vsprintf: %pf(%p)

This implements two new format strings: both do the same thing, one more
compatible with current gcc format string checking, the other that we'd
like to standardize:

 %pf(%p) - more compatible
 %(%p)  - more prettier

Both can take variable numbers of arguments, i.e. %(%p,%p,%p).

They're used to indicate that snprintf or pr_buf should interpret the
next argument as a pretty-printer function to call, and subsequent
arguments within the parentheses should be passed to the pretty-printer.

A pretty printer takes as its first argument a printbuf, and then zero
or more pointer arguments - integer arguments are not (currently) supported.

Example usage:

static void foo_to_text(struct printbuf *out, struct foo *foo)
{
	pr_buf(out, "bar=%u baz=%u", foo->bar, foo->baz);
}

printf("%(%p)", foo_to_text, foo);

The goal is to replace most of our %p format extensions with this
interface, and to move pretty-printers out of the core vsprintf.c code -
this will get us better organization and better discoverability (you'll
be able to cscope to pretty printer calls!), as well as eliminate a lot
of dispatch code in vsprintf.c.

Currently, we can only call pretty printers with pointer arguments. This
could be changed to also allow at least integer arguments in the future
by using libffi.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

1 files changed, 79 insertions, 2 deletions

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 09b259e03041..7fbeaf50d11c 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -431,7 +431,8 @@ enum format_type {
 	FORMAT_TYPE_UINT,
 	FORMAT_TYPE_INT,
 	FORMAT_TYPE_SIZE_T,
-	FORMAT_TYPE_PTRDIFF
+	FORMAT_TYPE_PTRDIFF,
+	FORMAT_TYPE_FN,
 };
 
 struct printf_spec {
@@ -2512,7 +2513,16 @@ qualifier:
 		return ++fmt - start;
 
 	case 'p':
-		spec->type = FORMAT_TYPE_PTR;
+		fmt++;
+		if (fmt[0] == 'f' &&
+		    fmt[1] == '(') {
+			fmt += 2;
+			spec->type = FORMAT_TYPE_FN;
+		} else
+			spec->type = FORMAT_TYPE_PTR;
+		return fmt - start;
+	case '(':
+		spec->type = FORMAT_TYPE_FN;
 		return ++fmt - start;
 
 	case '%':
@@ -2594,6 +2604,49 @@ set_precision(struct printf_spec *spec, int prec)
 	}
 }
 
+static void call_pr_fn(struct printbuf *out, void *fn, void **fn_args, unsigned nr_args)
+{
+	typedef void (*printf_fn_0)(struct printbuf *);
+	typedef void (*printf_fn_1)(struct printbuf *, void *);
+	typedef void (*printf_fn_2)(struct printbuf *, void *, void *);
+	typedef void (*printf_fn_3)(struct printbuf *, void *, void *, void *);
+	typedef void (*printf_fn_4)(struct printbuf *, void *, void *, void *, void *);
+	typedef void (*printf_fn_5)(struct printbuf *, void *, void *, void *, void *, void *);
+	typedef void (*printf_fn_6)(struct printbuf *, void *, void *, void *, void *, void *, void *);
+	typedef void (*printf_fn_7)(struct printbuf *, void *, void *, void *, void *, void *, void *, void *);
+	typedef void (*printf_fn_8)(struct printbuf *, void *, void *, void *, void *, void *, void *, void *, void *);
+
+	switch (nr_args) {
+	case 0:
+		((printf_fn_0)fn)(out);
+		break;
+	case 1:
+		((printf_fn_1)fn)(out, fn_args[0]);
+		break;
+	case 2:
+		((printf_fn_2)fn)(out, fn_args[0], fn_args[1]);
+		break;
+	case 3:
+		((printf_fn_3)fn)(out, fn_args[0], fn_args[1], fn_args[2]);
+		break;
+	case 4:
+		((printf_fn_4)fn)(out, fn_args[0], fn_args[1], fn_args[2], fn_args[3]);
+		break;
+	case 5:
+		((printf_fn_5)fn)(out, fn_args[0], fn_args[1], fn_args[2], fn_args[3], fn_args[4]);
+		break;
+	case 6:
+		((printf_fn_6)fn)(out, fn_args[0], fn_args[1], fn_args[2], fn_args[3], fn_args[4], fn_args[5]);
+		break;
+	case 7:
+		((printf_fn_7)fn)(out, fn_args[0], fn_args[1], fn_args[2], fn_args[3], fn_args[4], fn_args[5], fn_args[6]);
+		break;
+	case 8:
+		((printf_fn_8)fn)(out, fn_args[0], fn_args[1], fn_args[2], fn_args[3], fn_args[4], fn_args[5], fn_args[6], fn_args[7]);
+		break;
+	}
+}
+
 /**
  * vpr_buf - Format a string, outputting to a printbuf
  * @out: The printbuf to output to
@@ -2657,6 +2710,30 @@ void vpr_buf(struct printbuf *out, const char *fmt, va_list args)
 				fmt++;
 			break;
 
+		case FORMAT_TYPE_FN: {
+			unsigned nr_args = 0;
+			void *fn_args[8];
+			void *fn = va_arg(args, void *);
+
+			while (1) {
+				if (WARN_ON_ONCE(nr_args == ARRAY_SIZE(fn_args)))
+					goto out;
+				if (*fmt++ != '%')
+					goto out;
+				if (*fmt++ != 'p')
+					goto out;
+				fn_args[nr_args++] = va_arg(args, void *);
+				if (*fmt == ')')
+					break;
+				if (*fmt++ != ',')
+					goto out;
+			}
+
+			call_pr_fn(out, fn, fn_args, nr_args);
+			fmt++; /* past trailing ) */
+			break;
+		}
+
 		case FORMAT_TYPE_PERCENT_CHAR:
 			__pr_char(out, '%');
 			break;