Hi Mark,
Thank you for your reply and no problem -- I chose to benchmark ssh-add
with it. It contains no `.plt`.
The results are as follows:
Without prelink (ran prelink -auv):
26019:
26019: runtime linker statistics:
26019: total startup time in dynamic loader: 1321674 cycles
26019: time needed for relocation: 797948 cycles
(60.3%)
26019: number of relocations: 624
26019: number of relocations from cache: 3
26019: number of relative relocations: 9691
26019: time needed to load objects: 389972 cycles
(29.5%)
Could not open a connection to your authentication agent.
26019:
26019: runtime linker statistics:
26019: final number of relocations: 630
26019: final number of relocations from cache: 3
With prelink (ran prelink -av):
1930:
1930: runtime linker statistics:
1930: total startup time in dynamic loader: 462288 cycles
1930: time needed for relocation: 48730 cycles (10.5%)
1930: number of relocations: 7
1930: number of relocations from cache: 134
1930: number of relative relocations: 0
1930: time needed to load objects: 286076 cycles
(61.8%)
Could not open a connection to your authentication agent.
1930:
1930: runtime linker statistics:
1930: final number of relocations: 9
1930: final number of relocations from cache: 134
I also tested against execstack, which for sure had the assertion fire on.
Without prelink:
27736:
27736: runtime linker statistics:
27736: total startup time in dynamic loader: 1955954 cycles
27736: time needed for relocation: 755440 cycles
(38.6%)
27736: number of relocations: 247
27736: number of relocations from cache: 3
27736: number of relative relocations: 1353
27736: time needed to load objects: 710384 cycles
(36.3%)
/usr/bin/execstack: no files given
27736:
27736: runtime linker statistics:
27736: final number of relocations: 251
27736: final number of relocations from cache: 3
With prelink:
3268:
3268: runtime linker statistics:
3268: total startup time in dynamic loader: 1421206 cycles
3268: time needed for relocation: 199396 cycles
(14.0%)
3268: number of relocations: 3
3268: number of relocations from cache: 88
3268: number of relative relocations: 0
3268: time needed to load objects: 696886 cycles
(49.0%)
/usr/bin/execstack: no files given
3268:
3268: runtime linker statistics:
3268: final number of relocations: 5
3268: final number of relocations from cache: 88
So, it looks like prelink is working on these :)
On Tue, May 28, 2019 at 2:57 PM Mark Hatle <mark.ha...@windriver.com> wrote:
> 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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