Re: fstat(2) penalized by using newfstatat(6, "", buf, AT_EMPTY_PATH)

[Rich Felker <dalias@libc.org>]

On Tue, Sep 05, 2023 at 10:45:17AM -0700, Linus Torvalds wrote:
> On Tue, 5 Sept 2023 at 10:28, Adhemerval Zanella Netto
> <adhemerval.zanella@linaro.org> wrote:
> >
> > I think we can still make it in a generic way, stat family would use more
> > syscall (which some filters might complain) but it should be ok:
> 
> Ugh. That patch of yours certainly seems to avoid the kernel overhead
> (where that "fetch a byte from user space just to check it is '\0'" is
> much more expensive than it is in user space), but why does glibc do
> that whole "turn fstat() into fstatat(), and then turn it back again"?
> 
> It seems just stupid. If the user asked for 'fstat()', just give the
> user 'fstat()'.

We do it in musl, but we also do the "turn it back again" in
userspace. musl's __fstatat_kstat checks if AT_EMPTY_PATH is set, and
if so, calls SYS_fstat. I don't see why glibc couldn't do the same.

However, we don't do this, and this does not seem to be possible, in
the case where statx has to be used (32-bit archs). Is there an
analogous issue for statx?

Rich