Re: [PATCH v5 8/8] linux: Add mount_setattr

[Christian Brauner <brauner@kernel.org>]

On Tue, Jun 07, 2022 at 06:34:46PM +0200, Yann Droneaud wrote:
> Hi,
> 
> Le 06/06/2022 à 15:44, Adhemerval Zanella via Libc-alpha a écrit :
> > It was added on Linux 5.12 (2a1867219c7b27f928e2545782b86daaf9ad50bd)
> > to allow change the properties of a mount or a mount tree using file
> > descriptors which the new mount api is based on.
> > 
> > Checked on x86_64-linux-gnu.
> > ---
> >   NEWS                                          |  6 ++--
> >   sysdeps/unix/sysv/linux/Versions              |  1 +
> >   sysdeps/unix/sysv/linux/aarch64/libc.abilist  |  1 +
> >   sysdeps/unix/sysv/linux/alpha/libc.abilist    |  1 +
> >   sysdeps/unix/sysv/linux/arc/libc.abilist      |  1 +
> >   sysdeps/unix/sysv/linux/arm/be/libc.abilist   |  1 +
> >   sysdeps/unix/sysv/linux/arm/le/libc.abilist   |  1 +
> >   sysdeps/unix/sysv/linux/csky/libc.abilist     |  1 +
> >   sysdeps/unix/sysv/linux/hppa/libc.abilist     |  1 +
> >   sysdeps/unix/sysv/linux/i386/libc.abilist     |  1 +
> >   sysdeps/unix/sysv/linux/ia64/libc.abilist     |  1 +
> >   .../sysv/linux/m68k/coldfire/libc.abilist     |  1 +
> >   .../unix/sysv/linux/m68k/m680x0/libc.abilist  |  1 +
> >   .../sysv/linux/microblaze/be/libc.abilist     |  1 +
> >   .../sysv/linux/microblaze/le/libc.abilist     |  1 +
> >   .../sysv/linux/mips/mips32/fpu/libc.abilist   |  1 +
> >   .../sysv/linux/mips/mips32/nofpu/libc.abilist |  1 +
> >   .../sysv/linux/mips/mips64/n32/libc.abilist   |  1 +
> >   .../sysv/linux/mips/mips64/n64/libc.abilist   |  1 +
> >   sysdeps/unix/sysv/linux/nios2/libc.abilist    |  1 +
> >   sysdeps/unix/sysv/linux/or1k/libc.abilist     |  1 +
> >   .../linux/powerpc/powerpc32/fpu/libc.abilist  |  1 +
> >   .../powerpc/powerpc32/nofpu/libc.abilist      |  1 +
> >   .../linux/powerpc/powerpc64/be/libc.abilist   |  1 +
> >   .../linux/powerpc/powerpc64/le/libc.abilist   |  1 +
> >   .../unix/sysv/linux/riscv/rv32/libc.abilist   |  1 +
> >   .../unix/sysv/linux/riscv/rv64/libc.abilist   |  1 +
> >   .../unix/sysv/linux/s390/s390-32/libc.abilist |  1 +
> >   .../unix/sysv/linux/s390/s390-64/libc.abilist |  1 +
> >   sysdeps/unix/sysv/linux/sh/be/libc.abilist    |  1 +
> >   sysdeps/unix/sysv/linux/sh/le/libc.abilist    |  1 +
> >   .../sysv/linux/sparc/sparc32/libc.abilist     |  1 +
> >   .../sysv/linux/sparc/sparc64/libc.abilist     |  1 +
> >   sysdeps/unix/sysv/linux/sys/mount.h           | 22 ++++++++++++++
> >   sysdeps/unix/sysv/linux/syscalls.list         |  1 +
> >   sysdeps/unix/sysv/linux/tst-mount.c           | 30 +++++++++++++++++--
> >   .../unix/sysv/linux/x86_64/64/libc.abilist    |  1 +
> >   .../unix/sysv/linux/x86_64/x32/libc.abilist   |  1 +
> >   38 files changed, 87 insertions(+), 6 deletions(-)
> > 
> > diff --git a/NEWS b/NEWS
> > index de12657449..696de3962b 100644
> > --- a/NEWS
> > +++ b/NEWS
> > @@ -29,9 +29,9 @@ Major new features:
> >     memory is carried out in the context of the caller, using the caller's
