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From: Szabolcs Nagy <szabolcs.nagy@arm.com>
To: Andrew Pinski <pinskia@gmail.com>, wangboshi <wangboshi@huawei.com>
Cc: nd@arm.com, GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [RFC][PATCH] AArch64: use movz/movk instead of literal pools in start.S
Date: Mon, 11 Sep 2017 09:24:00 -0000	[thread overview]
Message-ID: <59B65662.3080106@arm.com> (raw)
In-Reply-To: <CA+=Sn1n7edmzjTSDatyY1tfi3Lz70m3XjeXfgKbgDt_GCZE2NQ@mail.gmail.com>

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

  reply	other threads:[~2017-09-11  9:24 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-09-07  7:34 wangboshi
2017-09-07 11:43 ` Florian Weimer
2017-09-11  8:58   ` Szabolcs Nagy
2017-09-11  9:09 ` Andrew Pinski
2017-09-11  9:24   ` Szabolcs Nagy [this message]
2017-09-12  8:54   ` Boshi Wang
2017-09-11  9:11 ` Szabolcs Nagy
     [not found]   ` <336560e5-c9de-08bc-c850-28994cac2c33@huawei.com>
2017-09-14  8:35     ` Szabolcs Nagy
2017-09-15  1:16       ` Boshi Wang
2017-09-18 17:21         ` Szabolcs Nagy

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