linux-kernel - Re: [PATCH v4 4/9] vsprintf: Consolidate handling of unknown pointer specifiers

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Date:   Thu, 5 Apr 2018 16:25:41 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Petr Mladek <pmladek@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Tobin C . Harding" <me@...in.cc>, Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/9] vsprintf: Consolidate handling of unknown pointer
 specifiers

On 2018-04-04 10:58, Petr Mladek wrote:
> There are few printk formats that make sense only with two or more
> specifiers. Also some specifiers make sense only when a kernel feature
> is enabled.
> 
> The handling of unknown specifiers is strange, inconsistent, and
> even leaking the address. For example, netdev_bits() prints the
> non-hashed pointer value or clock() prints "(null)".
> 
> The best solution seems to be in flags_string(). It does not print any
> misleading value. Instead it calls WARN_ONCE() describing the unknown
> specifier. Therefore it clearly shows the problem and helps to find it.
>

I'm not sure it's actually worth WARNing about the unknown variants
since we have static analysis (at least checkpatch and smatch) that
should catch that. Even just git grep -1 -E '%p"$' finds %pt and %po
which should get fixed before somebody claims those extensions.

But, I don't disagree with trying to fix up the inconsistency, and
certainly not with fixing netdev_bits(), but it seems you've missed that
e.g. the "case: 'g'" is completely compiled out for !CONFIG_BLOCK.
There's also %pOF which is effectively disabled for !CONFIG_OF (which
obviously makes sense), but with yet a different fallback behaviour.

Hm. I think we should somehow distinguish between the cases of "%po" and
"%pNX", i.e. specifiers/variants that are always bogus, and the cases of
a %pOF or %pC that somehow happens even though nobody should have a
struct device_node* or struct clk* to pass.

Rasmus