Re: [PATCH] kasan: fix stack traces dependency for HW_TAGS

[Marco Elver <elver@google.com>]

On Mon, 8 Feb 2021 at 19:40, Andrey Konovalov <andreyknvl@google.com> wrote:
>
> Currently, whether the alloc/free stack traces collection is enabled by
> default for hardware tag-based KASAN depends on CONFIG_DEBUG_KERNEL.
> The intention for this dependency was to only enable collection on slow
> debug kernels due to a significant perf and memory impact.
>
> As it turns out, CONFIG_DEBUG_KERNEL is not considered a debug option
> and is enabled on many productions kernels including Android and Ubuntu.
> As the result, this dependency is pointless and only complicates the code
> and documentation.
>
> Having stack traces collection disabled by default would make the hardware
> mode work differently to to the software ones, which is confusing.
>
> This change removes the dependency and enables stack traces collection
> by default.
>
> Looking into the future, this default might makes sense for production
> kernels, assuming we implement a fast stack trace collection approach.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

Reviewed-by: Marco Elver <elver@google.com>

I'm in favor of this simplification.

The fact that CONFIG_DEBUG_KERNEL cannot be relied upon to determine
if we're running a debug kernel or not is a bit unfortunate though.

Thanks!

> ---
>  Documentation/dev-tools/kasan.rst | 3 +--
>  mm/kasan/hw_tags.c                | 8 ++------
>  2 files changed, 3 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index 1651d961f06a..a248ac3941be 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -163,8 +163,7 @@ particular KASAN features.
>  - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
>
>  - ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
> -  traces collection (default: ``on`` for ``CONFIG_DEBUG_KERNEL=y``, otherwise
> -  ``off``).
> +  traces collection (default: ``on``).
>
>  - ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN
>    report or also panic the kernel (default: ``report``).
> diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> index e529428e7a11..d558799b25b3 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -134,12 +134,8 @@ void __init kasan_init_hw_tags(void)
>
>         switch (kasan_arg_stacktrace) {
>         case KASAN_ARG_STACKTRACE_DEFAULT:
> -               /*
> -                * Default to enabling stack trace collection for
> -                * debug kernels.
> -                */
> -               if (IS_ENABLED(CONFIG_DEBUG_KERNEL))
> -                       static_branch_enable(&kasan_flag_stacktrace);
> +               /* Default to enabling stack trace collection. */
> +               static_branch_enable(&kasan_flag_stacktrace);
>                 break;
>         case KASAN_ARG_STACKTRACE_OFF:
>                 /* Do nothing, kasan_flag_stacktrace keeps its default value. */
> --
> 2.30.0.478.g8a0d178c01-goog
>