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

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Date:   Wed, 2 Mar 2022 20:32:45 +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 09:47:03PM +0530, Naveen N. Rao wrote:

> With respect to kprobe_lookup_name(), one of the primary motivations there
> was the issue with function descriptors for the previous elf v1 ABI (it
> likely also affects ia64/parisc). I'm thinking it'll be simpler if we have a
> way to obtain function entry point. Something like this:
> 
> diff --git a/include/linux/kallsyms.h b/include/linux/kallsyms.h
> index 4176c7eca7b5aa..8c57cc5b77f9ae 100644
> --- a/include/linux/kallsyms.h
> +++ b/include/linux/kallsyms.h
> @@ -73,6 +73,12 @@ int kallsyms_on_each_symbol(int (*fn)(void *, const char *, struct module *,
> /* Lookup the address for a symbol. Returns 0 if not found. */
> unsigned long kallsyms_lookup_name(const char *name);
> 
> +/* Return function entry point by additionally dereferencing function descriptor */
> +static inline unsigned long kallsyms_lookup_function(const char *name)
> +{
> +	return (unsigned long)dereference_symbol_descriptor((void *)kallsyms_lookup_name(name));
> +}
> +
> extern int kallsyms_lookup_size_offset(unsigned long addr,
> 				  unsigned long *symbolsize,
> 				  unsigned long *offset);
> @@ -103,6 +109,11 @@ static inline unsigned long kallsyms_lookup_name(const char *name)
> 	return 0;
> }
> 
> +static inline unsigned long kallsyms_lookup_function(const char *name)
> +{
> +	return 0;
> +}
> +
> static inline int kallsyms_lookup_size_offset(unsigned long addr,
> 					      unsigned long *symbolsize,
> 					      unsigned long *offset)
> 
> 
> With that, we can fold some of the code from kprobe_lookup_name() into
> arch_adjust_kprobe_addr() and remove kprobe_lookup_name(). This should also
> address Masami's concerns with powerpc promoting all probes at function
> entry into a probe at the ftrace location.

Yes, this looks entirely reasonable to me.

> ---

> kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
> 					 bool *on_func_entry)
> {
> 	*on_func_entry = arch_kprobe_on_func_entry(offset);
> +#ifdef PPC64_ELF_ABI_v2
> +	/* Promote probes on the GEP to the LEP */
> +	if (!offset)
> +		addr = ppc_function_entry((void *)addr);
> +#endif
> 	return (kprobe_opcode_t *)(addr + offset);
> }

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);
}

Then you can also go and delete the arch_kprobe_on_func_entry rudment.

For the rest, Yay at cleanups!, lots of magic code gone.