[PATCH v1 1/8] powerpc/lib/code-patching: Enhance code patching

[Christophe LEROY]

Le 29/05/2017 à 00:50, Balbir Singh a écrit :
> On Sun, 2017-05-28 at 17:59 +0200, christophe leroy wrote:
>>
>> Le 25/05/2017 à 05:36, Balbir Singh a écrit :
>>> Today our patching happens via direct copy and
>>> patch_instruction. The patching code is well
>>> contained in the sense that copying bits are limited.
>>>
>>> While considering implementation of CONFIG_STRICT_RWX,
>>> the first requirement is to a create another mapping
>>> that will allow for patching. We create the window using
>>> text_poke_area, allocated via get_vm_area(), which might
>>> be an overkill. We can do per-cpu stuff as well. The
>>> downside of these patches that patch_instruction is
>>> now synchornized using a lock. Other arches do similar
>>> things, but use fixmaps. The reason for not using
>>> fixmaps is to make use of any randomization in the
>>> future. The code also relies on set_pte_at and pte_clear
>>> to do the appropriate tlb flushing.
>>
>> Isn't it overkill to remap the text in another area ?
>>
>> Among the 6 arches implementing CONFIG_STRICT_KERNEL_RWX (arm, arm64,
>> parisc, s390, x86/32, x86/64):
>> - arm, x86/32 and x86/64 set text RW during the modification
> 
> x86 uses set_fixmap() in text_poke(), am I missing something?
> 

Indeed I looked how it is done in ftrace. On x86 text modifications are 
done using ftrace_write() which calls probe_kernel_write() which doesn't 
remap anything. It first calls ftrace_arch_code_modify_prepare() which 
sets the kernel text to rw.

Indeed you are right, text_poke() remaps via fixmap. However it looks 
like text_poke() is used only for kgdb and kprobe

Christophe