linux-kernel - Re: [PATCH v3] kernel: add kcov code coverage

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Message-ID: <20160114162542.GC28386@node.shutemov.name>
Date:	Thu, 14 Jan 2016 18:25:43 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	akpm@...ux-foundation.org, drysdale@...gle.com,
	keescook@...gle.com, quentin.casasnovas@...cle.com,
	sasha.levin@...cle.com, vegard.nossum@...cle.com,
	linux-kernel@...r.kernel.org, edumazet@...gle.com,
	taviso@...gle.com, bhelgaas@...gle.com, syzkaller@...glegroups.com,
	kcc@...gle.com, glider@...gle.com, ryabinin.a.a@...il.com
Subject: Re: [PATCH v3] kernel: add kcov code coverage

On Thu, Jan 14, 2016 at 03:22:21PM +0100, Dmitry Vyukov wrote:
> +The following program demonstrates kcov usage from within a test program:
> +
> +#include <stdio.h>
> +#include <stddef.h>
> +#include <stdint.h>
> +#include <sys/types.h>
> +#include <sys/stat.h>
> +#include <sys/ioctl.h>
> +#include <sys/mman.h>
> +#include <fcntl.h>
> +
> +#define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
> +#define KCOV_ENABLE			_IO('c', 100)
> +#define KCOV_DISABLE			_IO('c', 101)
> +#define COVER_SIZE			(64<<10)
> +
> +int main(int argc, char **argv)
> +{
> +	int fd;
> +	uint32_t *cover, n, i;
> +
> +	/* A single fd descriptor allows coverage collection on a single
> +	 * thread.
> +	 */
> +	fd = open("/sys/kernel/debug/kcov", O_RDWR);
> +	if (fd == -1)
> +		perror("open");
> +	/* Setup trace mode and trace size. */
> +	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
> +		perror("ioctl");
> +	/* Mmap buffer shared between kernel- and user-space. */
> +	cover = (uint32_t*)mmap(NULL, COVER_SIZE * sizeof(uint32_t),
> +				PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> +	if ((void*)cover == MAP_FAILED)
> +		perror("mmap");
> +	/* Enable coverage collection on the current thread. */
> +	if (ioctl(fd, KCOV_ENABLE, 0))
> +		perror("ioctl");
> +	/* Reset coverage from the tail of the ioctl() call. */
> +	__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
> +	/* That's the target syscal call. */
> +	read(-1, NULL, 0);
> +	/* Read number of PCs collected. */
> +	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> +	/* PCs are shorten to uint32_t, so we need to restore the upper part. */
> +	for (i = 0; i < n; i++)
> +		printf("0xffffffff%0lx\n", (unsigned long)cover[i + 1]);
> +	/* Disable coverage collection for the current thread. After this call
> +	 * coverage can be enabled for a different thread.
> +	 */
> +	if (ioctl(fd, KCOV_DISABLE, 0))
> +		perror("ioctl");
> +	/* Free resources. */
> +	if (munmap(cover, COVER_SIZE * sizeof(uint32_t)))
> +		perror("munmap");
> +	if (close(fd))
> +		perror("close");
> +	return 0;
> +}

Do we really need ioctl in this interface? Why not just plain write(2)?

-- 
 Kirill A. Shutemov