Messages in this thread | | | From | Alan Mikhak <> | Date | Tue, 1 Oct 2019 10:09:48 -0700 | Subject | Re: [PATCH] scatterlist: Validate page before calling PageSlab() |
| |
On Tue, Oct 1, 2019 at 5:16 AM Jason Gunthorpe <jgg@ziepe.ca> wrote: > > On Mon, Sep 30, 2019 at 04:22:35PM -0700, Alan Mikhak wrote: > > From: Alan Mikhak <alan.mikhak@sifive.com> > > > > Modify sg_miter_stop() to validate the page pointer > > before calling PageSlab(). This check prevents a crash > > that will occur if PageSlab() gets called with a page > > pointer that is not backed by page struct. > > > > A virtual address obtained from ioremap() for a physical > > address in PCI address space can be assigned to a > > scatterlist segment using the public scatterlist API > > as in the following example: > > > > my_sg_set_page(struct scatterlist *sg, > > const void __iomem *ioaddr, > > size_t iosize) > > { > > sg_set_page(sg, > > virt_to_page(ioaddr), > > (unsigned int)iosize, > > offset_in_page(ioaddr)); > > sg_init_marker(sg, 1); > > } > > > > If the virtual address obtained from ioremap() is not > > backed by a page struct, virt_to_page() returns an > > invalid page pointer. However, sg_copy_buffer() can > > correctly recover the original virtual address. Such > > addresses can successfully be assigned to scatterlist > > segments to transfer data across the PCI bus with > > sg_copy_buffer() if it were not for the crash in > > PageSlab() when called by sg_miter_stop(). > > I thought we already agreed in general that putting things that don't > have struct page into the scatter list was not allowed? > > Jason
Thanks Jason for your comment.
Cost of adding page structs to a large PCI I/O address range can be quite substantial. Allowing PCI I/O pages without page structs may be desirable. Perhaps it is worth considering this cost.
Scatterlist has no problem doing its memcpy() from pages without a page struct that were obtained from ioremap(). It is only at sg_miter_stop() that the call to PageSlab() prevents such use by crashing. This seems accidental, not by design. Scatterlist API function sg_set_page() allows any page pointer to be assigned even if it has no page struct. It doesn't check if the page pointer is valid. Its description doesn't say that a page struct is required.
Regards, Alan
| |