Re: [PATCH] kcsan: Add option for verbose reporting

From: Paul E. McKenney
Date: Wed Feb 19 2020 - 13:01:57 EST


On Wed, Feb 19, 2020 at 04:15:31PM +0100, Marco Elver wrote:
> Adds CONFIG_KCSAN_VERBOSE to optionally enable more verbose reports.
> Currently information about the reporting task's held locks and IRQ
> trace events are shown, if they are enabled.
>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> Suggested-by: Qian Cai <cai@xxxxxx>

Queued for testing and review, thank you!

Thanx, Paul

> ---
> kernel/kcsan/report.c | 48 +++++++++++++++++++++++++++++++++++++++++++
> lib/Kconfig.kcsan | 13 ++++++++++++
> 2 files changed, 61 insertions(+)
>
> diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
> index 11c791b886f3c..f14becb6f1537 100644
> --- a/kernel/kcsan/report.c
> +++ b/kernel/kcsan/report.c
> @@ -1,10 +1,12 @@
> // SPDX-License-Identifier: GPL-2.0
>
> +#include <linux/debug_locks.h>
> #include <linux/jiffies.h>
> #include <linux/kernel.h>
> #include <linux/lockdep.h>
> #include <linux/preempt.h>
> #include <linux/printk.h>
> +#include <linux/rcupdate.h>
> #include <linux/sched.h>
> #include <linux/spinlock.h>
> #include <linux/stacktrace.h>
> @@ -245,6 +247,29 @@ static int sym_strcmp(void *addr1, void *addr2)
> return strncmp(buf1, buf2, sizeof(buf1));
> }
>
> +static void print_verbose_info(struct task_struct *task)
> +{
> + if (!task)
> + return;
> +
> + if (task != current && task->state == TASK_RUNNING)
> + /*
> + * Showing held locks for a running task is unreliable, so just
> + * skip this. The printed locks are very likely inconsistent,
> + * since the stack trace was obtained when the actual race
> + * occurred and the task has since continued execution. Since we
> + * cannot display the below information from the racing thread,
> + * but must print it all from the watcher thread, bail out.
> + * Note: Even if the task is not running, there is a chance that
> + * the locks held may be inconsistent.
> + */
> + return;
> +
> + pr_err("\n");
> + debug_show_held_locks(task);
> + print_irqtrace_events(task);
> +}
> +
> /*
> * Returns true if a report was generated, false otherwise.
> */
> @@ -319,6 +344,26 @@ static bool print_report(const volatile void *ptr, size_t size, int access_type,
> other_info.num_stack_entries - other_skipnr,
> 0);
>
> + if (IS_ENABLED(CONFIG_KCSAN_VERBOSE) && other_info.task_pid != -1) {
> + struct task_struct *other_task;
> +
> + /*
> + * Rather than passing @current from the other task via
> + * @other_info, obtain task_struct here. The problem
> + * with passing @current via @other_info is that, we
> + * would have to get_task_struct/put_task_struct, and if
> + * we race with a task being released, we would have to
> + * release it in release_report(). This may result in
> + * deadlock if we want to use KCSAN on the allocators.
> + * Instead, make this best-effort, and if the task was
> + * already released, we just do not print anything here.
> + */
> + rcu_read_lock();
> + other_task = find_task_by_pid_ns(other_info.task_pid, &init_pid_ns);
> + print_verbose_info(other_task);
> + rcu_read_unlock();
> + }
> +
> pr_err("\n");
> pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
> get_access_type(access_type), ptr, size,
> @@ -340,6 +385,9 @@ static bool print_report(const volatile void *ptr, size_t size, int access_type,
> stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr,
> 0);
>
> + if (IS_ENABLED(CONFIG_KCSAN_VERBOSE))
> + print_verbose_info(current);
> +
> /* Print report footer. */
> pr_err("\n");
> pr_err("Reported by Kernel Concurrency Sanitizer on:\n");
> diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
> index f0b791143c6ab..ba9268076cfbc 100644
> --- a/lib/Kconfig.kcsan
> +++ b/lib/Kconfig.kcsan
> @@ -20,6 +20,19 @@ menuconfig KCSAN
>
> if KCSAN
>
> +config KCSAN_VERBOSE
> + bool "Show verbose reports with more information about system state"
> + depends on PROVE_LOCKING
> + help
> + If enabled, reports show more information about the system state that
> + may help better analyze and debug races. This includes held locks and
> + IRQ trace events.
> +
> + While this option should generally be benign, we call into more
> + external functions on report generation; if a race report is
> + generated from any one of them, system stability may suffer due to
> + deadlocks or recursion. If in doubt, say N.
> +
> config KCSAN_DEBUG
> bool "Debugging of KCSAN internals"
>
> --
> 2.25.0.265.gbab2e86ba0-goog
>