Re: Alloc cap limit for 9p xattrs (Was: WARNING in __alloc_frozen_pages_noprof)

[Al Viro]

On Wed, Dec 11, 2024 at 01:32:26PM -0800, Linus Torvalds wrote:
> On Wed, 11 Dec 2024 at 13:04, <asmadeus@xxxxxxxxxxxxx> wrote:
> >
> > Christian Schoenebeck's suggestion was something like this -- I guess
> > that's good enough for now and won't break anything (e.g. ACLs bigger
> > than XATTR_SIZE_MAX), so shall we go with that instead?
> 
> Please use XATTR_SIZE_MAX. The KMALLOC_MAX_SIZE limit seems to make no
> sense in this context.
> 
> Afaik the VFS layer doesn't allow getting an xattr bigger than
> XATTR_SIZE_MAX anyway, and would return E2BIG for them later
> regardless, so returning anything bigger wouldn't work anyway, even if
> p9 tried to return such a thing up to some bigger limit.

E2BIG on attempt to set, quiet cap to XATTR_SIZE_MAX on attempt to get
(i.e. never asking more than that from fs) and if filesystem complains
about XATTR_SIZE_MAX not being enough, E2BIG it is (instead of ERANGE
normally expected on "your buffer is too small for that").