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Subject: Re: [syzbot] KASAN: stack-out-of-bounds Read in profile_pc
To:     Josh Poimboeuf <jpoimboe@redhat.com>,
        syzbot <syzbot+84fe685c02cd112a2ac3@syzkaller.appspotmail.com>
Cc:     bp@alien8.de, hpa@zytor.com, inglorion@google.com,
        linux-kernel@vger.kernel.org, mingo@redhat.com,
        syzkaller-bugs@googlegroups.com, tglx@linutronix.de,
        x86@kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>
References: <00000000000030293b05c39afd6f@google.com>
 <20210602230054.vyqama2q3koc4bpo@treble>
From:   Andi Kleen <ak@linux.intel.com>
Message-ID: <527ad07e-eec2-a211-03e7-afafe5196100@linux.intel.com>
Date:   Wed, 2 Jun 2021 16:35:11 -0700
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> profile_pc() assumes the return address is either directly at regs->sp,
> or one word adjacent to it due to saved flags, both of which are just
> completely wrong.  This code has probably never worked with ORC, and
> nobody noticed apparently.

I presume it used to work because the lock functions were really simple, 
but that's not true anymore.

>
> We could just use ORC to unwind to the next frame.  Though, isn't
> /proc/profile redundant, compared to all the more sophisticated options
> nowadays?  Is there still a distinct use case for it or can we just
> remove it?

It's still needed for some special cases. For example there is no other 
viable way to profile early boot without a VM

I would just drop the hack to unwind, at least for the early boot 
profile use case locking profiling is usually not needed.

-Andi