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Date:   Thu, 6 Oct 2022 16:32:19 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Guilherme G. Piccoli" <gpiccoli@igalia.com>
Cc:     linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        kernel-dev@igalia.com, kernel@gpiccoli.net, anton@enomsg.org,
        ccross@android.com, tony.luck@intel.com
Subject: Re: [PATCH 2/8] pstore: Expose kmsg_bytes as a module parameter
Message-ID: <202210061628.76EAEB8@keescook>
References: <20221006224212.569555-1-gpiccoli@igalia.com>
 <20221006224212.569555-3-gpiccoli@igalia.com>
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On Thu, Oct 06, 2022 at 07:42:06PM -0300, Guilherme G. Piccoli wrote:
> Currently this tuning is only exposed as a filesystem option,
> but most Linux distros automatically mount pstore, hence changing
> this setting requires remounting it. Also, if that mount option
> wasn't explicitly set it doesn't show up in mount information,
> so users cannot check what is the current value of kmsg_bytes.
> 
> Let's then expose it as a module parameter, allowing both user
> visibility at all times (even if not manually set) and also the
> possibility of setting that as a boot/module parameter.

I've been meaning to do this too. :)

> 
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> ---
>  fs/pstore/platform.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
> index c32957e4b256..be05090076ce 100644
> --- a/fs/pstore/platform.c
> +++ b/fs/pstore/platform.c
> @@ -89,6 +89,11 @@ static char *compress =
>  module_param(compress, charp, 0444);
>  MODULE_PARM_DESC(compress, "compression to use");
>  
> +/* How much of the kernel log to snapshot */
> +unsigned long kmsg_bytes = CONFIG_PSTORE_DEFAULT_KMSG_BYTES;
> +module_param(kmsg_bytes, ulong, 0444);
> +MODULE_PARM_DESC(kmsg_bytes, "amount of kernel log to snapshot (in bytes)");
> +
>  /* Compression parameters */
>  static struct crypto_comp *tfm;
>  
> @@ -100,9 +105,6 @@ struct pstore_zbackend {
>  static char *big_oops_buf;
>  static size_t big_oops_buf_sz;
>  
> -/* How much of the console log to snapshot */
> -unsigned long kmsg_bytes = CONFIG_PSTORE_DEFAULT_KMSG_BYTES;
> -
>  void pstore_set_kmsg_bytes(int bytes)
>  {
>  	kmsg_bytes = bytes;
> -- 
> 2.38.0

Doing a mount will override the result, so I wonder if there should be
two variables, etc... not a concern for the normal use case.

Also, I've kind of wanted to get rid of a "default" for this and instead
use a value based on the compression vs record sizes, etc. But I didn't
explore it.

-- 
Kees Cook