9

I have shared object A.so which statically links to libssl.a & another shared object B.so which also statically links libssl.a .

A.so & B.so has symbols from libssl.a in GLOBAL scope. I checked this by readelf -s A.so

I have an executable a.out which loads A.so and B.so. When a.out terminated I get a double free error in one of the symbols from libssl.a in A.so.

Even though libssl.a is statically linked to each shared object, since they are exposed globally is it possible that the same symbol is shared instead of picking it's local copy.

What is the workaround this ? How to make the symbols local here ?

Please help

KodeWarrior
  • 3,538
  • 3
  • 26
  • 40
  • I would recommend using a debugger to confirm your theory. – jdigital Jun 12 '12 at 22:43
  • Just a small thing, because I have no idea how to apply it to your situation: `dlopen` has a RTLD_LOCAL flag which would in some circumstances help in exactly this situation. So if you opened those libraries with `dlopen`, they probably shouldn't interfere then. – liori Jun 12 '12 at 23:03
  • I'm not using dlopen. The shared libs dynamically linked. – KodeWarrior Jun 12 '12 at 23:14

1 Answers1

7

This is indeed expected. One instance of libssl.a interposes (likely a subset of) the other, and the results are not pretty. You can use a version script (--version-script to ld, with -Wl, for cc) to control what is exported from A.so and B.so. If something is not exported, it cannot be interposed either.

Alternatively, you could compile libssl.a with visibility flags like -fvisibility=hidden. These flags only affect the dynamic linker and not static linking. You likely needed to compile it yourself anyway because shipped .a files tend to contain position-dependent code, meant for linking into executables. Only some platforms such as 32-bit x86 let you get away with linking such code into shared objects and only at the cost of text relocations.

The dlopen with RTLD_LOCAL as suggested in a comment should also work but it seems hackish to use dlopen for this purpose.

Another option is to use the same shared libssl.so in both libraries.

jilles
  • 10,509
  • 2
  • 26
  • 39