[Xen-devel] Weird overflowing user_desc assignment

["Stephen C. Tweedie" <sct@xxxxxxxxxx>]

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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