linux-kernel - Re: [PATCH v3 2/7] kprobes: Show blacklist addresses as same as kallsyms does

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Date:   Fri, 27 Apr 2018 09:14:17 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Ananth N Mavinakayanahalli <ananth@...ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jon Medhurst <tixy@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "Tobin C . Harding" <me@...in.cc>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        akpm@...ux-foundation.org, acme@...nel.org, rostedt@...dmis.org,
        brueckner@...ux.vnet.ibm.com, schwidefsky@...ibm.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v3 2/7] kprobes: Show blacklist addresses as same as
 kallsyms does


* Masami Hiramatsu <mhiramat@...nel.org> wrote:

> +	/*
> +	 * As long as kallsyms shows the address, kprobes blacklist also
> +	 * show it, Or, it shows null address and symbol.
> +	 */

Please _read_ the comments you write!

In which universe does a capitalized 'Or' make sense, even if we ignore the 
various other spelling mistakes?

Also, that sentence is unnecessarily complex, just say this:

> +	/*
> +	 * If /proc/kallsyms is not showing kernel addresses then we won't show 
> +      * them here either:
> +	 */

But I'm unhappy about the messy typing and the messy code flow:

+       void *start = (void *)ent->start_addr, *end = (void *)ent->end_addr;

+       /*
+        * As long as kallsyms shows the address, kprobes blacklist also
+        * show it, Or, it shows null address and symbol.
+        */
+       if (!kallsyms_show_value())
+               start = end = NULL;
+
+       seq_printf(m, "0x%px-0x%px\t%ps\n", start, end,
+                  (void *)ent->start_addr);


All three 'void *' type casts here are due to the bad type choices here:

struct kprobe_blacklist_entry {
        struct list_head list;
        unsigned long start_addr;
        unsigned long end_addr;
};

The natural type of ->start_addr and ->end_addr is 'void *', AFAICS this would 
remove some other type casts from the kprobes code as well, such as from the 
arch_deref_entry_point()...

But the whole code flow introduced by this patch is messy as hell as well.
Why cannot this do the obvious thing:

	if (!kallsyms_show_value())
		seq_printf(m, "0x%px-0x%px\t%ps\n", NULL, NULL, ent->start_addr);
	else
		seq_printf(m, "0x%px-0x%px\t%ps\n", ent->start_addr, ent->end_addr, ent->start_addr);

?

This variant eliminates the unnecessary complication over local variables and 
makes it abundantly clear what gets printed and how.

( Note that the kprobe_blacklist_entry type cleanup should still be done, 
  regardless of code flow choices. )

Thanks,

	Ingo