linux-kernel - Re: [PATCHv3 6/7] symbol lookup: use new kernel and module dereference functions

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Date:   Thu, 19 Oct 2017 15:42:35 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        James Bottomley <jejb@...isc-linux.org>,
        Helge Deller <deller@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3 6/7] symbol lookup: use new kernel and module
 dereference functions

Sorry for the delay and thanks for taking a look.

I'll try to re-spin the patch set by the end of this week/early next
week.


On (10/04/17 13:53), Petr Mladek wrote:
[..]
> Note that kallsyms_lookup() and module_address_lookup() is used
> in many other situations.

we dereference only things that can be dereferenced.
so calling it on already dereferenced address, or address
that does need to be dereferenced is OK.

besides, not all of those "other" places are available on
ppc64, ia64, parisc.

[..]
> > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> > index 127e7cfafa55..e2fc09ea9509 100644
> > --- a/kernel/kallsyms.c
> > +++ b/kernel/kallsyms.c
> > @@ -322,6 +322,7 @@ const char *kallsyms_lookup(unsigned long addr,
> >  	if (is_ksym_addr(addr)) {
> 
> is_ksym_addr() ignores the special .opd elf sections if
> CONFIG_KALLSYMS_ALL is disabled. We should dereference before
> this call.

I'll move it.

> >  		unsigned long pos;
> >  
> > +		addr = dereference_kernel_function_descriptor(addr);
> >  		pos = get_symbol_pos(addr, symbolsize, offset);
> 
> I still wonder if doing the dereference in the widely used kallsyms
> might cause any regression.

more testing wouldn't hurt, yes.

> Also get_symbol_pos() is called in several other helpers
> but the dereference is done only here. It would be
> confusing if for example kallsyms_lookup_size_offset()
> and kallsyms_lookup() give different result.

hm, so there is no change in this regard, right? there was no
deference before, there is no dereference now. what am I missing?


I'm touching the pf/pF part in this patch set. if there are cases
of missing dereferences anywhere else then we need to address it
in a separate patch set, I think.

> I would feel much more comfortable if we keep the derefenrece
> only in vsprintf.

at a price of extra module lookup, because we need `struct module *'
for module address dereference.

	-ss