Re: [PATCH] Make sure get_user_desc() doesn't sign extend.

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/21/2009 09:40 AM, Chris Lalancette wrote:
> The current implementation of get_user_desc() sign extends
> the return value because of integer promotion rules.  For
> the most part, this doesn't matter, because the top bit of
> base2 is usually 0.  If, however, that bit is 1, then the
> entire value will be 0xffff... which is probably not what
> the caller intended.  This patch casts the entire thing
> to unsigned before returning, which generates almost the
> same assembly as the current code but replaces the final
> "cltq" (sign extend) with a "mov %eax %eax" (zero-extend).
> This fixes booting certain guests under KVM.

For the record, the reason why this wasn't noticed so far is that 
get_user_desc will be zero outside KVM except if used for FS and GS. 
KVM with the right guest will easily see a 0xC0000000 segment base, but 
you would need TLS data allocated above 2 GB to see the bug outside KVM. 
  TLS data is in the same mmap-ed memory that hosts the thread stacks, 
so it will typically be below the 2 GB mark and have its most 
significant bit cleared.

I suppose you could see the bug if you used pthread_attr_setstack, plus 
of course all the right circumstances---which are rare because all but 
the most obscure users anyway cast the result to u32.

Paolo