RE: [PATCH v8 06/22] perf arm-spe: Refactor printing string to buffer

[David Laight <David.Laight@ACULAB.COM>]

From: Dave Martin
> Sent: 11 November 2020 17:58
> 
> On Wed, Nov 11, 2020 at 05:39:22PM +0000, Arnaldo Carvalho de Melo wrote:
> > Em Wed, Nov 11, 2020 at 03:45:23PM +0000, Andr� Przywara escreveu:
> > > On 11/11/2020 15:35, Arnaldo Carvalho de Melo wrote:
> > >
> > > Hi Arnaldo,
> > >
> > > thanks for taking a look!
> > >
> > > > Em Wed, Nov 11, 2020 at 03:11:33PM +0800, Leo Yan escreveu:
> > > >> When outputs strings to the decoding buffer with function snprintf(),
> > > >> SPE decoder needs to detects if any error returns from snprintf() and if
> > > >> so needs to directly bail out.  If snprintf() returns success, it needs
> > > >> to update buffer pointer and reduce the buffer length so can continue to
> > > >> output the next string into the consequent memory space.
> > > >>
> > > >> This complex logics are spreading in the function arm_spe_pkt_desc() so
> > > >> there has many duplicate codes for handling error detecting, increment
> > > >> buffer pointer and decrement buffer size.
> > > >>
> > > >> To avoid the duplicate code, this patch introduces a new helper function
> > > >> arm_spe_pkt_snprintf() which is used to wrap up the complex logics, and
> > > >> it's used by the caller arm_spe_pkt_desc().
> > > >>
> > > >> This patch also moves the variable 'blen' as the function's local
> > > >> variable, this allows to remove the unnecessary braces and improve the
> > > >> readability.
> > > >>
> > > >> Suggested-by: Dave Martin <Dave.Martin@arm.com>
> > > >> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> > > >> Reviewed-by: Andre Przywara <andre.przywara@arm.com>
> > > >> ---
> > > >>  .../arm-spe-decoder/arm-spe-pkt-decoder.c     | 260 +++++++++---------
> > > >>  1 file changed, 126 insertions(+), 134 deletions(-)
> > > >>
> > > >> diff --git a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c b/tools/perf/util/arm-spe-
> decoder/arm-spe-pkt-decoder.c
> > > >> index 04fd7fd7c15f..1970686f7020 100644
> > > >> --- a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
> > > >> +++ b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
> > > >> @@ -9,6 +9,7 @@
> > > >>  #include <endian.h>
> > > >>  #include <byteswap.h>
> > > >>  #include <linux/bitops.h>
> > > >> +#include <stdarg.h>
> > > >>
> > > >>  #include "arm-spe-pkt-decoder.h"
> > > >>
> > > >> @@ -258,192 +259,183 @@ int arm_spe_get_packet(const unsigned char *buf, size_t len,
> > > >>  	return ret;
> > > >>  }
> > > >>
> > > >> +static int arm_spe_pkt_snprintf(int *err, char **buf_p, size_t *blen,
> > > >> +				const char *fmt, ...)
> > > >> +{
> > > >> +	va_list ap;
> > > >> +	int ret;
> > > >> +
> > > >> +	/* Bail out if any error occurred */
> > > >> +	if (err && *err)
> > > >> +		return *err;
> > > >> +
> > > >> +	va_start(ap, fmt);
> > > >> +	ret = vsnprintf(*buf_p, *blen, fmt, ap);
> > > >> +	va_end(ap);
> > > >> +
> > > >> +	if (ret < 0) {
> > > >> +		if (err && !*err)
> > > >> +			*err = ret;
> > > >> +
> > > >> +	/*
> > > >> +	 * A return value of (*blen - 1) or more means that the
> > > >> +	 * output was truncated and the buffer is overrun.
> > > >> +	 */
> > > >> +	} else if (ret >= ((int)*blen - 1)) {
> > > >> +		(*buf_p)[*blen - 1] = '\0';
> > > >> +
> > > >> +		/*
> > > >> +		 * Set *err to 'ret' to avoid overflow if tries to
> > > >> +		 * fill this buffer sequentially.
> > > >> +		 */
> > > >> +		if (err && !*err)
> > > >> +			*err = ret;
> > > >> +	} else {
> > > >> +		*buf_p += ret;
> > > >> +		*blen -= ret;
> > > >> +	}
> > > >> +
> > > >> +	return ret;
> > > >> +}
> > > >> +

I'm not entirely sure that snprintf() can actually return a negative value.

Every implementation (except the microsoft one) also always writes a '\0'
even when the buffer is too short.

A simple wrapper that lets you append output and detect overflow is:
	ret = vsnprintf(buf, len, ...);
	if (ret < 0)
		/* just in case */
		return 0;
	return ret > len ? len : ret;

So on overflow the sum of the lengths is equal to the buffer size
(ie includes the terminating '\0'.

	David

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