Re: [RFC PATCH 2/8] jump label v4 - x86: Introduce generic jump patching without stop_machine

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Steven Rostedt (rostedt@goodmis.org) wrote:
> On Tue, 2010-01-12 at 11:26 -0500, Jason Baron wrote:
> 
> > +/**
> > + * text_poke_fixup() -- cross-modifying kernel text with fixup address.
> > + * @addr:	Modifying address.
> > + * @opcode:	New instruction.
> > + * @len:	length of modifying bytes.
> > + * @fixup:	Fixup address.
> > + *
> > + * Note: You must backup replaced instructions before calling this,
> > + * if you need to recover it.
> > + * Note: Must be called under text_mutex.
> > + */
> > +void *__kprobes text_poke_fixup(void *addr, const void *opcode, size_t len,
> > +				void *fixup)
> > +{
> > +	static const unsigned char int3_insn = BREAKPOINT_INSTRUCTION;
> > +	static const int int3_size = sizeof(int3_insn);
> > +
> > +	/* Replacing 1 byte can be done atomically. */
> > +	if (unlikely(len <= 1))
> > +		return text_poke(addr, opcode, len);
> 
> This part bothers me. The text_poke just writes over the text directly
> (using a separate mapping). But if that memory is in the pipeline of
> another CPU, I think this could cause a GPF.

It looks like we are thinking along the same lines.

I'm under the impression that I pointed out this exact same issue in the
previous round of review a few weeks ago. Does this submission reflect
the up-to-date state of this patch ?

Thanks,

Mathieu

> 
> -- Steve
> 
> > +
> > +	/* Preparing */
> > +	patch_fixup_addr = fixup;
> > +	wmb();
> > +	patch_fixup_from = (u8 *)addr + int3_size; /* IP address after int3 */
> > +
> > +	/* Cap by an int3 */
> > +	text_poke(addr, &int3_insn, int3_size);
> > +	sync_core_all();
> > +
> > +	/* Replace tail bytes */
> > +	text_poke((char *)addr + int3_size, (const char *)opcode + int3_size,
> > +		  len - int3_size);
> > +	sync_core_all();
> > +
> > +	/* Replace int3 with head byte */
> > +	text_poke(addr, opcode, int3_size);
> > +	sync_core_all();
> > +
> > +	/* Cleanup */
> > +	patch_fixup_from = NULL;
> > +	wmb();
> > +	return addr;
> > +}
> > +
> 

-- 
Mathieu Desnoyers
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