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[61.68.186.125]) by smtp.gmail.com with ESMTPSA id e12sm17074775pfd.69.2020.07.05.18.15.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 05 Jul 2020 18:15:27 -0700 (PDT) Date: Mon, 06 Jul 2020 11:15:22 +1000 From: Nicholas Piggin Subject: Re: [RFC PATCH 4/5] powerpc/mm: Remove custom stack expansion checking To: Christophe Leroy , linuxppc-dev@ozlabs.org, Michael Ellerman Cc: hughd@google.com, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org References: <20200703141327.1732550-1-mpe@ellerman.id.au> <20200703141327.1732550-4-mpe@ellerman.id.au> In-Reply-To: MIME-Version: 1.0 Message-Id: <1593997323.8pwn48yz8u.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Excerpts from Christophe Leroy's message of July 6, 2020 3:49 am: >=20 >=20 > Le 03/07/2020 =C3=A0 16:13, Michael Ellerman a =C3=A9crit=C2=A0: >> We have powerpc specific logic in our page fault handling to decide if >> an access to an unmapped address below the stack pointer should expand >> the stack VMA. >>=20 >> The logic aims to prevent userspace from doing bad accesses below the >> stack pointer. However as long as the stack is < 1MB in size, we allow >> all accesses without further checks. Adding some debug I see that I >> can do a full kernel build and LTP run, and not a single process has >> used more than 1MB of stack. So for the majority of processes the >> logic never even fires. >>=20 >> We also recently found a nasty bug in this code which could cause >> userspace programs to be killed during signal delivery. It went >> unnoticed presumably because most processes use < 1MB of stack. >>=20 >> The generic mm code has also grown support for stack guard pages since >> this code was originally written, so the most heinous case of the >> stack expanding into other mappings is now handled for us. >>=20 >> Finally although some other arches have special logic in this path, >> from what I can tell none of x86, arm64, arm and s390 impose any extra >> checks other than those in expand_stack(). >>=20 >> So drop our complicated logic and like other architectures just let >> the stack expand as long as its within the rlimit. >=20 > I agree that's probably not worth a so complicated logic that is nowhere=20 > documented. Agreed. >> @@ -569,30 +488,15 @@ static int __do_page_fault(struct pt_regs *regs, u= nsigned long address, >> vma =3D find_vma(mm, address); >> if (unlikely(!vma)) >> return bad_area(regs, address); >> - if (likely(vma->vm_start <=3D address)) >> - goto good_area; >> - if (unlikely(!(vma->vm_flags & VM_GROWSDOWN))) >> - return bad_area(regs, address); >> =20 >> - /* The stack is being expanded, check if it's valid */ >> - if (unlikely(bad_stack_expansion(regs, address, vma, flags, >> - &must_retry))) { >> - if (!must_retry) >> + if (unlikely(vma->vm_start > address)) { >> + if (unlikely(!(vma->vm_flags & VM_GROWSDOWN))) >=20 > We are already in an unlikely() branch, I don't think it is worth having=20 > a second level of unlikely(), better let gcc decide what's most efficient= . I'm not sure being nested matters. It does in terms of how the code is=20 generated and how much it might acutally matter, but if we say we=20 optimise the expand stack case rather than the segfault case, then=20 unlikely is fine here. I find it can be a readability aid as well. Thanks, Nick