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

[Szabolcs Nagy]

On 11/09/17 10:09, Andrew Pinski wrote:
> 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 think the change makes sense even without xom, maybe
a better rationale is needed in the commit message.

(i don't know what the kernel does with the write, with
a non-readable page i'd expect the write to fail just
like on PROT_NONE pages: EFAULT).

>> +/* 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:

i think either is fine (the meaning of PTR_REG should
be obvious since it is used all over the place)

> #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