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[115.64.212.199]) by smtp.gmail.com with ESMTPSA id q18sm8868134pgt.74.2020.05.23.17.01.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 23 May 2020 17:01:18 -0700 (PDT) Date: Sun, 24 May 2020 10:01:12 +1000 From: Nicholas Piggin Subject: Re: [PATCH v2] powerpc: Add ppc_inst_next() To: linuxppc-dev@ozlabs.org, Michael Ellerman References: <20200522133318.1681406-1-mpe@ellerman.id.au> <1590277817.8ax5p94lur.astroid@bobo.none> In-Reply-To: <1590277817.8ax5p94lur.astroid@bobo.none> MIME-Version: 1.0 Message-Id: <1590278278.q0bfj10pkn.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: christophe.leroy@c-s.fr, jniethe5@gmail.com Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Excerpts from Nicholas Piggin's message of May 24, 2020 9:56 am: > Excerpts from Michael Ellerman's message of May 22, 2020 11:33 pm: >> In a few places we want to calculate the address of the next >> instruction. Previously that was simple, we just added 4 bytes, or if >> using a u32 * we incremented that pointer by 1. >>=20 >> But prefixed instructions make it more complicated, we need to advance >> by either 4 or 8 bytes depending on the actual instruction. We also >> can't do pointer arithmetic using struct ppc_inst, because it is >> always 8 bytes in size on 64-bit, even though we might only need to >> advance by 4 bytes. >>=20 >> So add a ppc_inst_next() helper which calculates the location of the >> next instruction, if the given instruction was located at the given >> address. Note the instruction doesn't need to actually be at the >> address in memory. >>=20 >> Although it would seem natural for the value to be passed by value, >> that makes it too easy to write a loop that will read off the end of a >> page, eg: >>=20 >> for (; src < end; src =3D ppc_inst_next(src, *src), >> dest =3D ppc_inst_next(dest, *dest)) >>=20 >> As noticed by Christophe and Jordan, if end is the exact end of a >> page, and the next page is not mapped, this will fault, because *dest >> will read 8 bytes, 4 bytes into the next page. >>=20 >> So value is passed by reference, so the helper can be careful to use >> ppc_inst_read() on it. >>=20 >> Signed-off-by: Michael Ellerman >> --- >> arch/powerpc/include/asm/inst.h | 13 +++++++++++++ >> arch/powerpc/kernel/uprobes.c | 2 +- >> arch/powerpc/lib/feature-fixups.c | 15 ++++++++------- >> arch/powerpc/xmon/xmon.c | 2 +- >> 4 files changed, 23 insertions(+), 9 deletions(-) >>=20 >> v2: Pass the value as a pointer and use ppc_inst_read() on it. >>=20 >> diff --git a/arch/powerpc/include/asm/inst.h b/arch/powerpc/include/asm/= inst.h >> index d82e0c99cfa1..5b756ba77ed2 100644 >> --- a/arch/powerpc/include/asm/inst.h >> +++ b/arch/powerpc/include/asm/inst.h >> @@ -100,6 +100,19 @@ static inline int ppc_inst_len(struct ppc_inst x) >> return ppc_inst_prefixed(x) ? 8 : 4; >> } >> =20 >> +/* >> + * Return the address of the next instruction, if the instruction @valu= e was >> + * located at @location. >> + */ >> +static inline struct ppc_inst *ppc_inst_next(void *location, struct ppc= _inst *value) >> +{ >> + struct ppc_inst tmp; >> + >> + tmp =3D ppc_inst_read(value); >> + >> + return location + ppc_inst_len(tmp); >> +} >> + >> int probe_user_read_inst(struct ppc_inst *inst, >> struct ppc_inst __user *nip); >> =20 >> diff --git a/arch/powerpc/kernel/uprobes.c b/arch/powerpc/kernel/uprobes= .c >> index 83e883e1a42d..d200e7df7167 100644 >> --- a/arch/powerpc/kernel/uprobes.c >> +++ b/arch/powerpc/kernel/uprobes.c >> @@ -112,7 +112,7 @@ int arch_uprobe_post_xol(struct arch_uprobe *auprobe= , struct pt_regs *regs) >> * support doesn't exist and have to fix-up the next instruction >> * to be executed. >> */ >> - regs->nip =3D utask->vaddr + ppc_inst_len(ppc_inst_read(&auprobe->insn= )); >> + regs->nip =3D (unsigned long)ppc_inst_next((void *)utask->vaddr, &aupr= obe->insn); >> =20 >> user_disable_single_step(current); >> return 0; >=20 > AFAIKS except for here, there is no need for the void *location arg. >=20 > I would prefer to drop that and keep this unchanged (it's a slightly=20 > special case anyway). Ooops, I didn't read the last thread. I don't think insert_bpts needs it=20 though, only this case. Anyway it's a minor nitpick so if it's already=20 been considered, fine. Thanks, Nick