[yocto] prelink-cross with -fno-plt

[Shane Peelar]

Patch is attached.  Thank you!

On Sat, May 25, 2019 at 2:30 AM Khem Raj <raj.khem at gmail.com> wrote:

> On Fri, May 24, 2019 at 6:58 PM Shane Peelar <lookatyouhacker at gmail.com>
> wrote:
> >
> > Great!  Would you be willing to accept a patch that makes arch-x86_64.c
> handle that condition like the other arches?
> >
>
> yes certainly.
>
> > -Shane
> >
> > On Fri, May 24, 2019 at 12:27 PM Khem Raj <raj.khem at gmail.com> wrote:
> >>
> >>
> >>
> >> On 5/24/19 8:10 AM, Shane Peelar wrote:
> >> > I did some reading into the sources in other architectures.  The
> closest
> >> > match, arch_i386.c, makes the write conditional as you say.
> >> > So do other arches, including |arch_arm.c, |arch_sh.c, |arch-mips.c,
> >> > |arch-s390.c, |arch-s390x.c, and |arch-ia64.c.||||||
> >> > ||||||
> >> > ||||||
> >> > Notably, |||||||arch-cris.c||||||| has the same assert as
> >> > |||||||arch-x86_64.c||||||| instead of the conditional.
> >> >
> >> > The code roughly looks like follows:||||||||||||||
> >> > ||||||||||||||
> >> > |||||||
> >> > |||||||
> >> > 1. Check for dso->info[DT_PLTGOT].  If it does not exist, return 0
> >> > 2. Call addr_to_sec on dso->info[DT_PLTGOT], return 1 if error
> >> > 3. Look for the section named ".plt" in the ELF.
> >> > 4. If the section cannot be found, return 0
> >> > 5. Otherwise, write the address of .plt + constant (dependent on arch)
> >> > to got[1]||||||||||||||
> >> > ||||||||||||||
> >> > |||||||
> >> > |||||||
> >> > In |||||||arch-x86_64.c and arch-cris.c|||||||, step (4) above is an
> >> > assert:|||||||
> >> >
> >> > |||||||1. Check for dso->info[DT_PLTGOT].  If it does not exist,
> return 0
> >> > 2. Call addr_to_sec on dso->info[DT_PLTGOT], return 1 if error
> >> > 3. Look for the section named ".plt" in the ELF.
> >> > 4. Assert that the section was found
> >> > 5. Write the address of .plt + constant (dependent on arch) to got[1]
> >> >
> >> > I tested out making the assert conditional and nothing seemed to break
> >> > at least.
> >> > |||||||
> >> > |||||||
> >>
> >> It seems ok to me.
> >>
> >> >
> >> > On Fri, May 24, 2019 at 12:08 AM Khem Raj <raj.khem at gmail.com
> >> > <mailto:raj.khem at gmail.com>> wrote:
> >> >
> >> >
> >> >
> >> >     On 5/23/19 7:53 PM, Shane Peelar wrote:
> >> >      > Any of them on the system pretty much, and yes they are also
> >> >     built with
> >> >      > -fno-plt.
> >> >
> >> >     OK, I think its better to them conditionally check for .plt
> section,
> >> >     can you describe more of whats going on when sections are checked.
> >> >
> >> >      >
> >> >      > On Thu, May 23, 2019 at 9:59 PM Khem Raj <raj.khem at gmail.com
> >> >     <mailto:raj.khem at gmail.com>
> >> >      > <mailto:raj.khem at gmail.com <mailto:raj.khem at gmail.com>>>
> wrote:
> >> >      >
> >> >      >
> >> >      >
> >> >      >     On 5/23/19 8:05 AM, Shane Peelar wrote:
> >> >      >      > Hi Everyone @ the Yocto project,
> >> >      >      >
> >> >      >      > I'm Shane Peelar, a PhD Candidate at the University of
> >> >     Windsor.
> >> >      >      > I'm writing to you about prelink-cross, as part of the
> >> >     Yocto project.
> >> >      >      > Specifically, I'm looking at using it with executables
> >> >     built using
> >> >      >      > `-fno-plt` under GCC.
> >> >      >      > I wasn't quite sure where to send this email to, so I
> >> >     figured I'd
> >> >      >     try
> >> >      >      > here.  If there's a better place to send this, please
> let
> >> >     me know.
> >> >      >      >
> >> >      >      > Right now, prelink-cross seems to fail an assertion in
> >> >      >     arch-x86_64.c,
> >> >      >      > line 421, when
> >> >      >      > using it with an executable built with `-fno-plt`:
> >> >      >      >
> >> >      >      > ...
> >> >      >      > assert (i < dso->ehdr.e_shnum)
> >> >      >      > ...
> >> >      >      >
> >> >      >      > This snippet seems to be looking for the ".plt" section
> and,
> >> >      >     since it
> >> >      >      > can't find it, the assertion fires.  This makes sense
> >> >     because in
> >> >      >      > `-fno-plt` executables, the `.plt` section is missing
> >> >     entirely.
> >> >      >      > I'm not an expert on ELF stuff, although I am learning
> >> >     quickly.  It
> >> >      >      > looks like
> >> >      >      > this code wants to write into GOT[1] the address of
> ".plt"
> >> >     + 0x16 --
> >> >      >      > since ".plt" doesn't
> >> >      >      > exist, does it make sense to just change this assert to
> an if
> >> >      >     statement
> >> >      >      > like so:
> >> >      >      >
> >> >      >      > ...
> >> >      >      >        if (i < dso->ehdr.e_shnum)
> >> >      >      >        { ... }
> >> >      >      > ...
> >> >      >      >
> >> >      >      > and skip over that part?  Or is this a real error
> >> >     condition for
> >> >      >      > prelink-cross and it really should not continue?  The
> >> >     executable in
> >> >      >      > question is also non-PIE, if that makes a difference.
> >> >      >      >
> >> >      >
> >> >      >     what shared libs is this linking to ? are they also built
> with
> >> >      >     -fno-plt ?
> >> >      >
> >> >      >      > Thanks for your time,
> >> >      >      > Shane
> >> >      >      >
> >> >      >
> >> >
>
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