On 25/07/04 09:54AM, Amir Goldstein wrote:
> On Thu, Jul 3, 2025 at 8:51 PM John Groves <j...@groves.net> wrote:
> >
> > * FUSE_DAX_FMAP flag in INIT request/reply
> >
> > * fuse_conn->famfs_iomap (enable famfs-mapped files) to denote a
> > famfs-enabled connection
> >
> > Signed-off-by: John Groves <j...@groves.net>
> > ---
> > fs/fuse/fuse_i.h | 3 +++
> > fs/fuse/inode.c | 14 ++++++++++++++
> > include/uapi/linux/fuse.h | 4 ++++
> > 3 files changed, 21 insertions(+)
> >
> > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> > index 9d87ac48d724..a592c1002861 100644
> > --- a/fs/fuse/fuse_i.h
> > +++ b/fs/fuse/fuse_i.h
> > @@ -873,6 +873,9 @@ struct fuse_conn {
> > /* Use io_uring for communication */
> > unsigned int io_uring;
> >
> > + /* dev_dax_iomap support for famfs */
> > + unsigned int famfs_iomap:1;
> > +
>
> pls move up to the bit fields members.
Oops, done, thanks.
>
> > /** Maximum stack depth for passthrough backing files */
> > int max_stack_depth;
> >
> > diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> > index 29147657a99f..e48e11c3f9f3 100644
> > --- a/fs/fuse/inode.c
> > +++ b/fs/fuse/inode.c
> > @@ -1392,6 +1392,18 @@ static void process_init_reply(struct fuse_mount
> > *fm, struct fuse_args *args,
> > }
> > if (flags & FUSE_OVER_IO_URING &&
> > fuse_uring_enabled())
> > fc->io_uring = 1;
> > + if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX) &&
> > + flags & FUSE_DAX_FMAP) {
> > + /* XXX: Should also check that fuse server
> > + * has CAP_SYS_RAWIO and/or CAP_SYS_ADMIN,
> > + * since it is directing the kernel to
> > access
> > + * dax memory directly - but this function
> > + * appears not to be called in fuse server
> > + * process context (b/c even if it drops
> > + * those capabilities, they are held here).
> > + */
> > + fc->famfs_iomap = 1;
> > + }
>
> 1. As long as the mapping requests are checking capabilities we should be ok
> Right?
It depends on the definition of "are", or maybe of "mapping requests" ;)
Forgive me if this *is* obvious, but the fuse server capabilities are what
I think need to be checked here - not the app that it accessing a file.
An app accessing a regular file doesn't need permission to do raw access to
the underlying block dev, but the fuse server does - becuase it is directing
the kernel to access that for apps.
> 2. What's the deal with capable(CAP_SYS_ADMIN) in process_init_limits then?
I *think* that's checking the capabilities of the app that is accessing the
file, and not the fuse server. But I might be wrong - I have not pulled very
hard on that thread yet.
> 3. Darrick mentioned the need for a synchronic INIT variant for his work on
> blockdev iomap support [1]
I'm not sure that's the same thing (Darrick?), but I do think Darrick's
use case probably needs to check capabilities for a server that is sending
apps (via files) off to access extents of block devices.
>
> I also wonder how much of your patches and Darrick's patches end up
> being an overlap?
Darrick and I spent some time hashing through this, and came to the conclusion
that the actual overlap is slim-to-none.
>
> Thanks,
> Amir.
>
> [1]
> https://lore.kernel.org/linux-fsdevel/20250613174413.GM6138@frogsfrogsfrogs/
Thank you!
John
