debugging link_map corruption

Maule Mark
Wed Jun 4 13:36:00 GMT 2014

Tried this on libc-help, no response, so trying gdb ...

I have a heavily threaded program linked against many (~50) shared libraries which occasionally experiences memory corruption such that the link_map list gets trashed rendering the core mostly undebuggable.  I'm looking for ways to debug these sorts of problems.  My operating environment is Linux.

One idea I am experimenting with is to create an audit library which saves the publicly available link_map (the one exposed through /usr/include/link.h on Linux) list to a write-protected area upon receipt of a LA_ACT_CONSISTENT activity callback.  The thinking is that if gdb can't follow the link_map from the core, at least I would be able to manually load the .so's at their correct addresses from gdb when debugging the 'corrupt' core.  In my current implementation, the audit library has a simple 8k buffer which it uses to store the public link_map structs in.

The problem I'm having with the above, is that I can't figure out how to expose information about the address of the audit library's link_map buffer to gdb when debugging the core.  I could issue a fprintf from my audit library to save that information in a file, but it would be much better if I could just figure that out with gdb.  I assume the issue is that the audit library symbols are in a separate namespace.

Anyway, I'm looking for guidance on how to gain access to audit library symbols from gdb when examining a program core.  Additionally, if there are better ideas for how to attack the problem of corrupt link_map lists in general, I'd appreciate those as well.

Mark Maule

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