[Xen-devel] [PATCH] linux: fix grant table bug

[Michael Abd-El-Malek <mabdelmalek@xxxxxxx>]

A PV OS has two grant table data structures: the grant table itself and a free 
list.  The free list is composed of an array of pages, which grow dynamically as 
the guest OS requires more grants.  While the grant table contains 8-byte 
entries, the free list contains 4-byte entries.  So we have half as many pages 
in the free list than in the grant table.
There was a bug in the free list allocation code. The free list was indexed as 
if it was the same size as the grant table.  But it's only half as large.  So 
memory got corrupted, and I was seeing crashes in the slab allocator later on.
Signed-off-by: Michael Abd-El-Malek <mabdelmalek@xxxxxxx>

diff -r 1721546b9277 drivers/xen/core/gnttab.c
--- a/drivers/xen/core/gnttab.c Fri Mar 28 01:39:23 2008 -0400
+++ b/drivers/xen/core/gnttab.c Sun Mar 30 23:24:42 2008 -0400
@@ -379,11 +379,15 @@ static int grow_gnttab_list(unsigned int
 static int grow_gnttab_list(unsigned int more_frames)
 {
        unsigned int new_nr_grant_frames, extra_entries, i;
+       unsigned int nr_glist_frames, new_nr_glist_frames;
 
        new_nr_grant_frames = nr_grant_frames + more_frames;
        extra_entries       = more_frames * GREFS_PER_GRANT_FRAME;
 
-       for (i = nr_grant_frames; i < new_nr_grant_frames; i++)
+       nr_glist_frames = (nr_grant_frames * GREFS_PER_GRANT_FRAME + RPP - 1) / 
RPP;
+       new_nr_glist_frames =
+               (new_nr_grant_frames * GREFS_PER_GRANT_FRAME + RPP - 1) / RPP;
+       for (i = nr_glist_frames; i < new_nr_glist_frames; i++)
        {
                gnttab_list[i] = (grant_ref_t *)__get_free_page(GFP_ATOMIC);
                if (!gnttab_list[i])
@@ -406,7 +410,7 @@ static int grow_gnttab_list(unsigned int
        return 0;
        
 grow_nomem:
-       for ( ; i >= nr_grant_frames; i--)
+       for ( ; i >= nr_glist_frames; i--)
                free_page((unsigned long) gnttab_list[i]);
        return -ENOMEM;
 }
@@ -720,7 +724,7 @@ int __devinit gnttab_init(void)
 int __devinit gnttab_init(void)
 {
        int i;
-       unsigned int max_nr_glist_frames;
+       unsigned int max_nr_glist_frames, nr_glist_frames;
        unsigned int nr_init_grefs;
 
        if (!is_running_on_xen())
@@ -733,15 +737,15 @@ int __devinit gnttab_init(void)
         * grant reference free list on the current hypervisor.
         */
        max_nr_glist_frames = (boot_max_nr_grant_frames *
-                              GREFS_PER_GRANT_FRAME /
-                              (PAGE_SIZE / sizeof(grant_ref_t)));
+                              GREFS_PER_GRANT_FRAME / RPP);
 
        gnttab_list = kmalloc(max_nr_glist_frames * sizeof(grant_ref_t *),
                              GFP_KERNEL);
        if (gnttab_list == NULL)
                return -ENOMEM;
 
-       for (i = 0; i < nr_grant_frames; i++) {
+       nr_glist_frames = (nr_grant_frames * GREFS_PER_GRANT_FRAME + RPP - 1) / 
RPP;
+       for (i = 0; i < nr_glist_frames; i++) {
                gnttab_list[i] = (grant_ref_t *)__get_free_page(GFP_KERNEL);
                if (gnttab_list[i] == NULL)
                        goto ini_nomem;

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