On Wed, Sep 18, 2019 at 09:32:13AM -0700, Darrick J. Wong wrote:
> On Wed, Sep 18, 2019 at 10:47:33AM -0500, Bill O'Donnell wrote:
> > Guarantee zeroed memory buffers for cases where potential memory
> > leak to disk can occur. In these cases, kmem_alloc is used and
> > doesn't zero the buffer, opening the possibility of information
> > leakage to disk.
> >
> > Introduce a xfs_buf_flag, XBF_ZERO, to indicate a request for a zeroed
> > buffer, and use existing infrastucture (xfs_buf_allocate_memory) to
> > obtain the already zeroed buffer from kernel memory.
> >
> > This solution avoids the performance issue that would occur if a
> > wholesale change to replace kmem_alloc with kmem_zalloc was done.
> >
> > Signed-off-by: Bill O'Donnell <billodo@xxxxxxxxxx>
> > ---
> > v2: zeroed buffer not required for XBF_READ case. Correct placement
> > and rename the XBF_ZERO flag.
> >
> > fs/xfs/xfs_buf.c | 9 +++++++--
> > fs/xfs/xfs_buf.h | 2 ++
> > fs/xfs/xfs_trans_buf.c | 2 ++
> > 3 files changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > index 120ef99d09e8..0d96efff451e 100644
> > --- a/fs/xfs/xfs_buf.c
> > +++ b/fs/xfs/xfs_buf.c
> > @@ -345,6 +345,10 @@ xfs_buf_allocate_memory(
> > unsigned short page_count, i;
> > xfs_off_t start, end;
> > int error;
> > + uint kmflag_mask = 0;
> > +
> > + if ((flags & XBF_ZERO) && !(flags & XBF_READ))
>
> The sole caller of xfs_buf_allocate_memory is xfs_buf_get_map. If
> _get_map is called from the *read_buf* functions, they pass in XBF_READ
> to ensure buffer contents are read. The other _get_map callers seem to
> be initializing metadata blocks and do not set XBF_READ.
>
> So I wonder, do you even need XBF_ZERO? Or could this be reduced to:
>
> if (!(flags & XBF_READ))
> km_flag_mask |= KM_ZERO;
>
> ?
>
> --D
Yeah, I see no cases of XBF_ZERO true AND XBF_READ true.
Anytime XBF_ZERO is true, XBF_READ is false.
So, yes, it could be reduced by eliminating XBF_ZERO.
Thanks,
Bill
>
> > + kmflag_mask |= KM_ZERO;
> >
> > /*
> > * for buffers that are contained within a single page, just allocate
> > @@ -354,7 +358,8 @@ xfs_buf_allocate_memory(
> > size = BBTOB(bp->b_length);
> > if (size < PAGE_SIZE) {
> > int align_mask = xfs_buftarg_dma_alignment(bp->b_target);
> > - bp->b_addr = kmem_alloc_io(size, align_mask, KM_NOFS);
> > + bp->b_addr = kmem_alloc_io(size, align_mask,
> > + KM_NOFS | kmflag_mask);
> > if (!bp->b_addr) {
> > /* low memory - use alloc_page loop instead */
> > goto use_alloc_page;
> > @@ -391,7 +396,7 @@ xfs_buf_allocate_memory(
> > struct page *page;
> > uint retries = 0;
> > retry:
> > - page = alloc_page(gfp_mask);
> > + page = alloc_page(gfp_mask | kmflag_mask);
> > if (unlikely(page == NULL)) {
> > if (flags & XBF_READ_AHEAD) {
> > bp->b_page_count = i;
> > diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
> > index f6ce17d8d848..dccdd653c9dc 100644
> > --- a/fs/xfs/xfs_buf.h
> > +++ b/fs/xfs/xfs_buf.h
> > @@ -33,6 +33,7 @@
> > /* flags used only as arguments to access routines */
> > #define XBF_TRYLOCK (1 << 16)/* lock requested, but do not wait */
> > #define XBF_UNMAPPED (1 << 17)/* do not map the buffer */
> > +#define XBF_ZERO (1 << 18)/* zeroed buffer required */
> >
> > /* flags used only internally */
> > #define _XBF_PAGES (1 << 20)/* backed by refcounted pages */
> > @@ -52,6 +53,7 @@ typedef unsigned int xfs_buf_flags_t;
> > { XBF_WRITE_FAIL, "WRITE_FAIL" }, \
> > { XBF_TRYLOCK, "TRYLOCK" }, /* should never be set */\
> > { XBF_UNMAPPED, "UNMAPPED" }, /* ditto */\
> > + { XBF_ZERO, "KMEM_ZERO" }, \
> > { _XBF_PAGES, "PAGES" }, \
> > { _XBF_KMEM, "KMEM" }, \
> > { _XBF_DELWRI_Q, "DELWRI_Q" }
> > diff --git a/fs/xfs/xfs_trans_buf.c b/fs/xfs/xfs_trans_buf.c
> > index b5b3a78ef31c..087d413c1490 100644
> > --- a/fs/xfs/xfs_trans_buf.c
> > +++ b/fs/xfs/xfs_trans_buf.c
> > @@ -123,6 +123,8 @@ xfs_trans_get_buf_map(
> > xfs_buf_t *bp;
> > struct xfs_buf_log_item *bip;
> >
> > + flags |= XBF_ZERO;
> > +
> > if (!tp)
> > return xfs_buf_get_map(target, map, nmaps, flags);
> >
> > --
> > 2.21.0
> >
