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Date: Sat, 31 Jul 2021 08:25:51 -0700 From: Kees Cook <keescook@...omium.org> To: dsterba@...e.cz, linux-hardening@...r.kernel.org, "Gustavo A. R. Silva" <gustavoars@...nel.org>, Keith Packard <keithpac@...zon.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org, netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org, linux-staging@...ts.linux.dev, linux-block@...r.kernel.org, linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com Subject: Re: [PATCH 47/64] btrfs: Use memset_after() to clear end of struct On Thu, Jul 29, 2021 at 12:33:37PM +0200, David Sterba wrote: > On Wed, Jul 28, 2021 at 02:56:31PM -0700, Kees Cook wrote: > > On Wed, Jul 28, 2021 at 11:42:15AM +0200, David Sterba wrote: > > > On Tue, Jul 27, 2021 at 01:58:38PM -0700, Kees Cook wrote: > > > > In preparation for FORTIFY_SOURCE performing compile-time and run-time > > > > field bounds checking for memset(), avoid intentionally writing across > > > > neighboring fields. > > > > > > > > Use memset_after() so memset() doesn't get confused about writing > > > > beyond the destination member that is intended to be the starting point > > > > of zeroing through the end of the struct. > > > > > > > > Signed-off-by: Kees Cook <keescook@...omium.org> > > > > --- > > > > fs/btrfs/root-tree.c | 5 +---- > > > > 1 file changed, 1 insertion(+), 4 deletions(-) > > > > > > > > diff --git a/fs/btrfs/root-tree.c b/fs/btrfs/root-tree.c > > > > index 702dc5441f03..ec9e78f65fca 100644 > > > > --- a/fs/btrfs/root-tree.c > > > > +++ b/fs/btrfs/root-tree.c > > > > @@ -39,10 +39,7 @@ static void btrfs_read_root_item(struct extent_buffer *eb, int slot, > > > > need_reset = 1; > > > > } > > > > if (need_reset) { > > > > - memset(&item->generation_v2, 0, > > > > - sizeof(*item) - offsetof(struct btrfs_root_item, > > > > - generation_v2)); > > > > - > > > > > > Please add > > > /* Clear all members from generation_v2 onwards */ > > > > > > > + memset_after(item, 0, level); > > > > Perhaps there should be another helper memset_starting()? That would > > make these cases a bit more self-documenting. > > That would be better, yes. > > > + memset_starting(item, 0, generation_v2); > > memset_from? For v2, I bikeshed this to "memset_startat" since "from" is semantically close to "source" which I thought might be confusing. (I, too, did not like "starting".) :) Can I make "bikeshed" a verb? :P -- Kees Cook
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