Re: [PATCH] kernel/sysctl.c: If "count" including the terminating byte '\0' the write system call should retrun success.

[Sean Fu]

An application from HuaWei which works fine on 2.6 encounters this
issue on 3.0 or later kernel.

Test code:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

#define MAXLEN (100)

int main(int argc, char** argv)
{
        int fd = 0;
        int len = 0;
        char pathname[MAXLEN] = {0};
        char buf[2] = {0};
        int ret = 0xF;
        int value = 0xF;

        if (argc < 2)
        {
                printf("Input param error, less 2 param: %s, %s.\n",
argv[0], argv[1]);
                return 1;
        }

        printf("Input: %s, %s \n", argv[0], argv[1]);

        ret = sprintf(pathname,
"/proc/sys/net/ipv4/conf/%s/rp_filter", argv[1]);
        if (ret < 0)
                printf("sprintf error, errno %d, %s\n", errno, strerror(errno));
        printf("file is: %s. \n", pathname);

        fd = open(pathname, O_RDWR, S_IRUSR);
        if (fd <=0 )
        {
                printf("open %s failed, errno=%d, %s.\n", pathname,
errno, strerror(errno));
                return 1;
        }
        printf("open %s ok, fd=%d\n", pathname, fd);

        len = write(fd, "0\0", 2);
        printf("write %d: len=%d, errno=%d, %s\n", fd, len, errno,
strerror(errno));

        close(fd);
        return 0;
}

On Tue, Aug 25, 2015 at 12:59 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Mon, 24 Aug 2015 16:56:13 +0800
> Sean Fu <fxinrong@xxxxxxxxx> wrote:
>
>> when the input argument "count" including the terminating byte "\0",
>> The write system call return EINVAL on proc file.
>> But it return success on regular file.
>>
>> E.g. Writting two bytes ("1\0") to "/proc/sys/net/ipv4/conf/eth0/rp_filter".
>> write(fd, "1\0", 2) return EINVAL.
>
> And what would do that? What tool broke because of this?
>
>  echo 1 > /proc/sys/net/ipv4/conf/eth0/rp_filter
>
> works just fine. strlen("string") would not include the nul character.
> The only thing I could think of would be a sizeof(str), but then that
> would include someone hardcoding an integer in a string, like:
>
>         char val[] = "1"
>
>         write(fd, val, sizeof(val));
>
> Again, what tool does that?
>
> If there is a tool out in the wild that use to work on 2.6 (and was
> running on 2.6 then, and not something that was created after that
> change), then we can consider this fix.
>
> -- Steve
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