False positive permanent breakpoints

Ofir Cohen ofircohenn@gmail.com
Wed Oct 26 14:04:00 GMT 2016


Hello,
We have debugging support for code running on the iGPU, and we've encountered
something that looks like a limitation of gdb.

Problem: gdb freaks out on the iGPU when trying to step over/continue when
               stopped on a permanent breakpoint.

Root-cause: gdb *falsely* identifies some instructions as permanent breakpoints.


Details
======
We don't call set_gdbarch_skip_permanent_breakpoint() on our target, so there
is no support for stepping over permanent breakpoints. Which is fine
since software breakpoints is not something that we support at the
moment.

While trying to fix the root-cause,
i.e. let gdb determine that instruction is indeed a breakpoint instruction,
I ran into a wall.

Looks like gdb is being too *coarse* for the check (whether it is a
breakpoint instruction):
if (err || memcmp (buf, bp_opcode (bp), bp_size (bp)) != 0)

in validate_inserted_breakpoint() function.

I'm saying coarse here because on the iGPU the breakpoint thingy is mandated
by a single bit, and does not have a unique *opcode*.


Proposed solution
==============
Could we abstract (delegate away) this predicate to the _target_ ?

Perhaps setting a default handler for other targets.

Thanks,
Ofir



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