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

[Lizhi Xu <lizhi.xu@windriver.com>]

On Thu, 1 Aug 2024 16:12:24 +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.
> read_folio, have you noticed that the file value was passed to read_folio is NULL? 
> fs/namei.c
> const char *page_get_link(struct dentry *dentry, struct inode *inode
> ...
> 5272  read_mapping_page(mapping, 0, NULL);
> 
> So, Therefore, no matter what, the value of folio will not be initialized by file
> in read_folio. 
Oh, in read_folio, it will use mapping->host to init folio, I will research
why not init in do_read_cache_folio.

--
Lizhi