linux-kernel - Re: [PATCH] elf: don't hash userspace addresses

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Message-ID: <20180509164017.GB16034@avx2>
Date:   Wed, 9 May 2018 19:40:17 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        mhocko@...e.com
Subject: Re: [PATCH] elf: don't hash userspace addresses

On Sat, May 05, 2018 at 10:19:43PM +0900, Tetsuo Handa wrote:
> Alexey Dobriyan wrote:
> > On Sat, May 05, 2018 at 09:31:50AM +0900, Tetsuo Handa wrote:
> > > Alexey Dobriyan wrote:
> > > > Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> > > > ---
> > > > 
> > > >  fs/binfmt_elf.c |    4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > 
> > > > --- a/fs/binfmt_elf.c
> > > > +++ b/fs/binfmt_elf.c
> > > > @@ -379,8 +379,8 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
> > > >  
> > > >  	if ((type & MAP_FIXED_NOREPLACE) &&
> > > >  	    PTR_ERR((void *)map_addr) == -EEXIST)
> > > > -		pr_info("%d (%s): Uhuuh, elf segment at %px requested but the memory is mapped already\n",
> > > > -			task_pid_nr(current), current->comm, (void *)addr);
> > > > +		pr_info("%d (%s): Uhuuh, elf segment at %lx requested but the memory is mapped already\n",
> > > > +			task_pid_nr(current), current->comm, addr);
> > > 
> > > "%px" does not hash. For what reason you replace %px with %lx ?
> > 
> > Duh. There is no reason to use non-standard %px.
> > 
> 
> See the comment for pointer() function in lib/vsprintf.c .
> In Linux kernel world, various extension for %p is supported.

%p has this funny property: scanf(printf("%p", ptr)) does not equal %p
because small pointer will become (null) or (errptr) or whatever is
decided in recent vsnprintf thread, therefore the less %p is used
the better.