Subject: Re: PIE binaries are no longer mapped below 4 GiB on ppc64le

From: Florian Weimer <[email-masked]>
Date: Thu, 01 Nov 2018 12:20:50 +0100

* Michael Ellerman:

> Hi Florian,
>
> Florian Weimer <[email-masked]> writes:
>> We tried to use Go to build PIE binaries, and while the Go toolchain is
>> definitely not ready (it produces text relocations and problematic
>> relocations in general), it exposed what could be an accidental
>> userspace ABI change.
>>
>> With our 4.10-derived kernel, PIE binaries are mapped below 4 GiB, so
>> relocations like R_PPC64_ADDR16_HA work:
>>
>> 21f00000-220d0000 r-xp 00000000 fd:00 36593493                           /root/extld
>> 220d0000-220e0000 r--p 001c0000 fd:00 36593493                           /root/extld
>> 220e0000-22100000 rw-p 001d0000 fd:00 36593493                           /root/extld
> ...
>>
>> With a 4.18-derived kernel (with the hashed mm), we get this instead:
>>
>> 120e60000-121030000 rw-p 00000000 fd:00 102447141                        /root/extld
>> 121030000-121060000 rw-p 001c0000 fd:00 102447141                        /root/extld
>> 121060000-121080000 rw-p 00000000 00:00 0 
>
> I assume that's caused by:
>
>   47ebb09d5485 ("powerpc: move ELF_ET_DYN_BASE to 4GB / 4MB")
>
> Which did roughly:
>
>   -#define ELF_ET_DYN_BASE	0x20000000
>   +#define ELF_ET_DYN_BASE		(is_32bit_task() ? 0x000400000UL : \
>   +					   0x100000000UL)
>
> And went into 4.13.
>
>> ...
>> I'm not entirely sure what to make of this, but I'm worried that this
>> could be a regression that matters to userspace.
>
> It was a deliberate change, and it seemed to not break anything so we
> merged it. But obviously we didn't test widely enough.

* Michael Ellerman:

>> I'm not entirely sure what to make of this, but I'm worried that this
>> could be a regression that matters to userspace.
>
> It was a deliberate change, and it seemed to not break anything so we
> merged it. But obviously we didn't test widely enough.

Thanks for moving back the discussion to kernel matters. 8-)

> So I guess it clearly can matter to userspace, and it used to work, so
> therefore it is a regression.

Is there a knob to get back the old base address?

> But at the same time we haven't had any other reports of breakage, so is
> this somehow specific to something Go is doing?

Go uses 32-bit run-time relocations which (I think) were primarily
designed as link-time relocations for programs mapped under 4 GiB.  It's
amazing that the binaries work at all under old kernels.  On other
targets, the link editor refuses to produce an executable, or may even
produce a binary which crashes at run time.

> Or did we just get lucky up until now? Or is no one actually testing
> on Power? ;)

I'm not too worried about it.  It looks like a well-understood change to
me.  The glibc dynamic linker prints a reasonably informative error
message (in the sense that it doesn't crash without printing anything).
I think we can wait and see if someone comes up with a more compelling
case for backwards compatibility than the broken Go binaries (which we
will rebuild anyway because we don't want text relocations).  I assume
that it will be possible to add a personality flag if it ever proves
necessary—or maybe map the executable below 4 GiB in case of ASLR is
disabled, so that people have at least a workaround to get old binaries
going again.

But right now, that doesn't seem necessary.

Thanks,
Florian