Sorry for my delayed reply. I was out on a business trip.
Did you try this with the ld.so statistics to see if the relocations were indeed
reduced at runtime?
One of my worries with these changes (since I am not an ELF expert either) is
that we make a change that doesn't actually do anything -- but people expect it
to.
$ LD_DEBUG=help /lib/ld-linux.so.2
Valid options for the LD_DEBUG environment variable are:
libs display library search paths
reloc display relocation processing
files display progress for input file
symbols display symbol table processing
bindings display information about symbol binding
versions display version dependencies
scopes display scope information
all all previous options combined
statistics display relocation statistics
unused determined unused DSOs
help display this help message and exit
To direct the debugging output into a file instead of standard output
a filename can be specified using the LD_DEBUG_OUTPUT environment variable.
I believe that it's the 'statistics' option.
LD_DEBUG=statistics <executable that's been prelinked>
Should result in something like:
128820: runtime linker statistics:
128820: total startup time in dynamic loader: 1974661 cycles
128820: time needed for relocation: 354639 cycles (17.9%)
128820: number of relocations: 90
128820: number of relocations from cache: 3
128820: number of relative relocations: 1201
128820: time needed to load objects: 1303654 cycles (66.0%)
128820:
128820: runtime linker statistics:
128820: final number of relocations: 94
128820: final number of relocations from cache: 3
If prelink is working, the number of relocations (relative or otherwise) will be
significantly reduced from the original non-relocated version.
If you can run this test, it would give me the assurance that the patch is safe,
and I'll get it incorporated into the prelink-cross sources.
--Mark
On 5/25/19 2:53 PM, Shane Peelar wrote:
> Patch is attached. Thank you!
>
> On Sat, May 25, 2019 at 2:30 AM Khem Raj <raj.k...@gmail.com
> <mailto:raj.k...@gmail.com>> wrote:
>
> On Fri, May 24, 2019 at 6:58 PM Shane Peelar <lookatyouhac...@gmail.com
> <mailto:lookatyouhac...@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.k...@gmail.com
> <mailto:raj.k...@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.k...@gmail.com
> <mailto:raj.k...@gmail.com>
> >> > <mailto:raj.k...@gmail.com <mailto:raj.k...@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.k...@gmail.com
> <mailto:raj.k...@gmail.com>
> >> > <mailto:raj.k...@gmail.com <mailto:raj.k...@gmail.com>>
> >> > > <mailto:raj.k...@gmail.com <mailto:raj.k...@gmail.com>
> <mailto:raj.k...@gmail.com <mailto:raj.k...@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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