[llvm-dev] Instruction boundaries

Muhui Jiang via llvm-dev
Tue, 26 Jun 2018 14:43:37 -0000

Hi paulr

Thanks for your very quick and detail reply. Now I understand something
more but I still have the following questions.

As I said before, my goal is to know where the instructions are in one
specific binary. In other words, instruction boundary.

Dwarf info will give us some information like function boundary. However,
there might be some inline data inside the function. Thus, I may need the
help from LLVM and Dwarf and combine them together.

Now according to my observation. Addresses listed in Dwarf line table are
not always instructions and not all the instructions are listed inside the
dwarf line table. Thus, Dwarf may not able to help me so much.

I try to use LLVM to write some pass. However, the LLVM pass is on the LLVM
IR level and it only provides source level debugging information.  I still
need Dwarf line mapping table for binary level information.

So generally speaking, I still don't know how or whether it is possible to
use LLVM to get the ground truth of instruction boundary.

For a specific example, you could observe in the previous email that 0xa3a4
is an instruction and 0xa3a8 is an data for jump table(the jump table will
start from 0xa3a8 and end at 0xa7d4). However, no information is related to
these two addresses(0xa3a4 and 0xa3a8). I cannot get the instruction's
ground truth. Thank you so much and if you need more information, please
tell me. Many Thanks

Regards
Muhui

2018-06-26 10:14 GMT-04:00 <paul.robinson at sony.com>:

