linux-kernel - RE: [PATCH 3/4] samples/bpf: Check the error of write() and read()

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Message-ID: <c8fd15978e6642158c7e64190e4dee13@AcuMS.aculab.com>
Date:   Mon, 2 Jul 2018 09:25:11 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Taeung Song' <treeze.taeung@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
CC:     Teng Qin <qinteng@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 3/4] samples/bpf: Check the error of write() and read()

From: Taeung Song
> Sent: 02 July 2018 10:15
> test_task_rename() and test_urandom_read()
> can be failed during write() and read(),
> So check the result of them.
> 
> Signed-off-by: Taeung Song <treeze.taeung@...il.com>
> ---
>  samples/bpf/test_overhead_user.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/samples/bpf/test_overhead_user.c b/samples/bpf/test_overhead_user.c
> index 6caf47a..8a88d9c 100644
> --- a/samples/bpf/test_overhead_user.c
> +++ b/samples/bpf/test_overhead_user.c
> @@ -6,6 +6,7 @@
>   */
>  #define _GNU_SOURCE
>  #include <sched.h>
> +#include <errno.h>
>  #include <stdio.h>
>  #include <sys/types.h>
>  #include <asm/unistd.h>
> @@ -44,8 +45,12 @@ static void test_task_rename(int cpu)
>  		exit(1);
>  	}
>  	start_time = time_get_ns();
> -	for (i = 0; i < MAX_CNT; i++)
> -		write(fd, buf, sizeof(buf));
> +	for (i = 0; i < MAX_CNT; i++) {
> +		if (write(fd, buf, sizeof(buf)) < 0) {
> +			printf("task rename failed: %s\n", strerror(errno));
> +			break;

I'm not sure 'break' generates sensible output.

> +		}
> +	}

What about partial writes??

>  	printf("task_rename:%d: %lld events per sec\n",
>  	       cpu, MAX_CNT * 1000000000ll / (time_get_ns() - start_time));
>  	close(fd);
> @@ -55,7 +60,7 @@ static void test_urandom_read(int cpu)
>  {
>  	__u64 start_time;
>  	char buf[4];
> -	int i, fd;
> +	int i, fd, err = 0;
> 
>  	fd = open("/dev/urandom", O_RDONLY);
>  	if (fd < 0) {
> @@ -63,8 +68,13 @@ static void test_urandom_read(int cpu)
>  		exit(1);
>  	}
>  	start_time = time_get_ns();
> -	for (i = 0; i < MAX_CNT; i++)
> -		read(fd, buf, sizeof(buf));
> +	for (i = 0; i < MAX_CNT; i++) {
> +		err = read(fd, buf, sizeof(buf));
> +		if (err < 0 || err >= sizeof(buf)) {

Overlong reads indicate that something is seriously awry.
Short reads are valid - but maybe not expected.
> +			printf("failed to read from /dev/urandom: %s\n", strerror(errno));


strerror() won't give anything sensible unless err == -1;
You probably want to include the loop count.

> +			break;

The summary print will be gibberish after break.

> +		}
> +	}
>  	printf("urandom_read:%d: %lld events per sec\n",
>  	       cpu, MAX_CNT * 1000000000ll / (time_get_ns() - start_time));
>  	close(fd);
> --
> 2.7.4