[lldb-dev] Stepping into function generates EXC_BAD_INSTRUCTION signal
Mario Zechner
badlogicgames at gmail.com
Mon Dec 1 10:47:23 PST 2014
Thanks, i'm going to try that. I just wonder if it would make more sense to
consider the it instruction a branching instruction. Not sure what side
effects that may have.
Also, if i wrote a 4-byte breakpoint for blne, would it get hit if the it
branches over it? Guess i'll find out :)
On Dec 1, 2014 6:46 PM, "Stephane Sezer" <sas at fb.com> wrote:
> I remember fighting with this recently in our debug server (ds2), your
> understanding of the problem is correct I believe. What you need to do is
> to place a four-byte thumb breakpoint instead of a two-byte thumb
> breakpoint. I don’t know what the iOS kernel expects exactly, but for
> example the Linux kernel understands the following:
> - two-byte thumb breakpoint: 0xde01
> - four-byte thumb breakpoint: 0xa000f7f0
> - arm breakpoint: 0xe7f001f0
>
> If you insert a four-byte thumb breakpoint at 0x27b2ea, the it instruction
> will skip four bytes when skipping the breakpoint, and will end up at
> address 0x27b2ee, which is what you would expect.
>
> --
> Stephane Sezer
>
> > On Dec 1, 2014, at 8:13 AM, Mario Zechner <badlogicgames at gmail.com>
> wrote:
> >
> > I think i understand the issue now.
> ThreadPlanStepRange::SetNextBranchBreakpoint is falsely selecting the blne
> instruction instead of the it instruction. The condition is not meet, so
> the CPU jumps over the instruction after it. Since we have a trap there
> that's 2 bytes long, it will end up at 0x27b2ec (PC after 2 byte trap
> instruction) instead of 0x27b2ee (PC after 4 byte blne). So the CPU ends up
> in the middle of the blne instruction, which is of course not a valid
> instruction.
> >
> > I guess the next thing i have to figure out is why the it instruction
> isn't marked as a branch instruction, which is why it isn't selected by
> ThreadPlanStepRange::SetNextBranchBreakpoint as the next branch breakpoint.
> >
> > On Mon, Dec 1, 2014 at 4:59 PM, Mario Zechner <badlogicgames at gmail.com>
> wrote:
> > I traced through ThreadPlanStepRange and ThreadPlanStepRange for this
> piece of code:
> >
> > 0x27b2d4 <[J]java.lang.Object.<init>()V>: push {r7, lr}
> >
> > 0x27b2d6 <[J]java.lang.Object.<init>()V+2>: mov r7, sp
> >
> > 0x27b2d8 <[J]java.lang.Object.<init>()V+4>: sub sp, #0x4
> >
> > 0x27b2da <[J]java.lang.Object.<init>()V+6>: movs r2, #0x0
> >
> > 0x27b2dc <[J]java.lang.Object.<init>()V+8>: str r2, [sp]
> >
> > 0x27b2de <[J]java.lang.Object.<init>()V+10>: str r1, [sp]
> >
> > 0x27b2e0 <[J]java.lang.Object.<init>()V+12>: ldr r2, [r1]
> >
> > 0x27b2e2 <[J]java.lang.Object.<init>()V+14>: ldr r2, [r2, #0x30]
> >
> > 0x27b2e4 <[J]java.lang.Object.<init>()V+16>: tst.w r2, #0x100000
> >
> > 0x27b2e8 <[J]java.lang.Object.<init>()V+20>: it ne
> >
> > 0x27b2ea <[J]java.lang.Object.<init>()V+22>: blne 0x466290
> ; _bcRegisterFinalizer
> >
> > 0x27b2ee <[J]java.lang.Object.<init>()V+26>: add sp, #0x4
> >
> > 0x27b2f0 <[J]java.lang.Object.<init>()V+28>: pop {r7, pc}
> >
> > 0x27b2f2 <[J]java.lang.Object.<init>()V+30>: nop
> >
> >
> >
> > Execution is halted at 0x27b2e0 when i issue a source-level step. The
> ThreadPlanStepRange::DidPush method sets up a breakpoint at 0x27b2ea (2
> bytes) successfully after identifying the instruction at 0x27b2ea (blne) as
> the next branch instruction in ThreadPlanStepRange::SetNextBranchBreakpoint.
> >
> > Next, the threads are then resumed by the command interpreter. We
> receive an event from the inferior with stop reason eStopReasonException
> (EXC_BAD_INSTRUCTION) right after the resume, stopping the process.
> >
> > I guess this means i need to figure out how "it" and "blne" work
> together (my ARM assembler knowledge is minimal) to then understand why the
> breakpoint instruction that's written to the inferior results in a
> EXC_BAD_INSTRUCTION. If someone knows what could be the culprit let me know
> :)
> >
> > Thanks,
> >
> > Mario
> >
> >
> > On Mon, Dec 1, 2014 at 2:07 PM, Mario Zechner <badlogicgames at gmail.com>
> wrote:
> > Well, i wrote a very long mail detailing my journey to resolve issue #2
> (hanging after setting target.use-fast-stepping=false), only to eventually
> realize that it doesn't hang but instead just waits for the above loop to
> complete.
> >
> > This means turning off target.use-fast-stepping is not an option and i'm
> back to square one. I'd be grateful for any pointers on how to fix issue #1
> (EXC_BAD_INSTRUCTION). I guess i'll start by investigating the "run to next
> branch" stepping algorithm in LLDB, though my understanding is likely not
> sufficient to make a dent.
> >
> > Thanks,
> > Mario
> >
> >
> >
> >
> > On Mon, Dec 1, 2014 at 11:05 AM, Mario Zechner <badlogicgames at gmail.com>
> wrote:
> > Hi,
> >
> > setting target.use-fast-stepping to false did indeed solve this issue,
> albeit at the cost of increased runtime obviously. However, i ran into
> another issue right after i stepped out of the previously problematic
> function: http://sht.tl/bdAKRC
> >
> > Trying to source-level step this function (with use-fast-stepping=false)
> results in 1) the disassembly getting all kinds of messed up and 2) the
> process not stepping but hanging at the `cmp r1, #0` instruction. The
> original assembly code around that PC looks like this:
> >
> > LBB24_1: @ %label0
> > @ =>This Inner Loop Header:
> Depth=1
> > @DEBUG_VALUE:
> [J]java.lang.Thread.<init>(Ljava/lang/Runnable;Ljava/lang/String;)V:__$env
> <- R5
> > ldrexd r1, r2, [r0]
> > strexd r1, r6, r6, [r0]
> > cmp r1, #0
> > bne LBB24_1
> > @ BB#2: @ %label0
> > @DEBUG_VALUE:
> [J]java.lang.Thread.<init>(Ljava/lang/Runnable;Ljava/lang/String;)V:__$env
> <- R5
> > dmb ish
> > movs r1, #5
> >
> > A simple loop, which is actually part of an inlined function. We had
> some issues with inlined functions previously, i assume this issue is
> related. Interestingly enough, the back trace is also a bit wonky:
> >
> > (lldb) bt
> >
> > * thread #1: tid = 0x18082, 0x0021a9b4
> AttachTestIOSDev`[J]java.lang.Thread.<init>(Ljava/lang/Runnable;Ljava/lang/String;)V
> [inlined] [j]java.lang.Thread.threadPtr(J)[set] + 14 at Thread.java:1, stop
> reason = trace
> >
> > * frame #0: 0x0021a9b4
> AttachTestIOSDev`[J]java.lang.Thread.<init>(Ljava/lang/Runnable;Ljava/lang/String;)V
> [inlined] [j]java.lang.Thread.threadPtr(J)[set] + 14 at Thread.java:1
> >
> > frame #1: 0x0021a9a6
> AttachTestIOSDev`[J]java.lang.Thread.<init>(__$env=0x01662fc8,
> __$this=0x64da3833, runnable=0xa4f07400, threadName=0x00286000)V + 46 at
> Thread.java:138
> >
> > There should be a lot more frame. I'm gonna try to dig up some more
> details.
> >
> > Thanks a lot!
> > Mario
> >
> >
> >
> > On Sun, Nov 30, 2014 at 1:32 AM, Jason Molenda <jason at molenda.com>
> wrote:
> > The size of the breakpoint instruction is set by
> GetSoftwareBreakpointTrapOpcode(). In your case, most likely you're in
> PlatformDarwin::GetSoftwareBreakpointTrapOpcode() - lldb uses the symbol
> table (from the binary file) to determine if the code in a given function
> is arm or thumb. If it's arm, a 4 byte breakpoint is used. If it's thumb,
> a 2 byte breakpoint. Of course thumbv2 of T32 instructions can be 4 bytes
> -- the blne instruction is in your program -- but I assume the 2 byte
> breakpoint instruction still works correctly in these cases; the cpu sees
> the 2-byte instruction and stops execution.
> >
> > I am a little wary about the fact that this comes after an it
> instruction, I kind of vaguely remember issues with that instruction's
> behavior.
