Re: [Xen-devel] [PATCH v2 4/4] xen: use __symbol everywhere

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 17/10/2018 15:31, Stefano Stabellini wrote:
> Use __symbol everywhere _stext, _etext, etc. are used. Technically, it
> is only required when comparing pointers, but use it everywhere to avoid
Well no. It is also required when substracting pointers (see [1]).

> confusion.
[...]

> diff --git a/xen/arch/arm/alternative.c b/xen/arch/arm/alternative.c
> index 52ed7ed..305b337 100644
> --- a/xen/arch/arm/alternative.c
> +++ b/xen/arch/arm/alternative.c
> @@ -187,8 +187,8 @@ static int __apply_alternatives_multi_stop(void *unused)
>       {
>           int ret;
>           struct alt_region region;
> -        mfn_t xen_mfn = virt_to_mfn(_start);
> -        paddr_t xen_size = _end - _start;
> +        mfn_t xen_mfn = virt_to_mfn(__symbol(_start));
> +        paddr_t xen_size = __symbol(_end) - __symbol(_start);
>           unsigned int xen_order = get_order_from_bytes(xen_size);
>           void *xenmap;
>   
> @@ -206,7 +206,7 @@ static int __apply_alternatives_multi_stop(void *unused)
>           region.begin = __alt_instructions;
>           region.end = __alt_instructions_end;
>   
> -        ret = __apply_alternatives(&region, xenmap - (void *)_start);
> +        ret = __apply_alternatives(&region, xenmap - (void *)__symbol(_start));
For instance, I think this line would be contain undefined behavior. 
There are probably others below.
[...]

> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 7a06a33..d9d3948 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -620,7 +620,7 @@ void __init setup_pagetables(unsigned long 
> boot_phys_offset, paddr_t xen_paddr)
>       int i;
>   
>       /* Calculate virt-to-phys offset for the new location */
> -    phys_offset = xen_paddr - (unsigned long) _start;
> +    phys_offset = xen_paddr - (unsigned long) __symbol(_start);
>   
>   #ifdef CONFIG_ARM_64
>       p = (void *) xen_pgtable;
> @@ -681,7 +681,8 @@ void __init setup_pagetables(unsigned long 
> boot_phys_offset, paddr_t xen_paddr)
>       ttbr = (uintptr_t) cpu0_pgtable + phys_offset;
>   #endif
>   
> -    relocate_xen(ttbr, _start, (void*)dest_va, _end - _start);
> +    relocate_xen(ttbr, (void*)__symbol(_start), (void*)dest_va,
> +                            __symbol(_end) - __symbol(_start));
No hard tab.

[...]

> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> index ea2495a..9d0b915 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -394,7 +394,8 @@ static paddr_t __init get_xen_paddr(void)
>       paddr_t paddr = 0;
>       int i;
>   
> -    min_size = (_end - _start + (XEN_PADDR_ALIGN-1)) & ~(XEN_PADDR_ALIGN-1);
> +    min_size = (__symbol(_end) - __symbol(_start) +
> +                          (XEN_PADDR_ALIGN-1)) & ~(XEN_PADDR_ALIGN-1);
>   
>       /* Find the highest bank with enough space. */
>       for ( i = 0; i < mi->nr_banks; i++ )
> @@ -727,8 +728,9 @@ void __init start_xen(unsigned long boot_phys_offset,
>   
>       /* Register Xen's load address as a boot module. */
>       xen_bootmodule = add_boot_module(BOOTMOD_XEN,
> -                             (paddr_t)(uintptr_t)(_start + boot_phys_offset),
> -                             (paddr_t)(uintptr_t)(_end - _start + 1), NULL);
> +                             (paddr_t)(uintptr_t)(__symbol(_start) + 
> boot_phys_offset),
> +                             (paddr_t)(uintptr_t)(__symbol(_end) -
> +                                                                               
>   __symbol(_start) + 1), NULL);
No hard tab.

>       BUG_ON(!xen_bootmodule);
>   
>       xen_paddr = get_xen_paddr();
> diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
> index 93b79c7..0a7d6c0 100644
> --- a/xen/arch/x86/setup.c
> +++ b/xen/arch/x86/setup.c
[...]

> @@ -1390,22 +1393,22 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>       {
>           /* Mark .text as RX (avoiding the first 2M superpage). */
>           modify_xen_mappings(XEN_VIRT_START + MB(2),
> -                            (unsigned long)&__2M_text_end,
> +                            (unsigned long)__symbol(&__2M_text_end),
>                               PAGE_HYPERVISOR_RX);
>   
>           /* Mark .rodata as RO. */
> -        modify_xen_mappings((unsigned long)&__2M_rodata_start,
> -                            (unsigned long)&__2M_rodata_end,
> +        modify_xen_mappings((unsigned long)__symbol(&__2M_rodata_start),
> +                            (unsigned long)__symbol(&__2M_rodata_end),
AFAICT the return of __symbol should be unsigned long, right? If so, the 
cast could be removed.
Cheers,

[1] 
https://wiki.sei.cmu.edu/confluence/display/c/ARR36-C.+Do+not+subtract+or+compare+two+pointers+that+do+not+refer+to+the+same+array
--
Julien Grall

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