Re: [PATCH] binfmt_elf: fix PIE load with randomization disabled

[H.J. Lu]

On Mon, Dec 9, 2013 at 12:57 PM, Jiri Kosina <jkosina@xxxxxxx> wrote:
> On Mon, 9 Dec 2013, H.J. Lu wrote:
>
>> Normally, a PIE executable has zero virtual address on the first PT_LOAD
>> segment and kernel will load such executable at random address when
>> randomization is enabled.  If randomization is disabled, kernel will load
>> it at a fixed address.  But if a PIE executable has non-zero virtual
>> address on the first PT_LOAD segment, kernel will load such executable
>> at the non-zero virtual address when randomization is enabled. But when
>> randomization is disabled, kernel ignores the non-zero virtual address
>> at the non-zero virtual address when randomization is enabled.
>
> Hmm ... isn't actually this the thing that needs to be fixed instead?
>
> IOW, when randomization is enabled, is there a reason not to load on
> randomized address? (even if the first PT_LOAD segment has non-zero
> vaddr?)

No, please don't do that.  Normally, PIE has zero load address and kernel
can load it anywhere.  There are multiple reasons why PIE has non-zero
load address.  Saying you need to load a program above 4GB under x86-64,
you can't do that with normal dynamic executable.  PIE with non-zero load
address is the only way to do that on x86-64.

>> But when randomization is disabled, kernel ignores the non-zero virtual
>> address on the first PT_LOAD segment and loads it at the fixed address.
>> This patch makes kernel consistent by loading PIE executable with
>> non-zero virtual address at the non-zero virtual address, regardless if
>> randomization is enabled or disabled.
>


-- 
H.J.
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