> >
> > It shouldn't make any difference but you might want to try
> >
> > (lldb) settings set target.use-fast-stepping false
> >
> > which will force lldb to single instruction step through the function.
> Right now lldb is looking at the instruction stream and putting breakpoints
> on branch/call/jump instructions to do your high-level "step" command,
> instead of stopping on every instruction. It is possible there could be a
> problem with that approach and the it instruction. Please report back if
> this changes the behavior.
> >
> > J
> >
> >
> > > On Nov 26, 2014, at 9:22 AM, Mario Zechner <badlogicgames at gmail.com>
> wrote:
> > >
> > > I dug a little deeper, inspecting the GDB remote packets send by LLDB
> to perform the stepping. It appears when sending memory breakpoint commands
> used for stepping, the size of the instruction being replaced isn't taken
> into account, or writing back the original instruction isn't done properly.
> The following log shows what happens when stepping into the previously
> mentioned function:
> > >
> > > (lldb) s
> > > Process 166 stopped
> > > * thread #1: tid = 0x0fd9, 0x002602e0
> AttachTestIOSDev`[J]java.lang.Object.<init>(__$env=0x016bffc8,
> __$this=0x017864b0)V + 12 at Object.java:136, queue =
> 'com.apple.main-thread', stop reason = step in
> > > frame #0: 0x002602e0
> AttachTestIOSDev`[J]java.lang.Object.<init>(__$env=0x016bffc8,
> __$this=0x017864b0)V + 12 at Object.java:136
> > > (lldb) disassemble -p
> > > AttachTestIOSDev`[J]java.lang.Object.<init>()V + 12 at Object.java:136:
> > > -> 0x2602e0: ldr r2, [r1]
> > > 0x2602e2: ldr r2, [r2, #0x30]
> > > 0x2602e4: tst.w r2, #0x100000
> > > 0x2602e8: it ne
> > > (lldb) s
> > > Process 166 stopped
> > > * thread #1: tid = 0x0fd9, 0x002602ec
> AttachTestIOSDev`[J]java.lang.Object.<init>(__$env=0x016bffc8,
> __$this=0x017864b0)V + 24 at Object.java:136, queue =
> 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION
> (code=EXC_ARM_UNDEFINED, subcode=0xffd1b001)
> > > frame #0: 0x002602ec
> AttachTestIOSDev`[J]java.lang.Object.<init>(__$env=0x016bffc8,
> __$this=0x017864b0)V + 24 at Object.java:136
> > > (lldb) disassemble -p
> > > AttachTestIOSDev`[J]java.lang.Object.<init>()V + 24 at Object.java:136:
> > > -> 0x2602ec: .long 0xb001ffd1 ; unknown opcode
> > > 0x2602f0: pop {r7, pc}
> > >
> > > AttachTestIOSDev`[J]java.lang.Object.<init>()V + 30:
> > > 0x2602f2: nop
> > >
> > > AttachTestIOSDev`[J]java.lang.Object.clone()Ljava/lang/Object; at
> Object.java:154:
> > > 0x2602f4: push {r4, r5, r7, lr}
> > > (lldb) disassemble -f
> > > AttachTestIOSDev`[J]java.lang.Object.<init>()V at Object.java:136:
> > > 0x2602d4: push {r7, lr}
> > > 0x2602d6: mov r7, sp
> > > 0x2602d8: sub sp, #0x4
> > > 0x2602da: movs r2, #0x0
> > > 0x2602dc: str r2, [sp]
> > > 0x2602de: str r1, [sp]
> > > 0x2602e0: ldr r2, [r1]
> > > 0x2602e2: ldr r2, [r2, #0x30]
> > > 0x2602e4: tst.w r2, #0x100000
> > > 0x2602e8: it ne
> > > 0x2602ea: blne 0x44b290 ; _bcRegisterFinalizer
> > > 0x2602ee: add sp, #0x4
> > > 0x2602f0: pop {r7, pc}
> > >
> > > AttachTestIOSDev`[J]java.lang.Object.<init>()V + 30:
> > > 0x2602f2: nop
> > >
> > > The first step succeeds and ends up right after the prologue, at
> 0x2602e0: ldr r2, [r1]. The next step ends up at 0x2602ec: .long
> 0xb001ffd1 which is wrong, it should be 0x2602ea: blne 0x44b290.
