From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tulio Magno Quites Machado Filho Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and thread creation (v7) Date: Tue, 09 Apr 2019 10:58:34 -0300 Message-ID: <871s2bp9f9.fsf@linux.ibm.com> References: <5166fbe9-cfe0-8554-abc7-4fc844cf2765@redhat.com> <1965431879.7576.1553529272844.JavaMail.zimbra@efficios.com> <87lg0tosfz.fsf@concordia.ellerman.id.au> <87pnq4zxyj.fsf@oldenburg2.str.redhat.com> <87y34o4xt3.fsf@oldenburg2.str.redhat.com> <43f97ddb-c8df-27ea-9517-63252ebd3183@redhat.com> <877ec4pam2.fsf@linux.ibm.com> <877ec3yffq.fsf@concordia.ellerman.id.au> <20190409092948.GA14424@bubble.grove.modra.org> Mime-Version: 1.0 Content-Type: text/plain Return-path: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org In-Reply-To: <20190409092948.GA14424@bubble.grove.modra.org> To: Alan Modra , Michael Ellerman Cc: Carlos O'Donell , Florian Weimer , Michael Meissner , Peter Bergner , Mathieu Desnoyers , Paul Burton , Will Deacon , Boqun Feng , Heiko Carstens , Vasily Gorbik , Martin Schwidefsky , Russell King , Benjamin Herrenschmidt , Paul Mackerras , carlos , Joseph Myers , Szabolcs Nagy , libc-alpha , Thomas Gleixner , Ben Maurer , Peter Zijlstra , "Paul E. McKenney" , Dave Watson List-Id: linux-api@vger.kernel.org Alan Modra writes: > On Tue, Apr 09, 2019 at 02:23:53PM +1000, Michael Ellerman wrote: >> I'd much rather we use a trap with a specific immediate value. Otherwise >> someone's going to waste time one day puzzling over why userspace is >> doing mtmsr. > > It's data. We have other data in executable sections. Anyone who > wonders about odd disassembly just hasn't realized they are > disassembling data. > >> It would also complicate things if we ever wanted to emulate mtmsr. > > No, because it won't be executed. If I understand correctly, the only > reason to choose an illegal, trap or privileged insn is to halt > execution earlier rather than later when a program goes off in the > weeds. > >> If we want something that is a trap rather than a nop then use 0x0fe50553. >> >> That's "compare the value in r5 with 0x553 and then trap unconditionally". >> >> It shows up in objdump as: >> >> 10000000: 53 05 e5 0f twui r5,1363 >> >> >> The immediate can be anything, I chose that value to mimic the x86 value >> Mathieu mentioned. >> >> There's no reason that instruction would ever be generated because the >> immediate value serves no purpose. So it satisfies the "very unlikely >> to appear" criteria AFAICS. > > Yes, looks fine to me, except that in VLE mode (do we care?) > ".long 0x0fe50553" disassembles as > 0: 0f e5 se_cmphl r5,r30 > 2: 05 53 se_mullw r3,r5 > No illegal/trap/privileged insn there. > > ".long 0x0fe5000b" might be better to cover VLE. Looks good for me too. Actually, it better fits what Carlos O'Donnell had requested: >>> I think the order of preference is: >>> >>> 1. An uncommon insn (with random immediate values), in a literal pool, that is >>> not a useful ROP/JOP sequence (very uncommon) >>> 2a. A uncommon TRAP hopefully with some immediate data encoded (maybe uncommon) >>> 2b. A NOP to avoid affecting speculative execution (maybe uncommon) >>> >>> With 2a/2b being roughly equivalent depending on speculative execution policy. -- Tulio Magno