Improved linker-debugger interface
Paul Pluzhnikov
ppluzhnikov@google.com
Thu May 26 17:37:00 GMT 2011
Gary,
On Wed, May 25, 2011 at 8:36 AM, Gary Benson <gbenson@redhat.com> wrote:
> The solution I'm proposing is this:
It's great that you are working on this, thanks!
You might wish to consider the following points, which (AFAICT) you are
not currently addressing.
1. Stripped binaries.
There is a DT_DEBUG entry pointing to _dl_debug_state (set by ld-linux)
You might want to have a new dynamic tag, pointing to
_dl_debug_state_extended(), so the debugger would be able to track
shared libs using the new mechanism even when everything is stripped.
2. In-process debuggers.
There are many use cases for "self-aware" binaries. For example,
google-perftools collects profiling info with stack traces, and getting
a stack trace requires that you know which DSOs are loaded into the
process.
The current interface is very hostile to such debuggers, as
_dl_debug_state
A) is called directly by libc (so there is no way to interpose it), and
B) compiles down to a single 'ret', so there is no way to place a patch
on top of it.
This leads to all kinds of suboptimal solutions (such as scanning
/proc/self/maps; which doesn't work if /proc is not mounted).
A patch to make _dl_debug_state indirect through r_debug.r_brk has
been rejected:
https://bugzilla.redhat.com/show_bug.cgi?id=70407
Perhaps co-operation with "in-process" debuggers would be more acceptable
for a new interface?
Thanks,
--
Paul Pluzhnikov
More information about the Archer
mailing list