Re: [PATCH 04/17] vsprintf: add new `%pA` format specifier

Nick Desaulniers
2021-07-07T20:31:44Z

On Sun, Jul 4, 2021 at 1:29 PM <ojeda@kernel.org> wrote:
>
> From: Miguel Ojeda <ojeda@kernel.org>
>
> 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@gmail.com>
> Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
> Co-developed-by: Geoffrey Thomas <geofft@ldpreload.com>
> Signed-off-by: Geoffrey Thomas <geofft@ldpreload.com>
> Co-developed-by: Finn Behrens <me@kloenk.de>
> Signed-off-by: Finn Behrens <me@kloenk.de>
> Co-developed-by: Adam Bratschi-Kaye <ark.email@gmail.com>
> Signed-off-by: Adam Bratschi-Kaye <ark.email@gmail.com>
> Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
> Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> Co-developed-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
> Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
> Co-developed-by: Michael Ellerman <mpe@ellerman.id.au>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> Co-developed-by: Sven Van Asbroeck <thesven73@gmail.com>
> Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
> Co-developed-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> Co-developed-by: Boris-Chengbiao Zhou <bobo1239@web.de>
> Signed-off-by: Boris-Chengbiao Zhou <bobo1239@web.de>
> Co-developed-by: Fox Chen <foxhlchen@gmail.com>
> Signed-off-by: Fox Chen <foxhlchen@gmail.com>
> Co-developed-by: Ayaan Zaidi <zaidi.ayaan@gmail.com>
> Signed-off-by: Ayaan Zaidi <zaidi.ayaan@gmail.com>
> Co-developed-by: Douglas Su <d0u9.su@outlook.com>
> Signed-off-by: Douglas Su <d0u9.su@outlook.com>
> Co-developed-by: Yuki Okushi <jtitor@2k36.org>
> Signed-off-by: Yuki Okushi <jtitor@2k36.org>
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
> ---
> lib/vsprintf.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index f0c35d9b65b..e7afe954004 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2182,6 +2182,10 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
> return widen_string(buf, buf - buf_start, end, spec);
> }
>
> +#ifdef CONFIG_RUST
> +char *rust_fmt_argument(char* buf, char* end, void *ptr);

Which patch in the series adds the definition of rust_fmt_argument?
Sorry, I haven't looked through the entire series yet, but I don't
think it was an earlier patch in the series. If it's later in the
series, you may want to rebase this to be after (or combine it with
the patch that provides the definition). For instance, let's say the
first half of this series was accepted/merged, but not the latter
half. It would be weird to provide such definitions/calls to undefined
symbols.

> +#endif
> +
> /* Disable pointer hashing if requested */
> bool no_hash_pointers __ro_after_init;
> EXPORT_SYMBOL_GPL(no_hash_pointers);
> @@ -2335,6 +2339,10 @@ 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.
> */
> static noinline_for_stack
> char *pointer(const char *fmt, char *buf, char *end, void *ptr,
> @@ -2407,6 +2415,10 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
> return device_node_string(buf, end, ptr, spec, fmt + 1);
> case 'f':
> return fwnode_string(buf, end, ptr, spec, fmt + 1);
> +#ifdef CONFIG_RUST
> + case 'A':
> + return rust_fmt_argument(buf, end, ptr);
> +#endif
> case 'x':
> return pointer_string(buf, end, ptr, spec);
> case 'e':
> --
> 2.32.0
>

--
Thanks,
~Nick Desaulniers