Re: [PATCH] vsprintf: drop comment claiming %n is ignored

[Kees Cook]

On Fri, Sep 13, 2013 at 3:27 PM, Joe Perches <joe@xxxxxxxxxxx> wrote:
> On Fri, 2013-09-13 at 15:53 -0400, George Spelvin wrote:
>> > Maybe WARN_ONCE so it's easier to emit the format too.
>>
>> Good idea.  And, if it's not too much trouble, a comment explaining
>> why it's deliberately omitted so the issue doesn't arise again.
>
> Before any of the %n uses could be removed, I believe seq_printf
> could to be converted to return void and have a another mechanism
> to determine if any error occurred and the length of the output of
> seq_printf.
>
> I've done a preliminary conversion of seq_printf and seq_vprintf
> to return void and added last_ret and last_len to struct seq_file.
>
> If that's applied, it's trivial to convert vsnprintf to skip %n.
>
> Anyone have an opinion of a different conversion mechanism?

Maybe I missed this somewhere in the thread, but I'm not sure I
understand the move to "void". Here's what I see, please correct me:

1- seq_printf currently returns success/failure
2- some callers of seq_printf (correctly) use the return value as
success/failure indication
3- some callers of seq_printf (incorrectly) use the return value as a
length indication
4- both success/failure and length are important outputs from seq_printf
5- we need a way to access the length written during the call
6- want to minimize impact on the code base

Due to 1 and 2, it seems like there's no sense in changing the return
value to void. Success/failure is already returned, and there are
users of it. No sense changing them.

The normal way to handle multiple return values (4 and 5) is to add a
pointer argument. For example: seq_printf(s, &len, fmt, args...) where
len can be NULL. But this runs against 6.

Due to 6, to solve 4 and 5, usually macro or inline tricks are used,
for example:

__printf(3, 4) int seq_printf_len(struct seq_file *, size_t *len, ...);
#define seq_printf(s, fmt, ...) seq_printf_len(s, NULL, fmt, ##__VA_ARGS__)

With this, solving 3 becomes possible (your void patch has already
detected all the users of the return value, so we can sort out which
expect length and which expect success/failure), and lets us actually
remove the %n uses trivially too.

-Kees

-- 
Kees Cook
Chrome OS Security
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