[RFC][PATCHES v2] xattr stuff and interactions with io_uring

[Al Viro <viro@zeniv.linux.org.uk>]

	Changes since v1:
* moved on top of (and makes use of) getname_maybe_null() stuff
(first two commits here, form #base.getname-fixed)
* fixed a leak on io_uring side spotted by Jens
* putname(ERR_PTR(-E...)) is a no-op; allows to simplify things on
io_uring side.
* applied reviewed-by
* picked a generic_listxattr() cleanup from Colin Ian King

	Help with review and testing would be welcome; if nobody objects,
to #for-next it goes...

The branch lives in
git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git #work.xattr2
Individual patches in followups.
 
Diffstat:
 arch/alpha/kernel/syscalls/syscall.tbl      |   4 +
 arch/arm/tools/syscall.tbl                  |   4 +
 arch/m68k/kernel/syscalls/syscall.tbl       |   4 +
 arch/microblaze/kernel/syscalls/syscall.tbl |   4 +
 arch/mips/kernel/syscalls/syscall_n32.tbl   |   4 +
 arch/mips/kernel/syscalls/syscall_n64.tbl   |   4 +
 arch/mips/kernel/syscalls/syscall_o32.tbl   |   4 +
 arch/parisc/kernel/syscalls/syscall.tbl     |   4 +
 arch/powerpc/kernel/syscalls/syscall.tbl    |   4 +
 arch/s390/kernel/syscalls/syscall.tbl       |   4 +
 arch/sh/kernel/syscalls/syscall.tbl         |   4 +
 arch/sparc/kernel/syscalls/syscall.tbl      |   4 +
 arch/x86/entry/syscalls/syscall_32.tbl      |   4 +
 arch/x86/entry/syscalls/syscall_64.tbl      |   4 +
 arch/xtensa/kernel/syscalls/syscall.tbl     |   4 +
 fs/internal.h                               |  17 +-
 fs/namei.c                                  |  34 ++-
 fs/stat.c                                   |  28 +-
 fs/xattr.c                                  | 446 ++++++++++++++++++----------
 include/asm-generic/audit_change_attr.h     |   6 +
 include/linux/fs.h                          |  10 +
 include/linux/syscalls.h                    |  13 +
 include/linux/xattr.h                       |   4 +
 include/uapi/asm-generic/unistd.h           |  11 +-
 include/uapi/linux/xattr.h                  |   7 +
 io_uring/xattr.c                            |  97 ++----
 26 files changed, 466 insertions(+), 267 deletions(-)

Shortog:

Al Viro (9):
      teach filename_lookup() to treat NULL filename as ""
      getname_maybe_null() - the third variant of pathname copy-in
      io_[gs]etxattr_prep(): just use getname()
      xattr: switch to CLASS(fd)
      new helper: import_xattr_name()
      replace do_setxattr() with saner helpers.
      replace do_getxattr() with saner helpers.
      new helpers: file_listxattr(), filename_listxattr()
      new helpers: file_removexattr(), filename_removexattr()

Christian Göttsche (2):
      fs: rename struct xattr_ctx to kernel_xattr_ctx
      fs/xattr: add *at family syscalls

Colin Ian King (1):
      xattr: remove redundant check on variable err

Jens Axboe (1):
      io_uring: IORING_OP_F[GS]ETXATTR is fine with REQ_F_FIXED_FILE

Patch summaries:

getname_maybe_null() introduction (#getname.base-fixed):
01/13) teach filename_lookup() to treat NULL filename as ""
02/13) getname_maybe_null() - the third variant of pathname copy-in

io_uring-side prep:
03/13) io_uring: IORING_OP_F[GS]ETXATTR is fine with REQ_F_FIXED_FILE
04/13) io_[gs]etxattr_prep(): just use getname()
	getname_flags(_, LOOKUP_FOLLOW) is ridiculous.

05/13)	switch to CLASS(fd) use.  Obvious.

06/13)	rename struct xattr_ctx to kernel_xattr_ctx
prep from the ...xattrat() series, to reduce the PITA for rebase

07/13)	new helper: import_xattr_name()
exact same logics for copying the xattr name in, dealing with
overflows, etc. in a lot of places.  

08/13)	replace do_setxattr() with saner helpers
file_setxattr(file, ctx) and filename_setxattr(dfd, filename, lookup_flags, ctx).
Don't mess with do_setxattr() directly - use those.  In particular, don't
bother with the ESTALE loop in io_uring - it doesn't belong there.
 
09/13)	replace do_getxattr() with saner helpers
Same on getxattr() side.

10/13)	new helpers: file_listxattr(), filename_listxattr()
Same, except that io_uring doesn't use those, so the helpers are left static.

11/13)	new helpers: file_removexattr(), filename_removexattr()
Ditto

12/13)	fs/xattr: add *at family syscalls
Rebased patch introducing those, with a bunch of fixes folded in so we don't
create bisect hazard there.  Potentially interesting bit is the pathname-handling
logics - getname_maybe_null(pathname, flags) returns a struct filename reference
if no AT_EMPTY_PATH had been given or if the pathname was non-NULL and turned out
to be non-empty.  With (NULL,AT_EMPTY_PATH) or (empty string, AT_EMPTY_PATH) it
returns NULL (and it tries to skip the allocation using the same trick with
get_user() that vfs_empty_path() uses).  That helper simplifies a lot of things,
and it should be cheap enough to convert fsetxattr(2) et.al. to the unified
path_setxattrat() and its ilk.  IF we get a slowdown on those, we can always
expand and simplify the calls in fsetxattr(2) and friends.

13/13) xattr: remove redundant check on variable err