Re: [PATCH 2/2] vfs: support statx(..., NULL, AT_EMPTY_PATH, ...)

[Christian Brauner <brauner@kernel.org>]

On Tue, Jul 02, 2024 at 07:06:53PM GMT, Arnd Bergmann wrote:
> On Tue, Jul 2, 2024, at 17:36, Huacai Chen wrote:
> > On Mon, Jul 1, 2024 at 7:59 PM Arnd Bergmann <arnd@arndb.de> wrote:
> >> On Sun, Jun 30, 2024, at 04:39, Xi Ruoyao wrote:
> >> > On Sun, 2024-06-30 at 09:40 +0800, Huacai Chen wrote:
> >> >> >
> >> >> > Yes, both Linus and Christian hates introducing a new AT_ flag for
> >> >> > this.
> >> >> >
> >> >> > This patch just makes statx(fd, NULL, AT_EMPTY_PATH, ...) behave
> >> >> > like
> >> >> > statx(fd, "", AT_EMPTY_PATH, ...) instead.  NULL avoids the
> >> >> > performance
> >> >> > issue and it's also audit-able by seccomp BPF.
> >> >> To be honest, I still want to restore __ARCH_WANT_NEW_STAT. Because
> >> >> even if statx() becomes audit-able, it is still blacklisted now.
> >> >
> >> > Then patch the sandbox to allow it.
> >> >
> >> > The sandbox **must** be patched anyway or it'll be broken on all 32-bit
> >> > systems after 2037.  [Unless they'll unsupport all 32-bit systems before
> >> > 2037.]
> >>
> >> More importantly, the sandbox won't be able to support any 32-bit
> >> targets that support running after 2037, regardless of how long
> >> the sandbox supports them: if you turn off COMPAT_32BIT_TIME today
> >> in order to be sure those don't get called by accident, the
> >> fallback is immediately broken.
> > Would you mind if I restore newstat for LoongArch64 even if this patch exist?
> 
> I still prefer not add newstat back: it's easier to
> get applications to correctly implement the statx() code
> path if there are more architectures that only have that.

I agree.

We've now added AT_EMPTY_PATH support with NULL names because we want to
allow that generically. But I clearly remember that this was requested
to make statx() work with these sandboxes. So the kernel has done its
part. Now it's for the sandbox to allow statx() with NULL paths and
AT_EMPTY_PATH but certainly not for the kernel to start reenabling old
system calls.