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    SubjectRe: [BUGFIX][PATCH 2/3] kcore: fix vread/vwrite to be aware of holes.
    On Mon, Aug 03, 2009 at 08:18:45PM +0900, KAMEZAWA Hiroyuki wrote:
    >From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    >
    >vread/vwrite access vmalloc area without checking there is a page or not.
    >In most case, this works well.
    >
    >In old ages, the caller of get_vm_ara() is only IOREMAP and there is no
    >memory hole within vm_struct's [addr...addr + size - PAGE_SIZE]
    >( -PAGE_SIZE is for a guard page.)
    >
    >After per-cpu-alloc patch, it uses get_vm_area() for reserve continuous
    >virtual address but remap _later_. There tend to be a hole in valid vmalloc
    >area in vm_struct lists.
    >Then, skip the hole (not mapped page) is necessary.
    >This patch updates vread/vwrite() for avoiding memory hole.
    >
    >Routines which access vmalloc area without knowing for which addr is used
    >are
    > - /proc/kcore
    > - /dev/kmem
    >
    >kcore checks IOREMAP, /dev/kmem doesn't. After this patch, IOREMAP is
    >checked and /dev/kmem will avoid to read/write it.
    >Fixes to /proc/kcore will be in the next patch in series.
    >
    >Changelog v2->v3:
    > - fixed typos.
    > - use kmap. (if not using kmap, we have to add lock here.)


    Hmm.. I missed this.


    > - fixed PAGE_MASK miss-use.
    >Changelog v1->v2:
    > - enhanced comments.
    > - treat IOREMAP as hole always.
    > - zero-fill memory hole if [addr...addr+size] includes valid pages.
    > - returns 0 if [addr...addr+size) includes no valid pages.
    >
    >Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>


    This time it looks much better now, but it still has a small problem.
    Please check it below.


    >---
    > mm/vmalloc.c | 182 +++++++++++++++++++++++++++++++++++++++++++++++++++--------
    > 1 file changed, 159 insertions(+), 23 deletions(-)
    >

    <snip>


    >+
    >+/**
    >+ * vread() - read vmalloc area in a safe way.
    >+ * @buf: buffer for reading data
    >+ * @addr: vm address.
    >+ * @count: number of bytes to be read.
    >+ *
    >+ * Returns # of bytes which addr and buf should be increased.
    >+ * (same to count).
    >+ * If [addr...addr+count) doesn't includes any valid area, returns 0.


    If I read it correctly, your code doesn't do what you described here,
    it doesn't return 0 when there is no valid area.


    >+ *
    >+ * This function checks that addr is a valid vmalloc'ed area, and
    >+ * copy data from that area to a given buffer. If the given memory range of
    >+ * [addr...addr+count) includes some valid address, data is copied to
    >+ * proper area of @buf. If there are memory holes, they'll be zero-filled.
    >+ * IOREMAP area is treated as memory hole and no copy is done.
    >+ *
    >+ * Note: In usual ops, vread() is never necessary because the caller should
    >+ * know vmalloc() area is valid and can use memcpy(). This is for routines
    >+ * which have to access vmalloc area without any informaion, as /dev/kmem.
    >+ *
    >+ * The caller should guarantee KM_USER1 is not used.
    >+ */
    >+
    > long vread(char *buf, char *addr, unsigned long count)
    > {
    > struct vm_struct *tmp;
    > char *vaddr, *buf_start = buf;
    >+ unsigned long buflen = count;
    > unsigned long n;
    >
    > /* Don't allow overflow */
    >@@ -1640,7 +1739,7 @@
    > count = -(unsigned long) addr;
    >
    > read_lock(&vmlist_lock);
    >- for (tmp = vmlist; tmp; tmp = tmp->next) {
    >+ for (tmp = vmlist; count && tmp; tmp = tmp->next) {
    > vaddr = (char *) tmp->addr;
    > if (addr >= vaddr + tmp->size - PAGE_SIZE)
    > continue;
    >@@ -1653,32 +1752,66 @@
    > count--;
    > }
    > n = vaddr + tmp->size - PAGE_SIZE - addr;
    >- do {
    >- if (count == 0)
    >- goto finished;
    >- *buf = *addr;
    >- buf++;
    >- addr++;
    >- count--;
    >- } while (--n > 0);
    >+ if (n > count)
    >+ n = count;
    >+ if (!(tmp->flags & VM_IOREMAP))
    >+ aligned_vread(buf, addr, n);
    >+ else /* IOREMAP area is treated as memory hole */
    >+ memset(buf, 0, n);
    >+ buf += n;
    >+ addr += n;
    >+ count -= n;
    > }
    > finished:
    > read_unlock(&vmlist_lock);
    >- return buf - buf_start;
    >+
    >+ if (buf == buf_start)
    >+ return 0;
    >+ /* zero-fill memory holes */
    >+ if (buf != buf_start + buflen)
    >+ memset(buf, 0, buflen - (buf - buf_start));
    >+
    >+ return buflen;
    > }


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