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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Sep 18, 2015 at 09:52:44AM +0100, Ian Campbell wrote:
> On Thu, 2015-09-17 at 18:42 +0100, Julien Grall wrote:
> > On 17/09/15 18:36, Julien Grall wrote:
> > > 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".
> > 
> > Hmmmm I forgot my Signed-off-by :(.
> > 
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> 
> Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> 
> This is a no brainer for 4.6 (and further) IMHO and with Wei not being
> around I shall plan to apply later today unless there are objections.
> 

No objection from me of course.

Wei.

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