linux-kernel - Re: [PATCH v6 14/23] vsprintf: add new `%pA` format specifier

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Date:   Tue, 10 May 2022 10:38:35 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Miguel Ojeda <ojeda@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Gary Guo <gary@...yguo.net>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v6 14/23] vsprintf: add new `%pA` format specifier

On Sat 2022-05-07 07:24:12, Miguel Ojeda wrote:
> From: Gary Guo <gary@...yguo.net>
> 
> This patch adds a format specifier `%pA` to `vsprintf` which formats
> a pointer as `core::fmt::Arguments`. Doing so allows us to directly
> format to the internal buffer of `printf`, so we do not have to use
> a temporary buffer on the stack to pre-assemble the message on
> the Rust side.
> 
> This specifier is intended only to be used from Rust and not for C, so
> `checkpatch.pl` is intentionally unchanged to catch any misuse.
> 
> Co-developed-by: Alex Gaynor <alex.gaynor@...il.com>
> Signed-off-by: Alex Gaynor <alex.gaynor@...il.com>
> Co-developed-by: Wedson Almeida Filho <wedsonaf@...gle.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@...gle.com>
> Signed-off-by: Gary Guo <gary@...yguo.net>
> Co-developed-by: Miguel Ojeda <ojeda@...nel.org>
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>

Feel free to use:

Acked-by: Petr Mladek <pmladek@...e.com>

See a comment below.

> ---
>  lib/vsprintf.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 40d26a07a133..00f71f91d991 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2270,6 +2270,9 @@ int __init no_hash_pointers_enable(char *str)
>  }
>  early_param("no_hash_pointers", no_hash_pointers_enable);
>  
> +/* Used for Rust formatting ('%pA'). */
> +char *rust_fmt_argument(char *buf, char *end, void *ptr);
> +
>  /*
>   * Show a '%p' thing.  A kernel extension is that the '%p' is followed
>   * by an extra set of alphanumeric characters that are extended format
> @@ -2396,6 +2399,10 @@ early_param("no_hash_pointers", no_hash_pointers_enable);
>   *
>   * Note: The default behaviour (unadorned %p) is to hash the address,
>   * rendering it useful as a unique identifier.
> + *
> + * There is also a '%pA' format specifier, but it is only intended to be used
> + * from Rust code to format core::fmt::Arguments. Do *not* use it from C.
> + * See rust/kernel/print.rs for details.
>   */

It would be great to document it also in
Documentation/core-api/printk-formats.rst
I am sorry that I did not mentioned it earlier.

Also the check in checkpatch.pl, mentioned in another reply, sounds
like a good idea.

Best Regards,
Petr