[Xen-devel] [PATCH for-4.6] tools/libxc: arm: Check the index before accessing the bank

[Julien Grall <julien.grall@xxxxxxxxxx>]

When creating a guest with more than 3GB of memory, the 2 banks will be
used and the loop with overrunning. The code will fail later on because
Xen will deny to populate the region:

domainbuilder: detail: xc_dom_devicetree_mem: called
domainbuilder: detail: xc_dom_mem_init: mem 3096 MB, pages 0xc1800 pages, 4k 
each
domainbuilder: detail: xc_dom_mem_init: 0xc1800 pages
domainbuilder: detail: xc_dom_boot_mem_init: called
domainbuilder: detail: set_mode: guest xen-3.0-aarch64, address size 64
domainbuilder: detail: xc_dom_malloc            : 14384 kB
domainbuilder: detail: populate_guest_memory: populating RAM 
@0000000040000000-0000000100000000 (3072MB)
domainbuilder: detail: populate_one_size: populated 0x3/0x3 entries with shift 
18
domainbuilder: detail: populate_guest_memory: populating RAM 
@0000000200000000-0000000201800000 (24MB)
domainbuilder: detail: populate_one_size: populated 0xc/0xc entries with shift 9
domainbuilder: detail: populate_guest_memory: populating RAM 
@0000007fad41c000-0007fb39dd42c000 (2141954816MB)
domainbuilder: detail: populate_one_size: populated 0x100/0x1e4 entries with 
shift 0
domainbuilder: detail: populate_guest_memory: Not enough RAM

This is because we are currently accessing the bank before checking the
validity of the index. AFAICT, on  Debian Jessie, the compiler (gcc 4.9.2) is
assuming that it's not necessary to verify the index because it's used
before. This is a valid assumption because the operand of && are
execute from from left to right.

Re-order the checks to verify the validity of the index before accessing
the bank.

The problem has been present since the introduction of the multi-bank
feature in commit 45d9867837f099e9eed4189dac5ed39d1fe2ed49 " tools: arm:
prepare domain builder for multiple banks of guest RAM".

---
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

This patch is a candidate for Xen 4.6 and backport to Xen 4.5. Without
it, it's impossible to boot guest using more than 3GB (limit after which
the memory bank is used).
---
 tools/libxc/xc_dom_arm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/libxc/xc_dom_arm.c b/tools/libxc/xc_dom_arm.c
index b00d667..aeaba54 100644
--- a/tools/libxc/xc_dom_arm.c
+++ b/tools/libxc/xc_dom_arm.c
@@ -460,7 +460,7 @@ int arch_setup_meminit(struct xc_dom_image *dom)
         dom->p2m_host[pfn] = INVALID_P2M_ENTRY;
 
     /* setup initial p2m and allocate guest memory */
-    for ( i = 0; dom->rambank_size[i] && i < GUEST_RAM_BANKS; i++ )
+    for ( i = 0; i < GUEST_RAM_BANKS && dom->rambank_size[i]; i++ )
     {
         if ((rc = populate_guest_memory(dom,
                                         bankbase[i] >> XC_PAGE_SHIFT,
-- 
2.1.4


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel