From b1a57bbfcc17c87e5cc76695ebb0565380c7501a Mon Sep 17 00:00:00 2001 From: Douglas Anderson Date: Thu, 7 May 2020 13:08:42 -0700 Subject: [PATCH] kgdb: Delay "kgdbwait" to dbg_late_init() by default 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(). Signed-off-by: Douglas Anderson Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Reviewed-by: Greg Kroah-Hartman Link: https://lore.kernel.org/r/20200507130644.v4.4.I3113aea1b08d8ce36dc3720209392ae8b815201b@changeid Signed-off-by: Daniel Thompson --- arch/x86/Kconfig | 1 + kernel/debug/debug_core.c | 25 +++++++++++++++---------- lib/Kconfig.kgdb | 18 ++++++++++++++++++ 3 files changed, 34 insertions(+), 10 deletions(-) diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 1197b5596d5a..5f44955ee21c 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -60,6 +60,7 @@ config X86 select ARCH_HAS_ACPI_TABLE_UPGRADE if ACPI select ARCH_HAS_DEBUG_VIRTUAL select ARCH_HAS_DEVMEM_IS_ALLOWED + select ARCH_HAS_EARLY_DEBUG if KGDB select ARCH_HAS_ELF_RANDOMIZE select ARCH_HAS_FAST_MULTIPLIER select ARCH_HAS_FILTER_PGPROT diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c index b542f36499c6..1cd8e8b41115 100644 --- a/kernel/debug/debug_core.c +++ b/kernel/debug/debug_core.c @@ -948,6 +948,14 @@ void kgdb_panic(const char *msg) kgdb_breakpoint(); } +static void kgdb_initial_breakpoint(void) +{ + kgdb_break_asap = 0; + + pr_crit("Waiting for connection from remote gdb...\n"); + kgdb_breakpoint(); +} + void __weak kgdb_arch_late(void) { } @@ -958,6 +966,9 @@ void __init dbg_late_init(void) if (kgdb_io_module_registered) kgdb_arch_late(); kdb_init(KDB_INIT_FULL); + + if (kgdb_io_module_registered && kgdb_break_asap) + kgdb_initial_breakpoint(); } static int @@ -1053,14 +1064,6 @@ void kgdb_schedule_breakpoint(void) } EXPORT_SYMBOL_GPL(kgdb_schedule_breakpoint); -static void kgdb_initial_breakpoint(void) -{ - kgdb_break_asap = 0; - - pr_crit("Waiting for connection from remote gdb...\n"); - kgdb_breakpoint(); -} - /** * kgdb_register_io_module - register KGDB IO module * @new_dbg_io_ops: the io ops vector @@ -1097,7 +1100,8 @@ int kgdb_register_io_module(struct kgdb_io *new_dbg_io_ops) /* Arm KGDB now. */ kgdb_register_callbacks(); - if (kgdb_break_asap) + if (kgdb_break_asap && + (!dbg_is_early || IS_ENABLED(CONFIG_ARCH_HAS_EARLY_DEBUG))) kgdb_initial_breakpoint(); return 0; @@ -1167,7 +1171,8 @@ static int __init opt_kgdb_wait(char *str) kgdb_break_asap = 1; kdb_init(KDB_INIT_EARLY); - if (kgdb_io_module_registered) + if (kgdb_io_module_registered && + IS_ENABLED(CONFIG_ARCH_HAS_EARLY_DEBUG)) kgdb_initial_breakpoint(); return 0; diff --git a/lib/Kconfig.kgdb b/lib/Kconfig.kgdb index 933680b59e2d..ffa7a76de086 100644 --- a/lib/Kconfig.kgdb +++ b/lib/Kconfig.kgdb @@ -124,4 +124,22 @@ config KDB_CONTINUE_CATASTROPHIC CONFIG_KDB_CONTINUE_CATASTROPHIC == 2. KDB forces a reboot. If you are not sure, say 0. +config ARCH_HAS_EARLY_DEBUG + bool + default n + help + If an architecture can definitely handle entering the debugger + when early_param's are parsed then it select this config. + Otherwise, if "kgdbwait" is passed on the kernel command line it + won't actually be processed until dbg_late_init() just after the + call to kgdb_arch_late() is made. + + NOTE: Even if this isn't selected by an architecture we will + still try to register kgdb to handle breakpoints and crashes + when early_param's are parsed, we just won't act on the + "kgdbwait" parameter until dbg_late_init(). If you get a + crash and try to drop into kgdb somewhere between these two + places you might or might not end up being able to use kgdb + depending on exactly how far along the architecture has initted. + endif # KGDB