,7 +1171,8 @@ static int __init opt_kgdb_wait(char *str) · 63 unmodified lines | ||
| 1167 | 1171 | kgdb_break_asap = 1; |
| 1168 | 1172 | |
| 1169 | 1173 | kdb_init(KDB_INIT_EARLY); |
| 1170 | − if (kgdb_io_module_registered) | |
| 1174 | + if (kgdb_io_module_registered && | |
| 1175 | + IS_ENABLED(CONFIG_ARCH_HAS_EARLY_DEBUG)) | |
| 1171 | 1176 | kgdb_initial_breakpoint(); |
| 1172 | 1177 | |
| 1173 | 1178 | return 0; |