linux-kernel - Re: [PATCH v3] vsprintf: Prevent crash when dereferencing invalid pointers

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Date:   Tue, 03 Apr 2018 14:54:18 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Tobin C . Harding" <me@...in.cc>, Joe Perches <joe@...ches.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        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>
Subject: Re: [PATCH v3] vsprintf: Prevent crash when dereferencing invalid
 pointers

On Tue, 2018-04-03 at 13:46 +0200, Petr Mladek wrote:
> On Mon 2018-04-02 17:15:23, Andy Shevchenko wrote:
> > On Thu, 2018-03-29 at 16:53 +0200, Petr Mladek wrote:
> > > On Fri 2018-03-16 20:19:35, Andy Shevchenko wrote:
> > > > On Thu, 2018-03-15 at 16:26 +0100, Petr Mladek wrote:
> > > > > On Thu 2018-03-15 15:09:03, Andy Shevchenko wrote:
> > > > > > I still think that printing a hex value of the error code is
> > > > > > much
> > > > > > better
> > > > > > than some odd "(efault)".
> > > > > 
> > > > > Do you mean (err:0e)? Google gives rather confusing answers
> > > > > for
> > > > > this.
> > > > 
> > > > More like "(0xHHHH)" (we have already more than 512 error code
> > > > numbers.
> > > 
> > > Hmm, I have never seen the error code in this form.
> > 
> > We have limited space to print it and error numbers currently can be
> > up
> > to 0xfff (4095). So, I have no better idea how to squeeze them while
> > thinking that "(efault)" is much harder to parse in case of error
> > pointer.
> 
> But this will not be used instead of address value. It is used in
> situations
> where we print the information that is stored at the address, for
> example,
> string, IP address, dentry name.

We have a lot of API functions which returns:
-ERR_PTR
NULL
struct foo *

There is no guarantee that one of that API won't be used as a supplier
for printf().

You can't dereference ERR_PTR value, but anything else except the actual
error value is worse than value itself...

> 
> > >  Also google gives
> > > rather confusing results when searching, for example for
> > > "(0x000E)".
> > 
> > It's not primarily for google, though yeah, people would google for
> > error messages...
> > 
> > Another question is what the format: decimal versus hex for errors.
> > Maybe just "(-DDDDD)"?
> 
> This still looks confusing and google does not help.

...then we have a last option just to print a value as a pointer
address.

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy