[PATCH] D23621: llvm-objdump: ELF: Handle code and data mix in all scenarios

[khemant@codeaurora.org via llvm-commits]

khemant created this revision.
khemant added reviewers: compnerd, echristo, davide.
khemant added subscribers: davide, echristo, compnerd, rafael, llvm-commits.
khemant set the repository for this revision to rL LLVM.
Herald added subscribers: mehdi_amini, aemerson.

The patch handls behavior of the disassembler when code and data are mixed in a text section.
When code an data are mixed in a section, the GNU disassembler acts in an interesting manner.
If told to disassemble text only (-d), it will interpret mixed data as data
$cat 1.c
int myInt = 1;
char myChar = 'b';
float myFloat = 1.2;
double myComm;

int main () {
  return myInt;
}

$cat 1.t
SECTIONS {
.text : { *(.text)
 *(.data.my* )
 *(.bss*) }
}

$clang -c 1.c -fdata-sections
$ld 1.o -T 1.t
$objdump -d a.out

a.out:     file format elf64-x86-64


Disassembly of section .text:

0000000000000040 <main>:
  40:   55                      push   %rbp
  41:   48 89 e5                mov    %rsp,%rbp
  44:   c7 45 fc 00 00 00 00    movl   $0x0,-0x4(%rbp)
  4b:   8b 04 25 54 00 00 00    mov    0x54,%eax
  52:   5d                      pop    %rbp
  53:   c3                      retq

0000000000000054 <myInt>:
  54:   01 00 00 00                                         ....

0000000000000058 <myChar>:
  58:   62 0f 1f 00                                         b...

000000000000005c <myFloat>:
  5c:   9a 99 99 3f                                         ...?

ARM variant has one more twist to it. When told to disassemble text only, it will interpret data if data is a standalone symbol, if it is part of a function, it marks it as a word/short etc.
$arm-none-eabi-gcc -c 1.c -fdata-sections
$arm-none-eabi-ld 1.o -t 1.t

$arm-none-eabi-objdump -d a.out

a.out:     file format elf32-littlearm


Disassembly of section .text:

00008000 <main>:
    8000:       e52db004        push    {fp}            ; (str fp, [sp, #-4]!)
    8004:       e28db000        add     fp, sp, #0
    8008:       e59f3010        ldr     r3, [pc, #16]   ; 8020 <main+0x20>
    800c:       e5933000        ldr     r3, [r3]
    8010:       e1a00003        mov     r0, r3
    8014:       e24bd000        sub     sp, fp, #0
    8018:       e49db004        pop     {fp}            ; (ldr fp, [sp], #4)
    801c:       e12fff1e        bx      lr
    8020:       00008024        .word   0x00008024  <=====marked as data

00008024 <myInt>:
    8024:       00000001                                ....    <====interpreted as data

00008028 <myChar>:
    8028:       00000062                                b...

0000802c <myFloat>:
    802c:       3f99999a                                ...?

If  told to disassemble all - all objdump variants really disassemble the data as instructions:
$arm-none-eabi-objdump -D a.out

a.out:     file format elf32-littlearm


Disassembly of section .text:

00008000 <main>:
    8000:       e52db004        push    {fp}            ; (str fp, [sp, #-4]!)
    8004:       e28db000        add     fp, sp, #0
    8008:       e59f3010        ldr     r3, [pc, #16]   ; 8020 <main+0x20>
    800c:       e5933000        ldr     r3, [r3]
    8010:       e1a00003        mov     r0, r3
    8014:       e24bd000        sub     sp, fp, #0
    8018:       e49db004        pop     {fp}            ; (ldr fp, [sp], #4)
    801c:       e12fff1e        bx      lr
    8020:       00008024        andeq   r8, r0, r4, lsr #32

00008024 <myInt>:
    8024:       00000001        andeq   r0, r0, r1

00008028 <myChar>:
    8028:       00000062        andeq   r0, r0, r2, rrx

0000802c <myFloat>:
    802c:       3f99999a        svccc   0x0099999a

Disassembly of section .bss:

00010030 <__bss_start>:
        ...


Repository:
  rL LLVM

https://reviews.llvm.org/D23621

Files:
  test/tools/llvm-objdump/X86/disassemble-code-data-mix.test
  tools/llvm-objdump/llvm-objdump.cpp

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