Re: [PATCH] 2.6.17.9 Incorrect string length checking in param_set_copystring()

From: Julio Auto
Date: Sun Aug 20 2006 - 17:37:06 EST


First of all, thanks for the reply. I'll take the afore mentioned
point of SubmittingPatches in consideration in the future.

And yes, you're correct.
I was misled by an incorrect use of sizeof() and it made believe that
its return would strip the null terminator. Once again, my bad. :)

Julio Auto

On 8/20/06, Luca <kronos.it@xxxxxxxxx> wrote:
Julio Auto <mindvortex@xxxxxxxxx> ha scritto:
> As for 2.6.17.9, linux/include/linux/moduleparam.h suggests the user
> of module_param_string() to set the maxlen parameter to
> strlen(string), ie. '\0' excluded. However the function that actually
> sets the string (param_set_copystring()), doesn't accept inputs with
> maxlen-1 characters, reporting that the supplied string should fit
> maxlen-1 chars.
> See patch below.
> Cheers,
>
> Julio Auto
>
> --- linux-2.6.17.9/kernel/params.c.old 2006-08-19 20:48:30.000000000 -0700
> +++ linux-2.6.17.9/kernel/params.c 2006-08-19 20:49:15.000000000 -0700
> @@ -351,9 +351,9 @@ int param_set_copystring(const char *val
> {
> struct kparam_string *kps = kp->arg;
>
> - if (strlen(val)+1 > kps->maxlen) {
> + if (strlen(val) > kps->maxlen) {
> printk(KERN_ERR "%s: string doesn't fit in %u chars.\n",
> - kp->name, kps->maxlen-1);
> + kp->name, kps->maxlen);
> return -ENOSPC;
> }
> strcpy(kps->string, val);

Hi,
I believe that the code is correct. kps->maxlen is the lenght of the
buffer; when dealing with a string of N chars you need an array of (N +
1) bytes in order to store the terminator ('\0').

Look again at the check:

if (strlen(val) > kps->maxlen) {

Suppose that val is a string of 10 chars (strlen(val) == 10), suppose
that the buffer holds 10 bytes (kps->maxlen == 10). The expression:

if (10 > 10) {

is false, so strcpy() ends up writing 11 bytes in a buffer of 10 bytes.

Also, for future patches see point 11 of
Documentation/SubmittingPatches.

Luca
--
Home: http://kronoz.cjb.net
"Su cio` di cui non si puo` parlare e` bene tacere".
Ludwig Wittgenstein

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