linux-kernel - Re: Alloc cap limit for 9p xattrs (Was: WARNING in __alloc_frozen_pages

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Message-ID: <20241211225500.GH3387508@ZenIV>
Date: Wed, 11 Dec 2024 22:55:00 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: asmadeus@...ewreck.org, Leo Stone <leocstone@...il.com>,
	syzbot+03fb58296859d8dbab4d@...kaller.appspotmail.com,
	ericvh@...il.com, ericvh@...nel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux_oss@...debyte.com,
	lucho@...kov.net, syzkaller-bugs@...glegroups.com,
	v9fs-developer@...ts.sourceforge.net, v9fs@...ts.linux.dev,
	Fedor Pchelkin <pchelkin@...ras.ru>,
	Seth Forshee <sforshee@...nel.org>,
	Christian Brauner <brauner@...nel.org>
Subject: Re: Alloc cap limit for 9p xattrs (Was: WARNING in
 __alloc_frozen_pages_noprof)

On Wed, Dec 11, 2024 at 01:32:26PM -0800, Linus Torvalds wrote:
> On Wed, 11 Dec 2024 at 13:04, <asmadeus@...ewreck.org> 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").