linux-kernel - Re: [PATCH v4 12/20] vsprintf: add new `%pA` format specifier

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Date:   Mon, 14 Feb 2022 11:52:37 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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,
        Gary Guo <gary@...yguo.net>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...gle.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v4 12/20] vsprintf: add new `%pA` format specifier

On 14/02/2022 11.18, Andy Shevchenko wrote:
> On Sat, Feb 12, 2022 at 02:03:38PM +0100, Miguel Ojeda wrote:
> 
>> From: Gary Guo <gary@...yguo.net>
> 
> Not sure I understand this...
> 
>> 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>
> 
> ...together with this in the current SoB chain.
> 
>> Co-developed-by: Miguel Ojeda <ojeda@...nel.org>
>> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
> 
> I'm wondering if you considered to use %pV.
> 

I think the point is for vsnprintf() to call (back) into Rust code.

That said, I don't like the !CONFIG_RUST version to return NULL, that
will surely crash moments later.

So I prefer something like

[rust.h]
// no CONFIG_RUST conditional
+char *rust_fmt_argument(char* buf, char* end, void *ptr);

[vsprintf.c]
+	case 'A':
+               if (IS_ENABLED(CONFIG_RUST))
+		    return rust_fmt_argument(buf, end, ptr);
+               else
+                   return string_nocheck(buf, end, "[%pA in non-Rust
code?!]", default_str_spec);


Rasmus