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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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