[llvm-dev] Instruction boundaries

via llvm-dev
Tue, 26 Jun 2018 15:36:29 -0000

Hi Muhui,
You are correct that the DWARF line table does not itemize every instruction.
This is to reduce the size of the table.  DWARF assumes that any consumer of the
line table will understand how to decode instructions on the target machine and
be able to find successive instructions.  However, every address that *is*
specified in the DWARF line table should be an actual instruction (except for an
end_sequence).

If you want to distinguish instructions from jump tables, I think your only
choice is to instrument the AsmPrinter or MC layer in the compiler.  DWARF does
not provide that level of detail.  An LLVM IR pass simply cannot provide you
with information about the final machine instructions.
--paulr

From: Muhui Jiang [mailto:jiangmuhui at gmail.com]
Sent: Tuesday, June 26, 2018 10:44 AM
To: Robinson, Paul
Cc: Yajin; llvm-dev
Subject: Re: [llvm-dev] Instruction boundaries

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<mailto: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<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<mailto: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<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<mailto: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<mailto: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<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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