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

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On Thu, 1 Aug 2024 08:10:16 +0100, Al Viro 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. 




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