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Date: Tue, 24 Nov 2015 01:05:08 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Maurizio Lombardi <mlombard@...hat.com>,
Tejun Heo <tj@...nel.org>, Joe Perches <joe@...ches.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/14] lib/vsprintf.c: expand field_width to 24 bits
On Mon, Nov 23, 2015 at 11:29 PM, Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> 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.
And any objections to put it into vsnprintf() ?
>
> [1] http://thread.gmane.org/gmane.linux.kernel/2034835
>
> Reported-by: Maurizio Lombardi <mlombard@...hat.com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Joe Perches <joe@...ches.com>
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.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;
u8 ?
>
> /* 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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--
With Best Regards,
Andy Shevchenko
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