False positive permanent breakpoints

Pedro Alves palves@redhat.com
Wed Oct 26 14:28:00 GMT 2016


On 10/26/2016 03:04 PM, Ofir Cohen wrote:
> Hello,
> We have debugging support for code running on the iGPU, and we've encountered
> something that looks like a limitation of gdb.

What's "the iGPU" ?  An integrated GPU?

> 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.

So this is in gdbserver, not gdb.

> 
> 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_ ?

All can be done, it's just software.  :-)  But not if no port
in the tree is using it...  Otherwise it'll just look like
unnecessary complication and will likely be "cleaned up" again
Are you planning on contributing your port upstream?

I'm a bit confused since you said you don't support software
breakpoints...

> Perhaps setting a default handler for other targets.

Thanks,
Pedro Alves



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