> > >
> > > The GDB remote conversation between lldb and the debugserver on the
> device (only relevant parts):
> > >
> > > # First step
> > > lldb->debugserver: $Z0,2602e0,2#73
> > > debugserver->lldb: $OK#00
> > > lldb->debugserver: $vCont;c:0fd9#15
> > > debugserver->lldb: (320)
> $T05thread:fd9;qaddr:37ebfad0;threads:fd9,ffa,ffb,ffd,fff,1009,100a,100b;00:c8ff6b01;01:b0647801;02:00000000;03:c87d6a00;04:00000000;05:c8ff6b01;06:fc6a6501;07:0c6a6501;08:90e96b01;09:28000000;0a:74a0ea37;0b:c8ff6b01;0c:b09e5b00;0d:086a6501;0e:d1b22000;0f:
> > >
> > > # Second step
> > > lldb->debugserver: $Z0,2602ea,2#a4
> > > debugserver->lldb: $OK#00
> > > lldb->debugserver: $vCont;c:0fd9#15
> > > debugserver->lldb: (324)
> $T92thread:fd9;qaddr:37ebfad0;threads:fd9,ffa,ffb,ffd,fff,1009,100a,100b;00:c8ff6b01;01:b0647801;02:01004300;03:c87d6a00;04:00000000;05:c8ff6b01;06:fc6a6501;07:0c6a6501;08:90e96b01;09:28000000;0a:74a0ea37;0b:c8ff6b01;0c:b09e5b00;0d:086a6501;0e:d1b22000;0f:
> > >
> > > For the first step, a 2 byte memory breakpoint is written to 0x2602e0
> ($Z0,2602e0,2#73), which is where the first step ended up. The instruction
> that got replaced is 2 bytes long. The GDB command wrote a 2 bytes memory
> breakpoint to the address, so all is good.
> > >
> > > For the second step, a 2 byte memory breakpoint is written to 0x2602ea
> ($Z0,2602ea,2#a4). But instead of ending up at 0x2602ec, which is in the
> middle of the 4-byte blne instruction.
> > >
> > > Is it correct for LLDB to set a 2 byte memory breakpoint instead of a
> 4-byte memory breakpoint in this case? The PC will be set to an invalid
> address, which then causes the EXC_BAD_INSTRUCTION.
> > >
> > > Am i understanding this correctly? Is there a way for me to fix this?
> > >
> > > On Wed, Nov 26, 2014 at 5:26 PM, Mario Zechner <
> badlogicgames at gmail.com> wrote:
> > > Hi,
> > >
> > > we generate thumbv7 binaries for iOS devices. We deploy, launch and
> debug those via LLDB. Stepping into functions seems to almost always
> generate a EXC_BAD_INSTRUCTION signal. The signal is not generated when
> running the app without the debugger attached. It is also not generated
> when we attach a debugger, but simply let the app run without breakpoints
> or any stepping.
> > >
> > > Here's one of these function's LLVM IR:
> > >
> > > =======================
> > > define external void @"[J]java.lang.Object.<init>()V"(%Env* %p0,
> %Object* %p1) nounwind noinline optsize {
> > > label0:
> > > call void @"llvm.dbg.declare"(metadata !{%Env* %p0}, metadata
> !19), !dbg !{i32 136, i32 0, metadata !{i32 786478, metadata !0, metadata
> !1, metadata !"[J]java.lang.Object.<init>()V", metadata
> !"[J]java.lang.Object.<init>()V", metadata !"", i32 136, metadata !15, i1
> false, i1 true, i32 0, i32 0, null, i32 256, i1 false, void (%Env*,
> %Object*)* @"[J]java.lang.Object.<init>()V", null, null, metadata !17, i32
> 136}, null}
> > > %r0 = alloca %Object*
> > > store %Object* null, %Object** %r0
> > > call void @"llvm.dbg.declare"(metadata !{%Object** %r0}, metadata
> !21), !dbg !{i32 136, i32 0, metadata !14, null}
> > > store %Object* %p1, %Object** %r0
> > > call void @"register_finalizable"(%Env* %p0, %Object* %p1), !dbg
> !{i32 136, i32 0, metadata !18, null}
> > > ret void, !dbg !{i32 136, i32 0, metadata !18, null}
> > > }
> > > =======================
> > >
> > > The corresponding thumbv7 assembler code as generated by LLVM:
> > >
> > > =======================
> > > .globl "_[J]java.lang.Object.<init>()V"
> > > .align 2
> > > .code 16 @
> @"[J]java.lang.Object.<init>()V"
