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Message-ID: <20230809113021.63e5ef66@gandalf.local.home>
Date: Wed, 9 Aug 2023 11:30:21 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Marco Elver <elver@...gle.com>
Cc: Kees Cook <keescook@...omium.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Guenter Roeck <linux@...ck-us.net>,
Peter Zijlstra <peterz@...radead.org>,
Mark Rutland <mark.rutland@....com>,
Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>,
James Morse <james.morse@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Zenghui Yu <yuzenghui@...wei.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Tom Rix <trix@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
Sami Tolvanen <samitolvanen@...gle.com>,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
Dmitry Vyukov <dvyukov@...gle.com>,
Alexander Potapenko <glider@...gle.com>,
kasan-dev@...glegroups.com, linux-toolchains@...r.kernel.org
Subject: Re: [PATCH v3 3/3] list_debug: Introduce CONFIG_DEBUG_LIST_MINIMAL
On Wed, 9 Aug 2023 11:57:19 +0200
Marco Elver <elver@...gle.com> wrote:
> static __always_inline bool __list_add_valid(struct list_head *new,
> struct list_head *prev,
> struct list_head *next)
> {
> - return __list_add_valid_or_report(new, prev, next);
> + bool ret = true;
> +
> + if (IS_ENABLED(CONFIG_HARDEN_LIST)) {
> + /*
> + * With the hardening version, elide checking if next and prev
> + * are NULL, since the immediate dereference of them below would
> + * result in a fault if NULL.
> + *
> + * With the reduced set of checks, we can afford to inline the
> + * checks, which also gives the compiler a chance to elide some
> + * of them completely if they can be proven at compile-time. If
> + * one of the pre-conditions does not hold, the slow-path will
> + * show a report which pre-condition failed.
> + */
> + if (likely(next->prev == prev && prev->next == next && new != prev && new != next))
> + return true;
> + ret = false;
> + }
> +
> + ret &= __list_add_valid_or_report(new, prev, next);
> + return ret;
> }
I would actually prefer DEBUG_LIST to select HARDEN_LIST and not the other
way around. It logically doesn't make sense that HARDEN_LIST would select
DEBUG_LIST. That is, I could by default want HARDEN_LIST always on, but not
DEBUG_LIST (because who knows, it may add other features I don't want). But
then, I may have stumbled over something and want more info, and enable
DEBUG_LIST (while still having HARDEN_LIST) enabled.
I think you are looking at this from an implementation perspective and not
the normal developer one.
This would mean the above function should get enabled by CONFIG_HARDEN_LIST
(and CONFIG_DEBUG would select CONFIG_HARDEN) and would look more like:
static __always_inline bool __list_add_valid(struct list_head *new,
struct list_head *prev,
struct list_head *next)
{
bool ret = true;
if (!IS_ENABLED(CONFIG_DEBUG_LIST)) {
/*
* With the hardening version, elide checking if next and prev
* are NULL, since the immediate dereference of them below would
* result in a fault if NULL.
*
* With the reduced set of checks, we can afford to inline the
* checks, which also gives the compiler a chance to elide some
* of them completely if they can be proven at compile-time. If
* one of the pre-conditions does not hold, the slow-path will
* show a report which pre-condition failed.
*/
if (likely(next->prev == prev && prev->next == next && new != prev && new != next))
return true;
ret = false;
}
ret &= __list_add_valid_or_report(new, prev, next);
return ret;
}
That is, if DEBUG_LIST is enabled, we always call the
__list_add_valid_or_report(), but if only HARDEN_LIST is enabled, then we
do the shortcut.
-- Steve
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