[lldb-dev] Not able to set breakpoints above line #70 in lldb-mi.

Michael Kuklinski via lldb-dev lldb-dev at lists.llvm.org
Tue Aug 30 11:07:59 PDT 2016

I never see a Z0 or a Z1 packet for those addresses. Anything above Line 70, it never tries to send a packet to my stub to indicate that it wishes to place a breakpoint. I get the same behavior using -H that I do without. The system the stub is attached to is an emulator, so there's absolutely nothing special about those addresses. The stub can also support an arbitrary number of breakpoints (hardware and software).

In fact, when I specify, say, Line 72, it sends absolutely nothing to the stub. When I specify Line 70, I see:

> \03
< S 11
> Z1,13488,4
< OK
> c
< OK

Which appears to be a sane sequence. However, it doesn’t appear to be doing anything for higher addresses. It doesn't ever try to read them or see if they're valid. This is also the behavior I see when Line 72 is the first breakpoint I try to set, so I'm not sure how LLDB is coming to the conclusion that it cannot resolve it, as it's never actually trying to set the breakpoint on the stub.

In this case, Line 72 reporting as address 0x13384 - 'memory read 0x13384' worked without issue on that address.


-----Original Message-----
From: jingham at apple.com [mailto:jingham at apple.com] 
Sent: Tuesday, August 30, 2016 11:04 AM
To: Michael Kuklinski <mike.k at pixelcarbide.com>
Cc: lldb-dev at lists.llvm.org
Subject: Re: [lldb-dev] Not able to set breakpoints above line #70 in lldb-mi.

"unresolved" means that we know what address we want to set a breakpoint at, but for some reason can't.  For instance, if you were using -H and ran out of hardware breakpoints, then the subsequent breakpoints would never get resolved, since we can't implement them.

Anyway, going from unresolved -> resolved has nothing to do with the line table.  That is only used to figure out what address we should use.  So it isn't surprising that that looks okay.

What you need to do instead is figure out why we can't implement the breakpoint at that address.  Do we send a Z0 packet for that address as well as the others?  Can you read from the memory at the address of the breakpoints that aren't getting set?  Write to them?  


> On Aug 30, 2016, at 2:45 AM, Michael Kuklinski via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> Hello,
> I’m having an issue setting breakpoints using lldb-mi.
> I am trying to run a C++ program (which is itself a brainfsck interpreter), compiled for MIPS32r6 using Clang, linked with lld, and am trying to use lldb-mi as the debugger. I am using lldb-mi as my primary use for it is as an intermediary for a visual debugger, but I get the same behavior when using it myself.
> Whenever I try to set a reasonable breakpoint at or below line #70 in Main.cpp, it works – it is resolved, it sends a Z0 packet to the gdb stub, and it works. However, when I attempt to set any breakpoint above line #70, it comes up as ‘unresolved’ (though the addresses shown in the list seem reasonable) and does not submit anything to the stub as a result.
> When I dumped the line table using both readelf and lldb, they were completely reasonable – almost every line until the end had an entry – there was no cut off at line #70.
> At this point, I’m unsure if I’m doing something wrong, or if lldb-mi is having some difficulty with my MIPS binary. 
> I’m building with the following options:
> clang -std=c++14 -mcompact-branches=always -fasynchronous-unwind-tables -funwind-tables -fexceptions -fcxx-exceptions -mips32r6 -O0 -g -glldb ...
> lld --discard-none -znorelro --eh-frame-hdr ...
> And the resultant binary is here: http://filebin.ca/2tJzo2LLBJWO/MipsTest.bin (if you wish something to analyze).
> I’ve also uploaded the rather poor code for Main.cpp here: 
> https://paste.ee/p/WYs8Y
> I’ve also tested this on a second source file in the ELF (‘Recoder.cpp’) which stops working at a lower line number, but has similar behavior – there’s a hard cut-off where lldb stops working with breakpoints. The line table for that file also seems fine.
> Just for example, when I set a breakpoint at or below 70 (note, I get the same behavior when not specifying -H):
> break set -H -f Main.cpp -l 70
> (gdb)
> =breakpoint-created,bkpt={number="1",type="breakpoint",disp="keep",ena
> bled="y",addr="0x0000000000013488",func="main",file="Main.cpp",fullnam
> e="D:\Proj 
> ects\MipsTest2\MipsTest/Main.cpp",line="70",times="0",original-locatio
> n="Main.cpp:70"}
> (gdb)
> =breakpoint-modified,bkpt={number="1",type="breakpoint",disp="keep",en
> abled="y",addr="0x0000000000013488",func="main",file="Main.cpp",fullna
> me="D:\Pro 
> jects\MipsTest2\MipsTest/Main.cpp",line="70",times="0",original-locati
> on="Main.cpp:70"}
> And this is what I see when I set a breakpoint after 70:
> break set -H -f Main.cpp -l 72
> (gdb)
> =breakpoint-created,bkpt={number="2",type="breakpoint",disp="keep",ena
> bled="y",addr="0x00000000000134a0",func="main",file="Main.cpp",fullnam
> e="D:\Proj 
> ects\MipsTest2\MipsTest/Main.cpp",line="72",times="0",original-locatio
> n="Main.cpp:72"}
> However, the list looks as such:
> break list
> Current breakpoints:
> 1: file = 'Main.cpp', line = 70, exact_match = 0, locations = 1, resolved = 1, hit count = 1
>   1.1: where = MipsTest.bin`main + 520 at Main.cpp:70, address = 
> 0x00013488, resolved, hit count = 1
> 2: file = 'Main.cpp', line = 72, exact_match = 0, locations = 1
>   2.1: where = MipsTest.bin`main + 544 at Main.cpp:72, address = 
> MipsTest.bin[0x000134a0], unresolved, hit count = 0
> Note that the address seems completely reasonable but it remains unresolved, and does not submit anything to the stub.
> Thanks for any help or insight you might be able to provide, Michael
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