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

[Xi Ruoyao <xry111@xry111.site>]

On Wed, 2024-07-03 at 09:31 -0700, Linus Torvalds wrote:
> On Wed, 3 Jul 2024 at 01:46, Christian Brauner <brauner@kernel.org>
> wrote:
> > 
> > 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.
> 
> Those old system calls are still used.
> 
> Just enable them.
> 
> statx isn't the promised land. Existing applications matter. And there
> is absolutely nothing wrong with plain old 'stat' (well, we call it
> "newstat" in the kernel for historical reasons) on 64-bit
> architectures.
> 
> Honestly, 'statx' is disgusting. I don't understand why anybody pushes
> that thing that nobody actually uses or cares about.

Hmm why it was added in the first place then?  Why not just NAK it?  If
someone tries to add a "seccomp sandbox" into my project I'll
immediately NAK it anyway :).

And should we add stat_time64, fstat_time64, and fstatat_time64 to stop
using statx on 32-bit platforms too as it's disgusting?

Also some bad news: Glibc has this:

#if (__WORDSIZE == 32 \
     && (!defined __SYSCALL_WORDSIZE || __SYSCALL_WORDSIZE == 32)) \
    || defined STAT_HAS_TIME32 \
    || (!defined __NR_newfstatat && !defined __NR_fstatat64)
# define FSTATAT_USE_STATX 1
#else
# define FSTATAT_USE_STATX 0
#endif

So if a LoongArch Glibc is built with Linux kernel headers >= 6.11,
it'll use fstatat **even configured --with-kernel=5.19** and fail to run
on Linux kernel <= 6.10.  This will immediately blow up building Linux
From Scratch on a host distro with an "old" kernel.

<sarcasm>Alright, some Google project matters but Glibc does not matter
because it uses a disgusting syscall in the first place.</sarcasm>

We have to add some __ASSUME_blah_blah here now.

To make things worse Glibc 2.40 is being frozen today :(.  Copying to
libc-alpha and the RM.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University