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

[Miguel Ojeda <ojeda@kernel.org>]

From: Gary Guo <gary@garyguo.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@gmail.com>
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
Co-developed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 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.
  */
 static noinline_for_stack
 char *pointer(const char *fmt, char *buf, char *end, void *ptr,
@@ -2468,6 +2475,12 @@ 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);
+	case 'A':
+		if (!IS_ENABLED(CONFIG_RUST)) {
+			WARN_ONCE(1, "Please remove %%pA from non-Rust code\n");
+			return error_string(buf, end, "(%pA?)", spec);
+		}
+		return rust_fmt_argument(buf, end, ptr);
 	case 'x':
 		return pointer_string(buf, end, ptr, spec);
 	case 'e':
-- 
2.35.3