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

On Thu, Nov 01, 2018 at 02:55:34PM +1100, Michael Ellerman wrote:

Hi Florian,

Florian Weimer [off-list ref] writes:

quoted

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

...

quoted

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.

quoted

...
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.

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

But at the same time we haven't had any other reports of breakage, so is
this somehow specific to something Go is doing? Or did we just get lucky
up until now? Or is no one actually testing on Power? ;)

Mapping PIEs above 4G should be fine.  It works for gcc C and C++
after all.  The problem is that ppc64le Go is generating code not
suitable for a PIE.  Dynamic text relocations are evidence of non-PIC
object files.

Quoting Lynn Boger [off-list ref]:
"When building a pie binary with golang, they should be using
-buildmode=pie and not just pass -pie to the linker".

-- 
Alan Modra
Australia Development Lab, IBM