Re: [patch] kernel events layer

Bernd Petrovitsch
2004-07-24T11:38:26Z

On Sat, 2004-07-24 at 06:42, Keith Owens wrote:
> On Fri, 23 Jul 2004 22:47:06 -0400,
> Robert Love <rml@ximian.com> wrote:
> >On Sat, 2004-07-24 at 00:32 +0300, Dan Aloni wrote:
> >
> >> IMHO you either should not assume anything about the length of the object
> >> string, _or_ do the complete safe string assembly e.g:
> >>
> >> len += snprintf(buffer, PAGE_SIZE, "From: %s\nSignal: %s\n",
> >> object, signal);
> >>
> >
> >Fair enough. I guess what we want, exactly, is:
> >
> > len = snprintf(buffer, PAGE_SIZE, "From: %s\n", object);
> > len += snprintf(&buffer[len], PAGE_SIZE - len "Signal: %s\n", signal);
> >
> >I will add that to the next revision.
>
> man snprintf
>
> "If the output was truncated due to this limit then the return value
> is the number of characters (not including the trailing '\0') which
> would have been written to the final string if enough space had been
> available. Thus, a return value of size or more means that the output
> was truncated".
>
> Never use the return value from snprintf to work out the next buffer
> position, it is not reliable when the data is truncated. The example
> above uses a second call to snprintf which will generate a warning for
> truncated data and fail safe, but not all code is that trustworthy. I

The kernel snprintf() is (and the shown warning is actually debatable.
Actually snprintf()s interface is not that optimal for easy handling,
since the size parameter is unsigned and not signed. But it is specified
in SUSv3 that way.).

> always use strlen to get the real buffer length.
>
> snprintf(buffer, PAGE_SIZE, "From: %s\n", object);
> len = strlen(buffer);
> snprintf(buffer+len, PAGE_SIZE - len, "Signal: %s\n", signal);

Since "overflows" only occur if the destination buffer is full, one
could avoid the following snppritnf()s completely. Let alone the call of
strlen()
---- snip ----
len = snprintf(buffer, PAGE_SIZE, "From: %s\n", object);
if (len < PAGE_SIZE) {
len += snprintf(buffer+len, PAGE_SIZE - len,
"Signal: %s\n", signal);
}
---- snip ----

Bernd
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