Re: static PIE

['Rich Felker' <dalias@aerifal.cx>]

On Wed, Jul 30, 2014 at 12:19:03PM -0700, Weiming Zhao wrote:
> I just find a very interesting article written by you:
> 
> http://www.openwall.com/lists/musl/2012/05/24/1

This method is somewhat outdated. In particular, requiring a custom
linker script is a pain.

The new method is to use -shared instead of -pie to trick gcc that
it's generating a shared library (this will cause it to use a linker
mode that does not add a PT_INTERP header, and to omit crt1) and
manually add the needed Zcrt[12].o (no need to use -nostartfiles to
suppress others). The command line should look like:

gcc -shared -static-libgcc -Wl,-static -Wl,-Bsymbolic \
    Zcrt1.o Zcrt2.o [your object files...]

> I want to do the similar thing on ARM linux. I see _static_pie_reloc does
> the relocation, which would be done by loader in dynamic PIE.

Nice! Are you interested in trying to get this 'upstream' in gcc?
Technically it's not needed, but it would be nice if "-pie -static"
just did the right thing without the command line hackery.

> But with "-static", those reloc entries has already been fixed by ld.
> Without that, my code can still run but at fixed address space.

I don't think that should happen. Static linking objects (as long as
they're PIC/PIE) into a ET_DYN ELF file (.so or PIE executable) should
not result in fixed addresses but "relative" type relocations for the
dynamic linker.

> To get the benefit of PIE, there should be address randomization (at least
> for data sections), which should be done in startup code. Is my
> understanding right?

No, the kernel does the address randomization (the random base address
it loads the program at). The userspace side is just applying this
base address to the relative relocations in the rel/rela tables.

Rich