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Wed, 5 May 2021 10:30:17 -0700 (PDT) Date: Wed, 5 May 2021 18:30:14 +0100 From: Mark Rutland To: He Zhe Subject: Re: [PATCH v2 2/3] arm64: syscall.h: Add sign extension handling in syscall_get_return_value for compat Message-ID: <20210505173014.GE5605@C02TD0UTHF1T.local> References: <20210423103533.30121-1-zhe.he@windriver.com> <20210423103533.30121-2-zhe.he@windriver.com> MIME-Version: 1.0 In-Reply-To: <20210423103533.30121-2-zhe.he@windriver.com> X-Mimecast-Impersonation-Protect: Policy=CLT - Impersonation Protection Definition; Similar Internal Domain=false; Similar Monitored External Domain=false; Custom External Domain=false; Mimecast External Domain=false; Newly Observed Domain=false; Internal User Name=false; Custom Display Name List=false; Reply-to Address Mismatch=false; Targeted Threat Dictionary=false; Mimecast Threat Dictionary=false; Custom Threat Dictionary=false X-Scanned-By: MIMEDefang 2.78 on 10.11.54.3 X-loop: linux-audit@redhat.com X-Mailman-Approved-At: Wed, 05 May 2021 14:30:25 -0400 Cc: catalin.marinas@arm.com, oleg@redhat.com, eparis@redhat.com, linux-kernel@vger.kernel.org, linux-audit@redhat.com, will@kernel.org, linux-arm-kernel@lists.infradead.org X-BeenThere: linux-audit@redhat.com X-Mailman-Version: 2.1.12 Precedence: junk List-Id: Linux Audit Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: linux-audit-bounces@redhat.com Errors-To: linux-audit-bounces@redhat.com X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=linux-audit-bounces@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Hi, On Fri, Apr 23, 2021 at 06:35:32PM +0800, He Zhe wrote: > Add sign extension handling in syscall_get_return_value so that it can > handle 32-bit compatible case and can be used by for example audit, just > like what syscall_get_error does. > > Suggested-by: Mark Rutland > Signed-off-by: He Zhe > --- > v1 to v2: Improve error code check suggested by Mark > > arch/arm64/include/asm/syscall.h | 15 ++++++++++++++- > 1 file changed, 14 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h > index cfc0672013f6..c3b5fca82ff4 100644 > --- a/arch/arm64/include/asm/syscall.h > +++ b/arch/arm64/include/asm/syscall.h > @@ -44,7 +44,20 @@ static inline long syscall_get_error(struct task_struct *task, > static inline long syscall_get_return_value(struct task_struct *task, > struct pt_regs *regs) > { > - return regs->regs[0]; > + long val = regs->regs[0]; > + long error = val; > + > + if (compat_user_mode(regs)) > + error = sign_extend64(error, 31); > + > + /* > + * Return codes with bit 31 set may or may not be an error code. > + * For example, mmap may return a legal 32 bit address with bit 31 set > + * for 32 bit thread, in which case the untouched val should be > + * returned. Otherwise, the sign-extended error should be returned if > + * it still falls in error number range. > + */ > + return IS_ERR_VALUE(error) ? error : val; I'm afraid I have misled you here. I wrote up a test that uses PTRACE_GET_SYSCALL_INFO, and I found that on a 32-bit arm (v5.12) kernel, *all* syscall return values get sign-extended after all. For example, if (on a 32-bit kernel) I use MAP_FIXED to mmap() at address 0x8bad0000, the return value reported in ptrace_syscall_info::exit::rval is 0xffffffff8bad0000. So for that we shoudn't have the IS_ERR_VALUE() check after all, but I'm not currently sure whether there are other cases where 32-bit arm wouldn't sign-extend, and I think we'll need to dig into this some more. Thanks, Mark. -- Linux-audit mailing list Linux-audit@redhat.com https://listman.redhat.com/mailman/listinfo/linux-audit