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Re: SH FDPIC ABI spec/binutils and kernel conflict on flag definitions

Hi Rich,

In the ELF Header part of the SH FDPIC ABI document, it's stated that
EF_SH_FDPIC|EF_SH_PIC means each LOAD segment can be independently
positioned, while EF_SH_FDPIC by itself (without EF_SH_PIC) means the
relative position of LOAD segments with respect to each other is fixed
(like in normal ELF usage). This seems to match what binutils outputs.

However the kernel contains the code (arch/sh/include/asm/elf.h):

#define elf_check_const_displacement(x) ((x)->e_flags & EF_SH_PIC)

and (fs/binfmt_elf_fdpic.c):

	if (elf_check_const_displacement(&exec_params.hdr))
		exec_params.flags |= ELF_FDPIC_FLAG_CONSTDISP;

which does exactly the opposite: the presence of the EF_SH_PIC flag,
rather than absence of it, causes the kernel to treat the binary as
one requiring "constant displacement" between LOAD segments.

If my analysis is correct, how should this be fixed? It seems to me
the kernel is clearly wrong, but it might also be considered the de
facto ABI.

I think that adopting the kernel's behaviour as correct would set a dangerous precedent. A published ABI should be followed, and if the kernel does not implement it, then the kernel is wrong.

Is there any way forward that allows fixing this bug (which
defeats the whole purpose of FDPIC) without breaking existing usage?

Maybe a kernel tuning option ? (I am not familiar with kernel development so maybe this idea is a non-starter).

Or is there no existing usage to care about?

I could not say for sure, but I imagine that there must be at least some people who are using SH linux at the moment.


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