Re: [PATCH 1/4] stringbuf: A string buffer implementation

Kyle Moffett
2007-10-25T00:07:44Z

On Oct 24, 2007, at 17:21:10, Matthew Wilcox wrote:
> On Wed, Oct 24, 2007 at 04:59:48PM -0400, Kyle Moffett wrote:
>> This seems unlikely to work reliably as the various "v*printf"
>> functions modify the va_list argument they are passed. It may
>> happen to work on your particular architecture depending on how
>> that argument data is passed and stored, but you probably actually
>> want to make a copy of the varargs list for the first vsnprintf call.
>
> I based what I did on how printk works:
>
> va_start(args, fmt);
> r = vprintk(fmt, args);
> va_end(args);
>
> It doesn't call va_* anywhere else. I don't claim to be a varargs
> expert, but if I'm wrong, I'm at least wrong the same way that
> printk is, so not in any way that's significant for any other
> architecture Linux runs on.

No, the problem is what happens when you don't have enough space
allocated: you call "vsnprintf(s, len, format, args);" and then
later call "vsprintf(s, format, args);" with the *SAME* "args".
That's what's broken.

So this is wrong:
> va_list args;
> va_start(args, fmt);
> r1 = vprintk(fmt, args);
> r2 = vprintk(fmt, args);
> va_end(args);

To fix it, you have 2 options.

Option 1:
> va_list args;
> va_start(args, fmt);
> r1 = vprintk(fmt, args);
> va_end(args);
> va_start(args, fmt);
> r2 = vprintk(fmt, args);
> va_end(args);

Option 2:
> va_list args, argscopy;
> va_start(args, fmt);
> va_copy(argscopy, args);
> r1 = vprintk(fmt, argscopy);
> va_end(argscopy);
> r2 = vprintk(fmt, args);
> va_end(args);

Now in a function which *receives* a va_list from one of its callers,
"Option 1" isn't an option because you don't have the original stack
frame, so the result looks like this:

> void func1(const char *fmt, ...)
> {
> va_list ap;
> va_start(ap, fmt);
> func2(fmt, ap);
> va_end(ap);
> }
>
> void func2(const char *fmt, va_list ap)
> {
> va_list ap2;
> va_copy(ap2, ap);
> vprintk(fmt, ap2);
> va_end(ap2);
> vprintk(fmt, ap);
> }

Cheers,
Kyle Moffett