Re: [PATCH v2] fs/binfmt_elf_fdpic.c: provide NOMMU loader for regular ELF binaries

[Rich Felker <dalias@libc.org>]

On Tue, Oct 13, 2015 at 10:55:45PM +1000, Greg Ungerer wrote:
> Hi Rich,
> 
> On 09/10/15 02:38, Rich Felker wrote:
> >From: Rich Felker <dalias@libc.org>
> >
> >The ELF binary loader in binfmt_elf.c requires an MMU, making it
> >impossible to use regular ELF binaries on NOMMU archs. However, the
> >FDPIC ELF loader in binfmt_elf_fdpic.c is fully capable as a loader
> >for plain ELF, which requires constant displacements between LOAD
> >segments, since it already supports FDPIC ELF files flagged as needing
> >constant displacement.
> >
> >This patch adjusts the FDPIC ELF loader to accept non-FDPIC ELF files
> >on NOMMU archs. They are treated identically to FDPIC ELF files with
> >the constant-displacement flag bit set, except for personality, which
> >must match the ABI of the program being loaded; the PER_LINUX_FDPIC
> >personality controls how the kernel interprets function pointers
> >passed to sigaction.
> >
> >Files that do not set a stack size requirement explicitly are given a
> >default stack size (matching the amount of committed stack the normal
> >ELF loader for MMU archs would give them) rather than being rejected;
> >this is necessary because plain ELF files generally do not declare
> >stack requirements in theit program headers.
> >
> >Only ET_DYN (PIE) format ELF files are supported, since loading at a
> >fixed virtual address is not possible on NOMMU.
> >
> >Signed-off-by: Rich Felker <dalias@libc.org>
> 
> I have no problem with this, so from me:
> 
> Acked-by: Greg Ungerer <gerg@uclinux.org>

Thanks!

> >---
> >
> >This patch was developed and tested on J2 (SH2-compatible) but should
> >be usable immediately on all archs where binfmt_elf_fdpic is
> >available. Moreover, by providing dummy definitions of the
> >elf_check_fdpic() and elf_check_const_displacement() macros for archs
> >which lack an FDPIC ABI, it should be possible to enable building of
> >binfmt_elf_fdpic on all other NOMMU archs and thereby give them ELF
> >binary support, but I have not yet tested this.
> 
> There is a couple of other details that will currently stop this from
> working on other arches too.
> 
> .. kernel/ptrace.c has some fdpic specific code (wanting PTRACE_GETFDPIC)
> .. arch specific mm_context_t may not have members
> ‘interp_fdpic_loadmap' or 'exec_fdpic_loadmap'
> 
> Should be easy to fix those.

I see. For archs that lack an FDPIC ABI, I'm not sure it makes sense
to add these things unless/until someone developes an FDPIC ABI. Would
it instead make sense to add a new kconfig switch
CONFIG_BINFMT_ELF_NOMMU ("NOMMU ELF loader") that's implied-on by
CONFIG_BINFMT_ELF_FDPIC but that can also be enabled independently on
archs where CONFIG_BINFMT_ELF_FDPIC is not available?

Right now these are just ideas. Unless there's a quick and easy
decision to be made, I'd like it if we could move forward with the
current patch (which only offers the feature on archs where
CONFIG_BINFMT_ELF_FDPIC is already available) first and continue to
explore options for making this available to other archs separately.

> It would be good to get some testing and verification on other
> fdpic supported arches (frv or blackfin or microblaze for example).

I wasn't aware Microblaze had an FDPIC ABI; are you sure it does?

Testing to make sure these aren't broken by the patch shouldn't be
hard to do; I'll start looking into getting a setup for it or finding
someone who has one. If you want to also test non-FDPIC ELF binaries,
I think just using the ELF output of a bFLT toolchain without running
elf2flt may work as a test case, but I'm not sure. Alternatively, any
FDPIC binary linked with -pie that doesn't use signals can run as a
non-FDPIC one just by clearing the FDPIC bit in the header.

Rich