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Message-ID: <20200420222428.GB77284@google.com>
Date: Mon, 20 Apr 2020 15:24:28 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
James Morse <james.morse@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Mark Rutland <mark.rutland@....com>,
Masahiro Yamada <masahiroy@...nel.org>,
Michal Marek <michal.lkml@...kovi.net>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dave Martin <Dave.Martin@....com>,
Kees Cook <keescook@...omium.org>,
Laura Abbott <labbott@...hat.com>, Marc Zyngier <maz@...nel.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Jann Horn <jannh@...gle.com>,
Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
clang-built-linux@...glegroups.com,
kernel-hardening@...ts.openwall.com,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 03/12] scs: add support for stack usage debugging
On Mon, Apr 20, 2020 at 06:17:42PM +0100, Will Deacon wrote:
> > +#ifdef CONFIG_DEBUG_STACK_USAGE
> > +static inline unsigned long scs_used(struct task_struct *tsk)
> > +{
> > + unsigned long *p = __scs_base(tsk);
> > + unsigned long *end = scs_magic(p);
> > + unsigned long s = (unsigned long)p;
> > +
> > + while (p < end && READ_ONCE_NOCHECK(*p))
> > + p++;
>
> I think the expectation is that the caller has already checked that the
> stack is not corrupted, so I'd probably throw a couple of underscores
> in front of the function name, along with a comment.
Correct. I'll do that.
> Also, is tsk ever != current?
This is only called from scs_release(), so tsk is never current.
> > +static void scs_check_usage(struct task_struct *tsk)
> > +{
> > + static DEFINE_SPINLOCK(lock);
> > + static unsigned long highest;
> > + unsigned long used = scs_used(tsk);
> > +
> > + if (used <= highest)
> > + return;
> > +
> > + spin_lock(&lock);
> > +
> > + if (used > highest) {
> > + pr_info("%s (%d): highest shadow stack usage: %lu bytes\n",
> > + tsk->comm, task_pid_nr(tsk), used);
> > + highest = used;
> > + }
> > +
> > + spin_unlock(&lock);
>
> Do you really need this lock? I'd have thought you could cmpxchg()
> highest instead.
This is similar to check_stack_usage in kernel/exit.c, but yes, I can
change this to a cmpxchg() loop instead.
Sami
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