[yocto] prelink-cross with -fno-plt
Shane Peelar
lookatyouhacker at gmail.com
Sun Jun 2 21:50:03 PDT 2019
Will do, thanks!
On Sat, Jun 1, 2019, 3:50 PM Mark Hatle, <mark.hatle at windriver.com> wrote:
> Thanks, this shows that the prelinking is still working in this case.
> I'll get
> you patch queued up. If you don't see any progress on it this coming week,
> please feel free to remind me.
>
> --Mark
>
> On 5/29/19 1:42 PM, Shane Peelar wrote:
> > 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.hatle at windriver.com
> > <mailto:mark.hatle at 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.khem at gmail.com
> > <mailto:raj.khem at gmail.com>
> > > <mailto:raj.khem at gmail.com <mailto:raj.khem at gmail.com>>> wrote:
> > >
> > > On Fri, May 24, 2019 at 6:58 PM Shane Peelar
> > <lookatyouhacker at gmail.com <mailto:lookatyouhacker at gmail.com>
> > > <mailto:lookatyouhacker at gmail.com <mailto:
> 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
> > <mailto:raj.khem at gmail.com>
> > > <mailto:raj.khem at gmail.com <mailto: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>
> > > <mailto:raj.khem at gmail.com <mailto:raj.khem at gmail.com>>
> > > >> > <mailto: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 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>>
> > > >> > <mailto:raj.khem at gmail.com <mailto:raj.khem at gmail.com
> >
> > <mailto:raj.khem at gmail.com <mailto:raj.khem at gmail.com>>>
> > > >> > > <mailto:raj.khem at gmail.com <mailto:
> raj.khem at gmail.com>
> > <mailto:raj.khem at gmail.com <mailto:raj.khem at gmail.com>>
> > > <mailto: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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