Re: Re: [PATCH] filemap: Init the newly allocated folio memory to 0 for the filemap

[Al Viro]

On Thu, Aug 01, 2024 at 01:28:37PM +0800, Lizhi Xu wrote:
> > > syzbot report KMSAN: uninit-value in pick_link, this is because the
> > > corresponding folio was not found from the mapping, and the memory was
> > > not initialized when allocating a new folio for the filemap.
> > >
> > > To avoid the occurrence of kmsan report uninit-value, initialize the
> > > newly allocated folio memory to 0.
> > 
> > NAK.
> > 
> > You are papering over the real bug here.
> Did you see the splat? I think you didn't see that.

Sigh...  It is stepping into uninitialized data in pick_link(), and by
the look of traces it's been created by page_get_link().

What page_get_link() does is reading from page cache of symlink;
the contents should have come from ->read_folio() (if it's really
a symlink on squashfs, that would be squashfs_symlink_read_folio()).

Uninit might have happened if
	* ->read_folio() hadn't been called at all (which is an obvios
bug - that's what should've read the symlink contents) or
	* ->read_folio() had been called, it failed and yet we are
still trying to use the resulting page.  Again, an obvious bug - if
trying to read fails, we should _not_ use the results or leave it
in page cache for subsequent callers.
	* ->read_folio() had been called, claimed to have succeeded and
yet it had left something in range 0..inode->i_size-1 uninitialized.
Again, a bug, this time in ->read_folio() instance.

Your patch is basically "fill the page with zeroes before reading anything
into it".  It makes KMSAM warning STFU, but it does not fix anything
in any of those cases.