linux-kernel - Re: [PATCH] Add a text

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Message-ID: <20131122032934.GK29695@two.firstfloor.org>
Date:	Fri, 22 Nov 2013 04:29:34 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] Add a text_poke syscall

Thanks for the code review.

> This looks a bit confusing, afaics we can avoid the duplications:
> 
> 	// also handles bp_target_mm == NULL case
> 	if (user_mode_vm(regs) && current->mm != bp_target_mm)
> 		return 0;

I had that originally in my code, but I was worried there
were any threads with mm == NULL and user_mode_vm still true
(maybe some BIOS code or somesuch)

So I ended up with the longer, but safer, variant.


> > +	err = 0;
> > +	mutex_lock(&text_mutex);
> > +	bp_target_mm = current->mm;
> > +	bp_int3_addr = (u8 *)addr + 1;
> > +	__text_poke_bp(pages, 
> > +		       (unsigned long)addr & ~PAGE_MASK,
> > +		       insn, len, handler);
> 
> This doesn't look right if npages == 2 but get_user_pages_fast() returns 1.

True, i'll add an error out.

> 
> __text_poke() checks pages[1] != NULL, but in this case it assumes
> that memcpy(vaddr, opcode, len) should fit into the 1st page.
> 
> 
> Ingo, Andi, I do not think that it is good idea to implement this
> via ptrace. If nothing else, you need to fork the tracer which can
> do PTRACE_POKETEXT.

Yes I agree, ptrace is not the right way to do this.


-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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