Xen devel - Oct 2006 - Weird overflowing user_desc assignment

Hi all,

davej just noticed gcc spitting a complaint about this:

linux/arch/x86-64/kernel/process-xen.c ---

static inline void set_32bit_tls(struct task_struct *t, int tls, u32
addr)
{
	struct user_desc ud = { 
		.base_addr = addr,
		.limit = 0xfffff,
		.contents = (3 << 3), /* user */
		.seg_32bit = 1,
		.limit_in_pages = 1,
		.useable = 1,
	};
	struct n_desc_struct *desc = (void *)t->thread.tls_array;
	desc += tls;
	desc->a = LDT_entry_a(&ud); 
	desc->b = LDT_entry_b(&ud); 
}

where we get an overflow assigning "3 << 3" to ud.contents,
which is a
2-bit bitfield defined in include/asm-x86_64/ldt.h:

struct user_desc {
...
	unsigned int  contents:2;
...
};

Any idea what the intention was here?

--Stephen



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* Stephen C. Tweedie (sct@redhat.com) wrote:
> static inline void set_32bit_tls(struct task_struct *t, int tls, u32
> addr)
> {
> 	struct user_desc ud = { 
> 		.base_addr = addr,
> 		.limit = 0xfffff,
> 		.contents = (3 << 3), /* user */
> 		.seg_32bit = 1,
> 		.limit_in_pages = 1,
> 		.useable = 1,
> 	};
> 	struct n_desc_struct *desc = (void *)t->thread.tls_array;
> 	desc += tls;
> 	desc->a = LDT_entry_a(&ud); 
> 	desc->b = LDT_entry_b(&ud); 
> }
> 
> where we get an overflow assigning "3 << 3" to ud.contents,
which is a
> 2-bit bitfield defined in include/asm-x86_64/ldt.h:
> 
> struct user_desc {
> ...
> 	unsigned int  contents:2;
> ...
> };
> 
> Any idea what the intention was here?
And why this was never picked up?

http://lists.xensource.com/archives/html/xen-devel/2006-02/msg00640.html

thanks,
-chris

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