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Re: [RFC][PATCH] AArch64: use movz/movk instead of literal pools in start.S
- From: Szabolcs Nagy <szabolcs dot nagy at arm dot com>
- To: wangboshi <wangboshi at huawei dot com>, libc-alpha at sourceware dot org
- Cc: nd at arm dot com
- Date: Mon, 11 Sep 2017 10:11:35 +0100
- Subject: Re: [RFC][PATCH] AArch64: use movz/movk instead of literal pools in start.S
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On 07/09/17 08:33, wangboshi 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.
>
>
> 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.
>
thanks, the patch looks good to me (except for a nit below),
do you have copyright assignment?
do you have commit rights to the glibc repo?
> 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. */
either rename the macro argument to R or use N in the comment.
> +#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 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. */
>
>