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SubjectRe: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions
On Tue, Jun 11, 2019 at 11:14:10AM -0400, Steven Rostedt wrote:
> On Wed, 05 Jun 2019 15:08:01 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
>
> > -void text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
> > +void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate)
> > {
> > unsigned char int3 = 0xcc;
> >
> > - bp_int3_handler = handler;
> > + bp_int3_opcode = emulate ?: opcode;
> > bp_int3_addr = (u8 *)addr + sizeof(int3);
> > bp_patching_in_progress = true;
> >
> > lockdep_assert_held(&text_mutex);
> >
> > /*
> > + * poke_int3_handler() relies on @opcode being a 5 byte instruction;
> > + * notably a JMP, CALL or NOP5_ATOMIC.
> > + */
> > + BUG_ON(len != 5);
>
> If we have a bug on here, why bother with passing in len at all? Just
> force it to be 5.

Masami said the same.

> We could make it a WARN_ON() and return without doing anything.
>
> This also prevents us from ever changing two byte jmps.

It doesn't; that is, we'd need to add emulation for the 3 byte jump, but
that'd be pretty trivial.

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