On Mon, May 26, 2025 at 10:51 AM Allison Karlitskaya <lis@xxxxxxxxxx> wrote:
>
> hi,
>
> Sorry for being so late to reply to this: it's been busy.
>
> On Fri, 16 May 2025 at 11:07, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> > Okay, let's add it to fuse_init_in as uint8_t.
>
> Is this to help save a few bytes? I'm not sure it's worth it, for a
> few reasons:
> - there are 11 reserved fields here, which is still quite a lot of room
> - even if we reduce this to a single byte, we need to add 3 extra
> bytes of padding, which is a bit awkward. We also only get to use
> this if we have something else that fits into a u8 or u16, otherwise
> it's wasted
> - we might imagine some sort of a beautiful future where the kernel
> figures out a way to increase this restriction considerably (ie: >
> 256). I'm not sure how that would look, but it seems foolish to not
> consider it.
>
> I'm happy to redo the patch if you're sure this is right (since I want
> to update the commit message a bit anyway), but how should I call the
> 3 extra bytes in that case? unused2? reserved?
>
Allison,
Sorry for joining so late.
I cannot help feeling that all this is really pointless.
The reality is that I don't imagine the kernel's FILESYSTEM_MAX_STACK_DEPTH
constant to grow any time soon and that for the foreseen future
the only valid values for arg->max_stack_depth are 1 or 2.
I can't remember why we did not use 0.
I must have inherited this from the Android patches or something.
As it is, with or without knowing FILESYSTEM_MAX_STACK_DEPTH
this argument is quite hard to document for userspace.
for this reason I left an explicit documentation for the only two
possible values in libfuse:
/**
* When FUSE_CAP_PASSTHROUGH is enabled, this is the maximum allowed
* stacking depth of the backing files. In current kernel, the maximum
* allowed stack depth if FILESYSTEM_MAX_STACK_DEPTH (2), which includes
* the FUSE passthrough layer, so the maximum stacking depth for backing
* files is 1.
*
* The default is FUSE_BACKING_STACKED_UNDER (0), meaning that the
* backing files cannot be on a stacked filesystem, but another stacked
* filesystem can be stacked over this FUSE passthrough filesystem.
*
* Set this to FUSE_BACKING_STACKED_OVER (1) if backing files may be on
* a stacked filesystem, such as overlayfs or another FUSE passthrough.
* In this configuration, another stacked filesystem cannot be stacked
* over this FUSE passthrough filesystem.
*/
#define FUSE_BACKING_STACKED_UNDER (0)
#define FUSE_BACKING_STACKED_OVER (1)
If the problem is that this is not defined in uapi/fuse.h we can define:
#define FUSE_STACKED_UNDER (1)
#define FUSE_STAKCED_OVER (2)
With the comment from libfuse.
If we ever need to pass values greater than 2 we can introduce a new
init flag for that.
What do you think?
Did I misunderstand the problem that you are trying to solve?
Thanks,
Amir.
