From mboxrd@z Thu Jan 1 00:00:00 1970 From: takahiro.akashi@linaro.org (AKASHI Takahiro) Date: Wed, 23 Jul 2014 18:09:11 +0900 Subject: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations In-Reply-To: <20140723082505.GB27260@arm.com> References: <1406020499-5537-1-git-send-email-takahiro.akashi@linaro.org> <1406020499-5537-2-git-send-email-takahiro.akashi@linaro.org> <53CF5E53.3060409@linaro.org> <20140723082505.GB27260@arm.com> Message-ID: <53CF7BB7.8080509@linaro.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 07/23/2014 05:25 PM, Will Deacon wrote: > On Wed, Jul 23, 2014 at 08:03:47AM +0100, AKASHI Takahiro wrote: >> On 07/23/2014 05:15 AM, Kees Cook wrote: >>> On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro >>> wrote: >>>> asmlinkage int syscall_trace_enter(struct pt_regs *regs) >>>> { >>>> + unsigned long saved_x0, saved_x8; >>>> + >>>> + saved_x0 = regs->regs[0]; >>>> + saved_x8 = regs->regs[8]; >>>> + >>>> if (test_thread_flag(TIF_SYSCALL_TRACE)) >>>> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); >>>> >>>> + regs->syscallno = regs->regs[8]; >>>> + if ((long)regs->syscallno == ~0UL) { /* skip this syscall */ >>>> + regs->regs[8] = saved_x8; >>>> + if (regs->regs[0] == saved_x0) /* not changed by user */ >>>> + regs->regs[0] = -ENOSYS; >>> >>> I'm not sure this is right compared to other architectures. Generally >>> when a tracer performs a syscall skip, it's up to them to also adjust >>> the return value. They may want to be faking a syscall, and what if >>> the value they want to return happens to be what x0 was going into the >>> tracer? It would have no way to avoid this -ENOSYS case. I think >>> things are fine without this test. >> >> Yeah, I know this issue, but was not sure that setting a return value >> is mandatory. (x86 seems to return -ENOSYS by default if not explicitly >> specified.) >> Is "fake a system call" a more appropriate word than "skip"? >> >> I will defer to Will. > > I agree with Kees -- iirc, I only suggested restoring x8. OK. -Takahiro AKASHI > Will >