On Mon, Oct 21, 2013 at 7:34 PM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
> __set_tss_desc has a complex calculation of the TSS segment limit,
> duplicating the quirky details of the I/O bitmap array length, and
> requiring a complex comment to explain. Replace that calculation with a
> simpler one based on the offsetof the "stack" field that follows the
> array.
>
> That then removes the last use of IO_BITMAP_OFFSET, so delete it.
>
> Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> ---
> arch/x86/include/asm/desc.h | 11 +----------
> arch/x86/include/asm/processor.h | 3 ++-
> 2 files changed, 3 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
> index b90e5df..17ac92f 100644
> --- a/arch/x86/include/asm/desc.h
> +++ b/arch/x86/include/asm/desc.h
> @@ -177,16 +177,7 @@ static inline void __set_tss_desc(unsigned cpu, unsigned int entry, void *addr)
> struct desc_struct *d = get_cpu_gdt_table(cpu);
> tss_desc tss;
>
> - /*
> - * sizeof(unsigned long) coming from an extra "long" at the end
> - * of the iobitmap. See tss_struct definition in processor.h
> - *
> - * -1? seg base+limit should be pointing to the address of the
> - * last valid byte
I think it might be better to keep at least a minimal comment near the
TSS_LIMIT declaration, just to explain the "-1" part, which is not
entirely obvious from just reading the code.
-Kees
> - */
> - set_tssldt_descriptor(&tss, (unsigned long)addr, DESC_TSS,
> - IO_BITMAP_OFFSET + IO_BITMAP_BYTES +
> - sizeof(unsigned long) - 1);
> + set_tssldt_descriptor(&tss, (unsigned long)addr, DESC_TSS, TSS_LIMIT);
> write_gdt_entry(d, entry, &tss, DESC_TSS);
> }
>
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 987c75e..03d3003 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -259,9 +259,10 @@ struct x86_hw_tss {
> #define IO_BITMAP_BITS 65536
> #define IO_BITMAP_BYTES (IO_BITMAP_BITS/8)
> #define IO_BITMAP_LONGS (IO_BITMAP_BYTES/sizeof(long))
> -#define IO_BITMAP_OFFSET offsetof(struct tss_struct, io_bitmap)
> #define INVALID_IO_BITMAP_OFFSET 0x8000
>
> +#define TSS_LIMIT (offsetof(struct tss_struct, stack) - 1)
> +
> struct tss_struct {
> /*
> * The hardware state:
> --
> 1.8.4.rc3
>
--
Kees Cook
Chrome OS Security
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