> I'm not familiar with the target instruction set, but if "MOV PC,
R0" is
> not a return instruction, I'm guessing that the sequence starting at
A39C
> is a dispatch through a jump table.  The jump table would be considered
> part of the instruction stream and included in the scope of the line
> table.  This is not a case where you would see end_sequence; my mistake.
>
>
>
> The line table does not list every individual instruction.  For
> compactness, it lists instructions only when the associated source location
> changes.  All instructions between successive entries of the line table
> would have the same source location.
>
>
>
> Line 0 indicates one of two things.  First, some optimizations might
> combine instructions from different source locations into a single new
> instruction.  Because the source location is now ambiguous, we report line
> 0 rather than arbitrarily choosing one of the original source locations.
> This is a known deficiency in how DWARF represents source locations for
> instructions.  It will require a change to the DWARF specification to
> address this problem.
>
> Second, there might not be a specific source location for some
> instructions. This is most common for things like code to call global
> initializers prior to the start of the 'main()' function, but can
occur in
> other situations.
>
>
>
> I hope this addresses at least some of your questions.  I am happy to
> continue this discussion if some questions remain or if you have new
> questions.
>
> --paulr
>
> *From:* Muhui Jiang [mailto:jiangmuhui at gmail.com]
> *Sent:* Tuesday, June 26, 2018 9:38 AM
>
> *To:* Robinson, Paul
> *Cc:* Yajin; llvm-dev
> *Subject:* Re: [llvm-dev] Instruction boundaries
>
>
>
> Hi Paulr
>
>
>
> I am using the llvm-dwarfdump. This is not the end of the function, thus
> there is no 'end_sequence' reported. Here I just want to give you
an
> exception because you said that all the instructions should be listed
> inside dwarf line table. However, I found that it will lost some
> instructions. And some instruction's source line is zero.
>
>
>
> Besides, I also noticed that inline data is listed inside the line table.
>
>
>
> Regards
>
> Muhui
>
>
>
> 2018-06-26 9:08 GMT-04:00 <paul.robinson at sony.com>:
>
> There should be a line-table entry for the end of the function, which
> appears to be missing from the dump you provided.  llvm-dwarfdump should
> report this address with 'end_sequence' in the Flags.  Are you
using a
> different dumper?
>
>
>
> I am not sure but my guess would be that inline data is not represented in
> the line table.  The line table's primary purpose is to inform the
debugger
> about good breakpoint locations, and clearly you do not want to set
> breakpoints in data.  Inline data is probably contained within the code
> ranges described in the DW_TAG_subprogram, however.
>
> --paulr
>
>
>
> *From:* Muhui Jiang [mailto:jiangmuhui at gmail.com]
> *Sent:* Tuesday, June 26, 2018 1:44 AM
> *To:* Robinson, Paul
> *Cc:* Yajin; llvm-dev
> *Subject:* Re: [llvm-dev] Instruction boundaries
>
>
>
> Hi Paulr
>
>
>
> According to my observation. Not all the instructions are listed in the
> line table.
>
>
>
> For example.  We have address 0xa3a0 and 0xa3a4 as the instructions
>
>
>
> .text:0000A394                 CMP     R1, #0x42
>
> .text:0000A398                 BHI     loc_AB70
>
> .text:0000A39C                 ADR     R1, off_A3A8
>
> .text:0000A3A0                 LDR     R0, [R1,R0,LSL#2]
>
> .text:0000A3A4                 MOV     PC, R0
>
> .text:0000A3A4 ; ------------------------------
> ---------------------------------------------
>
> .text:0000A3A8 off_A3A8        DCD loc_AB3C            ; DATA XREF:
> main+AC↑o
>
> .text:0000A3AC                 DCD loc_AB34
>
> .text:0000A3B0                 DCD loc_AB70
>
> .text:0000A3B4                 DCD loc_AB70
>
>
>
> However, inside the line table. The description is end at 0xa39c. Do you
> have any ideas?
>
>
>
>   196 0x000000000000a38c    956      7      1   0             0
>
>   197 0x000000000000a39c      0      7      1   0             0
>
>
>   198 0x000000000000a7d8    959     27      1   0             0  is_stmt
>
>   199 0x000000000000a7f8    959     25      1   0             0
>
>   200 0x000000000000a7fc    961     11      1   0             0  is_stmt
>
>   201 0x000000000000a800    964     15      1   0             0  is_stmt
>
>   202 0x000000000000a808    964     15      1   0             0
>
>
>
> Regards
>
> Muhui
>
>
>
>
>
> 2018-06-25 23:31 GMT-04:00 Muhui Jiang <jiangmuhui at gmail.com>:
>
> Hi paulr
>
>
>
> Thanks for your reply. Though DWARF info give me the code address ranges,
> there might be inline data. If so, how to handle this case?
>
>
>
> As for the dwarf line table. Sometimes, the source line might be zero. Do
> you know why? If all instructions should be describe in the line table, I
> think analyzing Dwarf line table is enough to get all the instructions
> addresses. Do you agree?
>
>
>
> I would also cc my supervisor for the discussion.
>
>
>
> Regards
>
> Muhui
>
>
>
> <paul.robinson at sony.com>于2018年6月26日 周二上午2:38写道：
>
> The main DWARF info should provide the code address ranges for each
> function, as well as the starting source location.  You could then use the
> line table to map code ranges to individual source lines.  That could give
> you a reasonable grasp of the source range for each function.
>
> All addresses in the DWARF line table will be instruction addresses.  And
> in fact, all instructions should be described in the line table (assuming
> all source was compiled with debug info).
>
> --paulr
>
>
>
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of
*Muhui
> Jiang via llvm-dev
> *Sent:* Monday, June 25, 2018 11:36 AM
> *To:* llvm-dev
> *Subject:* [llvm-dev] Instruction boundaries
>
>
>
> Hi
>
>
>
> I was wondering whether there are any methods to know what part of the
> target binary is code.
>
>
>
> I have some ideas and hope to get your comments.
>
>
>
> I would like to use LLVM's source level debugging information to
extract
> the source lines belonging to every functions. Then use the dwarf mapping
> table to transfer the  source level information to binary address. Are
> there any better methods?
>
>
>
> Besides, is the address listed inside dwarf line mapping table must be
> code rather than data?
>
>
>
> Regards
>
> Muhui
>
>
>
>
>
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