Re: [PATCH v2 2/4] Add fchmodat4(), a new syscall

Al Viro
2019-07-17T03:02:58Z

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;
}