> >     CPU affinity, and priority with CPU usage accounted to the caller.
> > -* On Linux, the fsopen, fsmount, move_mount, fsconfig, fspick, and open_tree
> > -  have been added.  It is a new mount API to allow more flexibility on mount
> > -  operations, specially when used along namespaces.
> > +* On Linux, the fsopen, fsmount, move_mount, fsconfig, fspick, open_tree,
> > +  and mount_setattr have been added.  It is a new mount API to allow more
> > +  flexibility on mount operations, specially when used along namespaces.
> >   Deprecated and removed features, and other changes affecting compatibility:
> > diff --git a/sysdeps/unix/sysv/linux/Versions b/sysdeps/unix/sysv/linux/Versions
> > index b70530ef40..65d2ceda2c 100644
> > --- a/sysdeps/unix/sysv/linux/Versions
> > +++ b/sysdeps/unix/sysv/linux/Versions
> > @@ -305,6 +305,7 @@ libc {
> >       fsopen;
> >       fspick;
> >       move_mount;
> > +    mount_setattr;
> >       open_tree;
> >       pidfd_open;
> >       pidfd_getfd;
> 
> > diff --git a/sysdeps/unix/sysv/linux/sys/mount.h b/sysdeps/unix/sysv/linux/sys/mount.h
> > index eb6705a091..4929d84027 100644
> > --- a/sysdeps/unix/sysv/linux/sys/mount.h
> > +++ b/sysdeps/unix/sysv/linux/sys/mount.h
> > @@ -23,6 +23,8 @@
> >   #include <fcntl.h>
> >   #include <features.h>
> > +#include <stdint.h>
> > +#include <stddef.h>
> >   #include <sys/ioctl.h>
> >   #define BLOCK_SIZE	1024
> > @@ -155,6 +157,17 @@ enum
> >   #define MOUNT_ATTR_NOSYMFOLLOW  0x00200000 /* Do not follow symlinks.  */
> > +/* For mount_setattr.  */
> > +struct mount_attr
> > +{
> > +  uint64_t attr_set;
> > +  uint64_t attr_clr;
> > +  uint64_t propagation;
> > +  uint64_t userns_fd;
> > +};
> > +
> > +#define MOUNT_ATTR_SIZE_VER0    32 /* sizeof first published struct */
> > +
> >   /* move_mount flags.  */
> >   #define MOVE_MOUNT_F_SYMLINKS   0x00000001 /* Follow symlinks on from path */
> >   #define MOVE_MOUNT_F_AUTOMOUNTS 0x00000002 /* Follow automounts on from path */
> > @@ -240,6 +253,15 @@ extern int fspick (int __dfd, const char *__path, unsigned int __flags)
> >   extern int open_tree (int __dfd, const char *__filename, unsigned int __flags)
> >     __THROW;
> > +/* Change the mount proprieties of the mount or an entire mount tree.  If
> > +   PATH is a relative pathname, then it is interpreted relative to the directory
> > +   referred to by the file descriptor dirfd.  Otherwise if DFD is the special
> > +   value AT_FDCWD then PATH is interpreted relative to the current working
> > +   directory of the calling process.  */
> > +extern int mount_setattr (int __dfd, const char *__path, unsigned int __flags,
> > +			  struct mount_attr *__uattr, size_t __usize)
> > +  __THROW;
> > +
> 
> 
> I wonder why __uattr is not declared const struct mount_attr * ?
> 
> Is it really expected that future mount_setattr() extensions to write back
> into the userspace structure ?

I don't think we can in principle rule this out. A const is a bit of a
scary commitment in this regard... I need to think about this a bit.