Re: [Xen-devel] [PATCH for-4.8] tools/oxenstored: Fix transaction handling in 32bit builds

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Oct 31, 2016 at 01:21:56PM +0000, Andrew Cooper wrote:
> In a 32bit build, the ocaml code 'proposed_id >= 0x7fffffff' compiles to:
> 
>   8055eac:       83 fb ff                cmp    $0xffffffff,%ebx
>   8055eaf:       7d 0f                   jge    8055ec0 <...+0x20>
> 
> which in C is 'proposed_id >= INT_MIN', or in other words, tautologically
> true.  As a result, 32bit builds of oxenstored always try to allocate the
> transaction id 1, and fall into an infinite loop of trying the next id if
> transaction 1 is already in use.
> 
> Restrict the range down to 1 billion, to sit in the positive half of a 31 bit
> ocaml integer.  The compiled code is now:
> 
>   8055eac:       b9 ff ff ff 7f          mov    $0x7fffffff,%ecx
>   8055eb1:       39 cb                   cmp    %ecx,%ebx
>   8055eb3:       7d 0b                   jge    8055ec0 <...+0x20>
> 
> which (other than non-optimal code generation because of the unnecessary use
> of %ecx), isn't unconditionally true.
> 
> In principle, the check could be changed to 'proposed_id == 0x7fffffff' which
> would still allow for 2 billion transaction in 32bit builds.  However, in
> 64bit builds, this reintroduces a risk that if proposed_id is initially
> greater than 0x7fffffff, it will not be clipped suitably into range.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>

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