On 2020/9/18 0:25, Borislav Petkov wrote:
> On Thu, Sep 17, 2020 at 07:38:57PM +0800, Xiongfeng Wang wrote:
>> I am not sure if snprintf will return a value larger than its second input
>> paramter 'size'.
>
> The comment over snprintf() says
>
> * The return value is the number of characters which would be
> * generated for the given input, excluding the trailing null,
> * as per ISO C99. If the return is greater than or equal to
> * @size, the resulting string is truncated.
>
> And let's try it, see diff at the end. Now look what that produces:
>
> [ 2.594796] kernel_init: len: 16, str: [A lo]
>
> it returns 16 for len even though the buffer is 5 chars long. So in our
> patch, we'd increment by 16 which would be wrong.
>
> Now let's use scnprintf():
>
> [ 2.700142] kernel_init: len: 4, str: [A lo]
>
> Much better. Lemme do that.
>
>> Not sure whether we need to check 'len' equals to 0.
>> if (len <= 0) ?
>
> Yeah, lemme fix that too, so we have now incrementally ontop:
Thansk a lot. I will send another version. Also I will change the 'snprintf' in
'dimmdev_location_show()' to 'scnprintf'
Thanks,
Xiongfeng
>
> ---
> diff --git a/drivers/edac/edac_mc_sysfs.c b/drivers/edac/edac_mc_sysfs.c
> index fa0551c81e63..c56e0004b39e 100644
> --- a/drivers/edac/edac_mc_sysfs.c
> +++ b/drivers/edac/edac_mc_sysfs.c
> @@ -822,18 +822,17 @@ static ssize_t mci_max_location_show(struct device *dev,
> int i, n;
>
> for (i = 0; i < mci->n_layers; i++) {
> - n = snprintf(p, len, "%s %d ",
> - edac_layer_name[mci->layers[i].type],
> - mci->layers[i].size - 1);
> -
> + n = scnprintf(p, len, "%s %d ",
> + edac_layer_name[mci->layers[i].type],
> + mci->layers[i].size - 1);
> len -= n;
> - if (len < 0)
> + if (len <= 0)
> goto out;
>
> p += n;
> }
>
> - p += snprintf(p, len, "\n");
> + p += scnprintf(p, len, "\n");
> out:
> return p - data;
> }
> ---
>
> Test diff:
>
> ---
> diff --git a/init/main.c b/init/main.c
> index ae78fb68d231..e2d6110d3a3d 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1397,7 +1397,8 @@ void __weak free_initmem(void)
>
> static int __ref kernel_init(void *unused)
> {
> - int ret;
> + char str[5];
> + int ret, len;
>
> kernel_init_freeable();
> /* need to finish all async __init code before freeing the memory */
> @@ -1419,6 +1420,11 @@ static int __ref kernel_init(void *unused)
>
> do_sysctl_args();
>
> + len = snprintf(str, 5, "A longer string\n");
> +
> + pr_info("%s: len: %d, str: [%s]\n",
> + __func__, len, str);
> +
> if (ramdisk_execute_command) {
> ret = run_init_process(ramdisk_execute_command);
> if (!ret)
>
