[yocto] prelink-cross with -fno-plt

[Khem Raj]

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
>      >      >
>      >
>