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[PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits

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Maurizio Lombardi reported a problem [1] with the %pb extension: It
doesn't work for sufficiently large bitmaps, since the size is stashed
in the field_width field of the struct printf_spec, which is currently
an s16. Concretely, this manifested itself in
/sys/bus/pseudo/drivers/scsi_debug/map being empty, since the bitmap
printer got a size of 0, which is the 16 bit truncation of the actual
bitmap size.

We do want to keep struct printf_spec at 8 bytes so that it can
cheaply be passed by value. The qualifier field is only used for
internal bookkeeping in format_decode, so we might as well use a local
variable for that. This gives us an additional 8 bits, which we can
then use for the field width.

To stay in 8 bytes, we need to do a little rearranging and make the
type member a bitfield as well. For consistency, change all the
members to bit fields. gcc doesn't generate much worse code with these
changes (in fact, bloat-o-meter says we save 300 bytes - which I think
is a little surprising).

I didn't find a BUILD_BUG/compiletime_assertion/... which would work
outside function context, so for now I just open-coded it.

[1] http://thread.gmane.org/gmane.linux.kernel/2034835

Reported-by: Maurizio Lombardi <mlombard@redhat.com>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Joe Perches <joe@perches.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 lib/vsprintf.c | 41 +++++++++++++++++++++--------------------
 1 file changed, 21 insertions(+), 20 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 63ca52366049..01c3aa638582 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -380,13 +380,13 @@ enum format_type {
 };
 
 struct printf_spec {
-	u8	type;		/* format_type enum */
-	u8	flags;		/* flags to number() */
-	u8	base;		/* number base, 8, 10 or 16 only */
-	u8	qualifier;	/* number qualifier, one of 'hHlLtzZ' */
-	s16	field_width;	/* width of output field */
-	s16	precision;	/* # of digits/chars */
-};
+	unsigned int	type:8;		/* format_type enum */
+	signed int	field_width:24;	/* width of output field */
+	unsigned int	flags:8;	/* flags to number() */
+	unsigned int	base:8;		/* number base, 8, 10 or 16 only */
+	signed int	precision:16;	/* # of digits/chars */
+} __packed;
+extern char __check_printf_spec[1-2*(sizeof(struct printf_spec) != 8)];
 
 static noinline_for_stack
 char *number(char *buf, char *end, unsigned long long num,
@@ -1641,6 +1641,7 @@ static noinline_for_stack
 int format_decode(const char *fmt, struct printf_spec *spec)
 {
 	const char *start = fmt;
+	char qualifier;
 
 	/* we finished early by reading the field width */
 	if (spec->type == FORMAT_TYPE_WIDTH) {
@@ -1723,16 +1724,16 @@ precision:
 
 qualifier:
 	/* get the conversion qualifier */
-	spec->qualifier = -1;
+	qualifier = 0;
 	if (*fmt == 'h' || _tolower(*fmt) == 'l' ||
 	    _tolower(*fmt) == 'z' || *fmt == 't') {
-		spec->qualifier = *fmt++;
-		if (unlikely(spec->qualifier == *fmt)) {
-			if (spec->qualifier == 'l') {
-				spec->qualifier = 'L';
+		qualifier = *fmt++;
+		if (unlikely(qualifier == *fmt)) {
+			if (qualifier == 'l') {
+				qualifier = 'L';
 				++fmt;
-			} else if (spec->qualifier == 'h') {
-				spec->qualifier = 'H';
+			} else if (qualifier == 'h') {
+				qualifier = 'H';
 				++fmt;
 			}
 		}
@@ -1789,19 +1790,19 @@ qualifier:
 		return fmt - start;
 	}
 
-	if (spec->qualifier == 'L')
+	if (qualifier == 'L')
 		spec->type = FORMAT_TYPE_LONG_LONG;
-	else if (spec->qualifier == 'l') {
+	else if (qualifier == 'l') {
 		BUILD_BUG_ON(FORMAT_TYPE_ULONG + SIGN != FORMAT_TYPE_LONG);
 		spec->type = FORMAT_TYPE_ULONG + (spec->flags & SIGN);
-	} else if (_tolower(spec->qualifier) == 'z') {
+	} else if (_tolower(qualifier) == 'z') {
 		spec->type = FORMAT_TYPE_SIZE_T;
-	} else if (spec->qualifier == 't') {
+	} else if (qualifier == 't') {
 		spec->type = FORMAT_TYPE_PTRDIFF;
-	} else if (spec->qualifier == 'H') {
+	} else if (qualifier == 'H') {
 		BUILD_BUG_ON(FORMAT_TYPE_UBYTE + SIGN != FORMAT_TYPE_BYTE);
 		spec->type = FORMAT_TYPE_UBYTE + (spec->flags & SIGN);
-	} else if (spec->qualifier == 'h') {
+	} else if (qualifier == 'h') {
 		BUILD_BUG_ON(FORMAT_TYPE_USHORT + SIGN != FORMAT_TYPE_SHORT);
 		spec->type = FORMAT_TYPE_USHORT + (spec->flags & SIGN);
 	} else {
-- 
2.6.1

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Thread

[PATCH v3 00/14] printf stuff for 4.5 Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 01/14] lib/vsprintf.c: pull out padding code from dentry_name() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Joe Perches <joe@perches.com> - 2015-12-03 22:00 +0100
    Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Andy Shevchenko <andriy.shevchenko@linux.intel.com> - 2015-12-03 22:40 +0100
      Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Andrew Morton <akpm@linux-foundation.org> - 2015-12-04 00:40 +0100
        Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Joe Perches <joe@perches.com> - 2015-12-04 01:10 +0100
          Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-04 10:10 +0100
        Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-04 10:10 +0100
      Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Joe Perches <joe@perches.com> - 2015-12-04 00:50 +0100
  [PATCH v3 08/14] lib/test_printf.c: don't BUG Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 03/14] lib/vsprintf.c: eliminate potential race in string() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 06/14] lib/vsprintf.c: warn about too large precisions and field widths Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 09/14] lib/test_printf.c: check for out-of-bound writes Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 02/14] lib/vsprintf.c: move string() below widen_string() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
  [PATCH v3 14/14] lib/test_printf.c: test dentry printing Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100
    Re: [PATCH v3 14/14] lib/test_printf.c: test dentry printing Andrew Morton <akpm@linux-foundation.org> - 2015-12-04 01:20 +0100
      Re: [PATCH v3 14/14] lib/test_printf.c: test dentry printing Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-04 09:20 +0100
        Re: [PATCH v3 14/14] lib/test_printf.c: test dentry printing Andrew Morton <akpm@linux-foundation.org> - 2015-12-04 09:50 +0100
  [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-12-03 22:00 +0100

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