From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: [PATCH v2 2/4] Add fchmodat4(), a new syscall Date: Wed, 17 Jul 2019 04:02:58 +0100 Message-ID: <20190717030258.GT17978@ZenIV.linux.org.uk> References: <20190717012719.5524-1-palmer@sifive.com> <20190717012719.5524-3-palmer@sifive.com> <20190717024046.GI1506@brightrain.aerifal.cx> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20190717024046.GI1506@brightrain.aerifal.cx> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Rich Felker Cc: Palmer Dabbelt , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann , rth@twiddle.net, ink@jurassic.park.msu.ru, mattst88@gmail.com, linux@armlinux.org.uk, catalin.marinas@arm.com, will@kernel.org, tony.luck@intel.com, fenghua.yu@intel.com, geert@linux-m68k.org, monstr@monstr.eu, ralf@linux-mips.org, paul.burton@mips.com, jhogan@kernel.org, James.Bottomley@hansenpartnership.com, deller@gmx.de, benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au, heiko.carstens@de.ibm.com, gor@linux.ibm.com, borntraeger@de.ibm.com, ysato@users.sourceforge.jp, davem@davemloft.net, luto@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org, peterz@infradead.org, On Tue, Jul 16, 2019 at 10:40:46PM -0400, Rich Felker wrote: > On Tue, Jul 16, 2019 at 06:27:17PM -0700, Palmer Dabbelt wrote: > > man 3p says that fchmodat() takes a flags argument, but the Linux > > syscall does not. There doesn't appear to be a good userspace > > workaround for this issue but the implementation in the kernel is pretty > > straight-forward. The specific use case where the missing flags came up > > was WRT a fuse filesystem implemenation, but the functionality is pretty > > generic so I'm assuming there would be other use cases. > > Note that we do have a workaround in musl libc with O_PATH and > /proc/self/fd, but a syscall that allows a proper fix with the ugly > workaround only in the fallback path for old kernels will be much > appreciated! > > What about also doing a new SYS_faccessat4 with working AT_EACCESS > flag? The workaround we have to do for it is far worse. Umm... That's doable, but getting into the "don't switch creds unless needed" territory. I'll need to play with that a bit and see what gives a tolerable variant... What of this part wrt AT_EACCESS? if (!issecure(SECURE_NO_SETUID_FIXUP)) { /* Clear the capabilities if we switch to a non-root user */ kuid_t root_uid = make_kuid(override_cred->user_ns, 0); if (!uid_eq(override_cred->uid, root_uid)) cap_clear(override_cred->cap_effective); else override_cred->cap_effective = override_cred->cap_permitted; }