Re: [PATCH v3 3/3] man2/fincore.2: document general description about fincore(2)

[Dave Hansen]

On 07/07/2014 11:00 AM, Naoya Horiguchi wrote:
> +.SH RETURN VALUE
> +On success,
> +.BR fincore ()
> +returns 0.
> +On error, \-1 is returned, and
> +.I errno
> +is set appropriately.

Is this accurate?  From reading the syscall itself, it looked like it
did this:

> + * Return value is the number of pages whose data is stored in fc->buffer.
> + */
> +static long do_fincore(struct fincore_control *fc, int nr_pages)

and:

> +SYSCALL_DEFINE6(fincore, int, fd, loff_t, start, long, nr_pages,
...
> +	while (fc.nr_pages > 0) {
> +		memset(fc.buffer, 0, fc.buffer_size);
> +		ret = do_fincore(&fc, min(step, fc.nr_pages));
> +		/* Reached the end of the file */
> +		if (ret == 0)
> +			break;
> +		if (ret < 0)
> +			break;
...
> +	}
...
> +	return ret;
> +}

Which seems that for a given loop of do_fincore(), you might end up
returning the result of that *single* iteration of do_fincore() instead
of the aggregate of the entire syscall.

So, it can return <0 on failure, 0 on success, or also an essentially
random >0 number on success too.

Why not just use the return value for something useful instead of
hacking in the extras->nr_entries stuff?  Oh, and what if that

> +	if (extra)
> +		__put_user(nr, &extra->nr_entries);

fails?  It seems like we might silently forget to tell userspace how
many entries we filled.
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