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

[Josef Söntgen]

Hello Alexander,

> […]
> So, this is inside dl_iterate_phdr under mutex, as usual, and double
> mutex call related to search of errno.
> I suspect other potential code path where we can hit «seach for
> address of some functions from shared libraries»... 
> 
> Finally I will remove, as suggested , mutex guard from dl_iterate_phdr
> completely - while not so sure that this will not lead to error.

Please replace your mutex-remove commit with the following commit [1],
which replaces the linker mutex with the shared-object mutex. This
behaviour is in line with other dynamic-linkers.

  [1] https://github.com/genodelabs/genode/commit/c45d964bc23f4f


(I moved the quotation down a bit to get concise block of text.)

> In short, it try to open ld.lib.so (from inside genode), open() fail,
> and it try to obtain libc errno (via jmp_slot/etc from shared library).
> So, this is inside dl_iterate_phdr under mutex, as usual, and double
> mutex call related to search of errno.
> I suspect other potential code path where we can hit «seach for address
> of some functions from shared libraries»...
> Also I add the following to the run file to allow read of ld.lib.so
> from inside (not sure that this is correct - may be need something
> else? like direct reference to rom session?)
>                     <dir name=«ld.lib.so»> </dir>

In case libbacktrace wants to access other shared-objects directly, besides
the binary itself, I would deal with it in the same way, e.g:

! […]
!     <vfs>
!       <dir name="dev"> <log/> </dir>
!
!       <rom name="ld.lib.so"/>
!       <rom name="xyz.lib.so"/>
!
!       <dir name="proc">
!         <dir name="self">
!           <rom name="exe" label="test-go"/>
!         </dir>
!       </dir>
!     </vfs>
! […]

This config operates under the assumption that libbacktrace wants to
access the shared-objects via 'open("/ld.lib.so")' - I have not checked
it that's true.


> Seems that after that operation my SMP small test with even 4 CPU
> works somehow (need more extensive tests to be run for golang and
> goroutunes).
> 
> Question: how to verify that removed mutex call does not break
> something important?

Please just use the commit I referenced above. As far as our automated
tests go, it did not introduce any regressions (of course that is no
guarantee that all potential corner-cases are covered).


Regards
Josef

-- 
Josef Söntgen
Genode Labs

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