> > > .thumb_func "_[J]java.lang.Object.<init>()V"
> > > "_[J]java.lang.Object.<init>()V":
> > > .cfi_startproc
> > > Lfunc_begin18:
> > > .loc 1 136 0 @ Object.java:136:0
> > > @ BB#0: @ %label0
> > > .loc 1 136 0 @ Object.java:136:0
> > > push {r7, lr}
> > > mov r7, sp
> > > sub sp, #4
> > > @DEBUG_VALUE: [J]java.lang.Object.<init>()V:__$env <- R0
> > > movs r2, #0
> > > str r2, [sp]
> > > str r1, [sp]
> > > .loc 1 136 0 prologue_end @ Object.java:136:0
> > > Ltmp6:
> > > ldr r2, [r1]
> > > ldr r2, [r2, #48]
> > > tst.w r2, #1048576
> > > Ltmp7:
> > > @DEBUG_VALUE: [J]java.lang.Object.<init>()V:__$env <- R0
> > > it ne
> > > blxne __bcRegisterFinalizer
> > > add sp, #4
> > > pop {r7, pc}
> > > Ltmp8:
> > > Lfunc_end18:
> > > "L_[J]java.lang.Object.<init>()V_end":
> > >
> > > .cfi_endproc
> > > =======================
> > >
> > > Now, when stepping into this function, LLDB receives a signal from the
> debug server:
> > >
> > > =======================
> > > (lldb) s
> > > Process 176 stopped
> > > * thread #1: tid = 0x11f5, 0x0023e2ec
> AttachTestIOSDev`[J]java.lang.Object.<init>(__$env=0x0169efc8,
> __$this=0x0174cd10)V + 24 at Object.java:136, queue =
> 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION
> (code=EXC_ARM_UNDEFINED, subcode=0xffd1b001)
> > > frame #0: 0x0023e2ec
> AttachTestIOSDev`[J]java.lang.Object.<init>(__$env=0x0169efc8,
> __$this=0x0174cd10)V + 24 at Object.java:136
> > > =======================
> > >
> > > Disassembling around the PC gives:
> > >
> > > =======================
> > > (lldb) disassemble --pc
> > > AttachTestIOSDev`[J]java.lang.Object.<init>()V + 24 at Object.java:136:
> > > -> 0x23e2ec: .long 0xb001ffd1 ; unknown opcode
> > > 0x23e2f0: pop {r7, pc}
> > >
> > > AttachTestIOSDev`[J]java.lang.Object.<init>()V + 30:
> > > 0x23e2f2: nop
> > >
> > > Disassembling until the beginning of the frame gives:
> > >
> > > (lldb) disassemble -f
> > > AttachTestIOSDev`[J]java.lang.Object.<init>()V at Object.java:136:
> > > 0x23e2d4: push {r7, lr}
> > > 0x23e2d6: mov r7, sp
> > > 0x23e2d8: sub sp, #0x4
> > > 0x23e2da: movs r2, #0x0
> > > 0x23e2dc: str r2, [sp]
> > > 0x23e2de: str r1, [sp]
> > > 0x23e2e0: ldr r2, [r1]
> > > 0x23e2e2: ldr r2, [r2, #0x30]
> > > 0x23e2e4: tst.w r2, #0x100000
> > > 0x23e2e8: it ne
> > > 0x23e2ea: blne 0x429290 ; _bcRegisterFinalizer
> > > 0x23e2ee: add sp, #0x4
> > > 0x23e2f0: pop {r7, pc}
> > >
> > > Accprding to this, execution should never end up at address 0x23e2ec.
> That's right in the middle of the blne and add instructions in the second
> disassembly. I have a hunch that the debugserver on the device may
> interfere here, e.g. add a trap instruction to implement the stepping. I'm
> not quite sure what to make of it.
> > >
> > > I'd appreciate any hints. If you require more information, i got
> plenty of logs :)
> > >
> > > Thanks,
> > > Mario
> > >
> > > _______________________________________________
> > > lldb-dev mailing list
> > > lldb-dev at cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> >
> >
> >
> >
> >
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at cs.uiuc.edu
> >
> https://urldefense.proofpoint.com/v1/url?u=http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev&k=ZVNjlDMF0FElm4dQtryO4A%3D%3D%0A&r=g1GoAnQQskSBaWLJWw6X6w%3D%3D%0A&m=Zl2rgz3vY3p3Z1gT4mYUogC%2B71s1vpu6iiR2%2BAqSFEs%3D%0A&s=3063d588fdc99fda75142f80da681ac13b53ba823de3e2221c1b01c0c7c54982
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/lldb-dev/attachments/20141201/b3044a92/attachment.html>
More information about the lldb-dev
mailing list