Re: [PATCH v1] fs: Add kfuncs to handle idmapped mounts

[Christian Brauner]

On Wed, Jul 05, 2023 at 03:43:09PM +0200, Alexey Gladkov wrote:
> On Tue, Jul 04, 2023 at 05:28:13PM +0200, Christian Brauner wrote:
> > On Tue, Jul 04, 2023 at 05:11:12PM +0200, Alexey Gladkov wrote:
> > > On Tue, Jul 04, 2023 at 07:42:53PM +0800, Hou Tao wrote:
> > > > Hi,
> > > > 
> > > > On 6/30/2023 7:08 PM, Alexey Gladkov wrote:
> > > > > Since the introduction of idmapped mounts, file handling has become
> > > > > somewhat more complicated. If the inode has been found through an
> > > > > idmapped mount the idmap of the vfsmount must be used to get proper
> > > > > i_uid / i_gid. This is important, for example, to correctly take into
> > > > > account idmapped files when caching, LSM or for an audit.
> > > > 
> > > > Could you please add a bpf selftest for these newly added kfuncs ?
> > > > >
> > > > > Signed-off-by: Alexey Gladkov <legion@xxxxxxxxxx>
> > > > > ---
> > > > >  fs/mnt_idmapping.c | 69 ++++++++++++++++++++++++++++++++++++++++++++++
> > > > >  1 file changed, 69 insertions(+)
> > > > >
> > > > > diff --git a/fs/mnt_idmapping.c b/fs/mnt_idmapping.c
> > > > > index 4905665c47d0..ba98ce26b883 100644
> > > > > --- a/fs/mnt_idmapping.c
> > > > > +++ b/fs/mnt_idmapping.c
> > > > > @@ -6,6 +6,7 @@
> > > > >  #include <linux/mnt_idmapping.h>
> > > > >  #include <linux/slab.h>
> > > > >  #include <linux/user_namespace.h>
> > > > > +#include <linux/bpf.h>
> > > > >  
> > > > >  #include "internal.h"
> > > > >  
> > > > > @@ -271,3 +272,71 @@ void mnt_idmap_put(struct mnt_idmap *idmap)
> > > > >  		kfree(idmap);
> > > > >  	}
> > > > >  }
> > > > > +
> > > > > +__diag_push();
> > > > > +__diag_ignore_all("-Wmissing-prototypes",
> > > > > +		  "Global functions as their definitions will be in vmlinux BTF");
> > > > > +
> > > > > +/**
> > > > > + * bpf_is_idmapped_mnt - check whether a mount is idmapped
> > > > > + * @mnt: the mount to check
> > > > > + *
> > > > > + * Return: true if mount is mapped, false if not.
> > > > > + */
> > > > > +__bpf_kfunc bool bpf_is_idmapped_mnt(struct vfsmount *mnt)
> > > > > +{
> > > > > +	return is_idmapped_mnt(mnt);
> > > > > +}
> > > > > +
> > > > > +/**
> > > > > + * bpf_file_mnt_idmap - get file idmapping
> > > > > + * @file: the file from which to get mapping
> > > > > + *
> > > > > + * Return: The idmap for the @file.
> > > > > + */
> > > > > +__bpf_kfunc struct mnt_idmap *bpf_file_mnt_idmap(struct file *file)
> > > > > +{
> > > > > +	return file_mnt_idmap(file);
> > > > > +}
> > > > 
> > > > A dummy question here: the implementation of file_mnt_idmap() is
> > > > file->f_path.mnt->mnt_idmap, so if the passed file is a BTF pointer, is
> > > > there any reason why we could not do such dereference directly in bpf
> > > > program ?
> > > 
> > > I wanted to provide a minimal API for bpf programs. I thought that this
> > > interface is stable enough, but after reading Christian's answer, it looks
> > > like I was wrong.
> > 
> > It isn't even about stability per se. It's unlikely that if we change
> > internal details that types or arguments to these helpers change. That's
> > why we did the work of abstracting this all away in the first place and
> > making this an opaque type.
> > 
> > The wider point is that according to the docs, kfuncs claim to have
> > equivalent status to EXPORT_SYMBOL_*() with the added complexity of
> > maybe having to take out of tree bpf programs into account.
> > 
> > Right now, we can look at the in-kernel users of is_idmapped_mnt(),
> > convert them and then kill this thing off if we wanted to. As soon as
> > this is a kfunc such an endeavour becomes a measure of "f**** around and
> > find out". That's an entirely avoidable conflict if we don't even expose
> > it in the first place.
> > 
> 
> I was hoping to make it possible to use is_idmapped_mnt or its equivalent
> to at least be able to distinguish a file with an idmapped mount from a
> regular one.

Afaict, you can do this today pretty easily. For example,

#!/usr/bin/env bpftrace

#include <linux/mount.h>
#include <linux/path.h>
#include <linux/dcache.h>

kfunc:do_move_mount
{
        printf("Target path           %s\n", str(args->new_path->dentry->d_name.name));
        printf("Target mount idmapped %d\n", args->new_path->mnt->mnt_idmap != kaddr("nop_mnt_idmap"));
}

sample output:

Target path           console
Target mount idmapped 0
Target path           rootfs
Target mount idmapped 1