Re: [RFC][PATCH] AArch64: use movz/movk instead of literal pools in start.S

[Andrew Pinski <pinskia at gmail dot com>]

On Thu, Sep 7, 2017 at 12:33 AM, wangboshi <wangboshi@huawei.com> wrote:
> eXecute-Only Memory (XOM) is a protection mechanism against some ROP
> attacks. XOM sets the code as executable and unreadable, so the access to
> any data, like literal pools, in the code section causes the fault with XOM.
> The compiler can disable literal pools for C source files, but not for
> assembly files, so I use movz/movk instead of literal pools in start.S for
> XOM.
>
> I add MOVL macro with movz/movk instructions like movl pseudo-instruction in
> armasm, and use the macro instead of literal pools.

I have a few comments about the overall design:
I don't know if this is a good idea, can the kernel override XOM anyways?
That is if you do write(N, &main, 1024);
That will write the main function out to the file?


I have one comment about the implementation too.

>
>
> 2017-09-07  Wang Boshi  <wangboshi@huawei.com>
>
>     * sysdeps/aarch64/start.S: Use MOVL instead of literal pools.
>     * sysdeps/aarch64/sysdep.h (MOVL): Add MOVL macro.
>
> diff --git a/sysdeps/aarch64/start.S b/sysdeps/aarch64/start.S
> index df1c642..51e8e82 100644
> --- a/sysdeps/aarch64/start.S
> +++ b/sysdeps/aarch64/start.S
> @@ -71,9 +71,9 @@ _start:
>      ldr     PTR_REG (4), [x4, #:got_lo12:__libc_csu_fini]
>  #else
>      /* Set up the other arguments in registers */
> -    ldr    PTR_REG (0), =main
> -    ldr    PTR_REG (3), =__libc_csu_init
> -    ldr    PTR_REG (4), =__libc_csu_fini
> +    MOVL(0, main)
> +    MOVL(3, __libc_csu_init)
> +    MOVL(4, __libc_csu_fini)
>  #endif
>
>      /* __libc_start_main (main, argc, argv, init, fini, rtld_fini,
> diff --git a/sysdeps/aarch64/sysdep.h b/sysdeps/aarch64/sysdep.h
> index a749a70..0a11b57 100644
> --- a/sysdeps/aarch64/sysdep.h
> +++ b/sysdeps/aarch64/sysdep.h
> @@ -137,6 +137,20 @@
>      ldr    PTR_REG (T), [x##T, #:got_lo12:EXPR];    \
>      OP    PTR_REG (R), [x##T];
>
> +/* Load an immediate into R.
> +   Note R is a register number and not a register name. */
> +#ifdef __LP64__
> +# define MOVL(n, name)                    \
> +    movz    PTR_REG(n), #:abs_g3:name;        \
> +    movk    PTR_REG(n), #:abs_g2_nc:name;        \
> +    movk    PTR_REG(n), #:abs_g1_nc:name;        \
> +    movk    PTR_REG(n), #:abs_g0_nc:name;
> +#else
> +# define MOVL(n, name)                    \
> +    movz    PTR_REG(n), #:abs_g1:name;        \
> +    movk    PTR_REG(n), #:abs_g0_nc:name;
> +#endif

Since PTR_REG is defined only based on __LP64__ already why don't you just do:
#ifdef __LP64__
# define MOVL(n, name)                    \
    movz    x##n, #:abs_g3:name;        \
    movk    x##n, #:abs_g2_nc:name;        \
    movk    x##n, #:abs_g1_nc:name;        \
    movk    x##n, #:abs_g0_nc:name;
#else
# define MOVL(n, name)                    \
    movz    w##n, #:abs_g1:name;        \
    movk    w##n, #:abs_g0_nc:name;
#endif


Thanks,
Andrew

> +
>  /* Since C identifiers are not normally prefixed with an underscore
>     on this system, the asm identifier `syscall_error' intrudes on the
>     C name space.  Make sure we use an innocuous name.  */
>
>