linux-kernel - Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions

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Date:   Wed, 2 Mar 2022 20:39:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     alexei.starovoitov@...il.com, alyssa.milburn@...el.com,
        andrew.cooper3@...rix.com, hjl.tools@...il.com,
        joao@...rdrivepizza.com, jpoimboe@...hat.com,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, mbenes@...e.cz,
        Masami Hiramatsu <mhiramat@...nel.org>,
        ndesaulniers@...gle.com, rostedt@...dmis.org,
        samitolvanen@...gle.com, x86@...nel.org
Subject: Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions

On Wed, Mar 02, 2022 at 08:32:45PM +0100, Peter Zijlstra wrote:
> I wonder if you also want to tighten up on_func_entry? Wouldn't the
> above suggest something like:
> 
> kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
> 					 bool *on_func_entry)
> {
> #ifdef PPC64_ELF_ABI_V2
> 	unsigned long entry = ppc_function_entry((void *)addr) - addr;
> 	*on_func_entry = !offset || offset == entry;
> 	if (*on_func_entry)
> 		offset = entry;
> #else
> 	*on_func_entry = !offset;
> #endif
> 	return (void *)(addr + offset);
> }

One question though; the above seems to work for +0 or +8 (IIRC your
instructions are 4 bytes each and the GEP is 2 instructions).

But what do we want to happen for +4 ?