Subject: [PATCHv3 3/7] arm64: Move cpu_resume into the text section

From: [email-masked] (Laura Abbott)
Date: Mon, 25 Aug 2014 17:43:32 -0700

On 8/25/2014 1:34 PM, Stephen Boyd wrote:
> On 08/20/14 18:20, Laura Abbott wrote:
>> The function cpu_resume currently lives in the .data
>> section. This breaks functionality to make .data no
>> execute. Move cpu_resume to .text which is where it
>> actually belongs.
>>
>> Signed-off-by: Laura Abbott <[email-masked]>
>> ---
> 
> This is similar to arm (where I don't see a similar patch to this in
> Kees' patchset for RO). In arm we have this comment:
> 
> /*
>  * Note: Yes, part of the following code is located into the .data section.
>  *       This is to allow sleep_save_sp to be accessed with a relative load
>  *       while we can't rely on any MMU translation.  We could have put
>  *       sleep_save_sp in the .text section as well, but some setups might
>  *       insist on it to be truly read-only.
>  */
> 
> Doesn't the same situation apply here? How is this problem overcome?
> 
> But I'm a little confused, shouldn't this code run with the MMU disabled?
> 
>>  ENDPROC(cpu_resume_after_mmu)
>>  
>> -	.data
>>  ENTRY(cpu_resume)
>>  	bl	el2_setup		// if in EL2 drop to EL1 cleanly
>>  #ifdef CONFIG_SMP
>>  	mrs	x1, mpidr_el1
>> -	adr	x4, mpidr_hash_ptr
>> +	adrp	x4, mpidr_hash_ptr
>> +	add	x4, x4, #:lo12:mpidr_hash_ptr
>>  	ldr	x5, [x4]
>>  	add	x8, x4, x5		// x8 = struct mpidr_hash phys address
>>          /* retrieve mpidr_hash members to compute the hash */
>> @@ -143,14 +143,15 @@ ENTRY(cpu_resume)
>>  #else
>>  	mov	x7, xzr
>>  #endif
>> -	adr	x0, sleep_save_sp
>> +	adrp	x0, sleep_save_sp
>> +	add	x0, x0, #:lo12:sleep_save_sp
>>  	ldr	x0, [x0, #SLEEP_SAVE_SP_PHYS]
>>  	ldr	x0, [x0, x7, lsl #3]
>>  	/* load sp from context */
>>  	ldr	x2, [x0, #CPU_CTX_SP]
>> -	adr	x1, sleep_idmap_phys
>> +	adrp	x1, sleep_idmap_phys
>>  	/* load physical address of identity map page table in x1 */
>> -	ldr	x1, [x1]
>> +	ldr	x1, [x1, #:lo12:sleep_idmap_phys]
>>  	mov	sp, x2
>>  	/*
>>  	 * cpu_do_resume expects x0 to contain context physical address
>> @@ -160,6 +161,7 @@ ENTRY(cpu_resume)
>>  	b	cpu_resume_mmu		// Resume MMU, never returns
>>  ENDPROC(cpu_resume)
>>  
>> +	.data
>>  	.align 3
>>  mpidr_hash_ptr:
>>  	/*
> 
> 

Good point. I think this was a patch I added when I was debugging other
issues and assumed it would be needed (code in .data segment, seems
naturally a problem, right?) . When I revert the patch though it seems
to work just fine. I suspect the comment about pc relative load is no
longer relevant since I use the relocation trick to properly access
sleep_save_sp in the data section.

Since it's not technically needed, I could drop the patch and add one
adding the comment back saying this was done on purpose. On the other
hand, I wonder if I could do something 'interesting' by modifying
the cpu_resume code since it's writable if I was a malicious
program.

Thanks,
Laura

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