how to open elf file image from inside genode? libbacktrace question

[Josef Söntgen]

Hallo Alexander,

> Returning to my question, could I use posix open(«/test-go») to open
> my binary from which I run executable, if I have the following code in
> run file:
> […]

Yes, using 'open()' is the way to go. As libbacktrace hardcodes a few
options you could piggy-back on one of them. I picked the '/proc/self/exe'
option:

! <start name="test-go" caps="250">
!   <resource name="RAM" quantum="12M"/>
!   <config verbose="yes" ld_verbose="yes">
!     <vfs>
!       <dir name="dev"> <log/> </dir>
!
!       <dir name="proc">
!         <dir name="self">
!           <rom name="exe" label="test-go"/>
!         </dir>
!       </dir>
!     </vfs>
!     <libc stdout="/dev/log" stderr="/dev/log"/>
!   </config>
! </start>

With this VFS configuration in place libbacktrace opens the binary
successfully. However, you will be greeted with the by now infamous
deadlock ahead error message when using '-smp 2':

! [init -> test-go] Error: deadlock ahead, mutex=0x164040, return ip=0xf3383

The mutex in question is the 'Linker::mutex()' object. Again this is due
to the fact that as side-effect of 'dl_iterate_phdr' a jump-slot
relocation for 'mknod' takes place (*). Why that is I have not checked.

Now we are not entirely sure if taking the mutex in 'dl_iterate_phdr' is
strictly necessary - that is something we have to investigate. Removing
the mutex allows the 'go_app' to run through. So you are free to do the
same. But to be clear, we do not know if this leads to other regressions.
We are going to look into that.


(*) You can instrument that by applying the patch I attached that
    makes the linker verbose and shows you the object and index for
    the relocation:

    ! […]
    ! [init -> test-go] LD: SLOT libc.lib.so 0x20c
    ! [init -> test-go] LD: SLOT binary 0xe
    ! [init -> test-go] Error: deadlock ahead, mutex=0x164040, return ip=0xf3383

    With this information you can check to which symbol the index
    belongs to:

    $ readelf --dyn-syms test-go|grep '\<14:'
        14: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND mknod


Regards
Josef

-- 
Josef Söntgen
Genode Labs

http://www.genode-labs.com/ · http://genode.org/
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