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SubjectRe: [PATCH v2 4/9] kgdb: Delay "kgdbwait" to dbg_late_init() by default
On Tue, Apr 21, 2020 at 02:14:42PM -0700, Douglas Anderson wrote:
> 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 implement the weak function
> kgdb_arch_can_debug_early() to return true. 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_params'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 <dianders@chromium.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>

Overall this looks good but there is a small quibble below...


> diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> index b072aeb1fd78..7371517aeacc 100644
> --- a/include/linux/kgdb.h
> +++ b/include/linux/kgdb.h
> @@ -226,6 +226,28 @@ extern int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt);
> */
> extern void kgdb_arch_late(void);
>
> +/**
> + * kgdb_arch_can_debug_early - Check if OK to break before dbg_late_init()
> + *
> + * If an architecture can definitely handle entering the debugger when
> + * early_param's are parsed then it can override this function to return
> + * true. 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 returns false we will still try to register kgdb to
> + * handle breakpoints and crashes when early_params'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.
> + *
> + * ALSO: dbg_late_init() is actually still fairly early in the system
> + * boot process.
> + *
> + * Return: true if platform can handle kgdb early.
> + */
> +extern bool kgdb_arch_can_debug_early(void);

Does this need to be a function? It looks like all implementations are
either return true or return false (e.g. CONFIG_ARCH_HAVE_EARLY_DEBUG
would do the same thing).


Daniel.

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