[Bug gas/2946] jmp to a weak symbol jumps to a wrong address

[vincent dot riviere at freesbee dot fr]

--- Additional Comments From vincent dot riviere at freesbee dot fr  
2006-07-22 12:52 ---
Sorry, you're right.
The jump is relocated.

But I'm very surprised.

Without the "weak" keyword, gas generates a relative jump and no relocation 
entry. Good.

With the "weak" keyword, gas generates an absolute jump and a relocation entry. 
In the special case described here, the jump and the weak symbol are in the 
same object file, so gas could generate a relative jump.

It seems that there is still a bug on m68k-*-netbsd (a.out format).

I'm going to post a detailed report.


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http://sourceware.org/bugzilla/show_bug.cgi?id=2946

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[Bug gas/2946] jmp to a weak symbol jumps to a wrong address

[amodra at bigpond dot net dot au]

--- Additional Comments From amodra at bigpond dot net dot au  2006-07-22 
13:00 ---
No, gas should not resolve this branch.  There may be a strong symbol "mylabel"
in another file, in which case the branch should go to that symbol.

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http://sourceware.org/bugzilla/show_bug.cgi?id=2946

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[Bug gas/2946] jmp to a weak symbol jumps to a wrong address

[vincent dot riviere at freesbee dot fr]

--- Additional Comments From vincent dot riviere at freesbee dot fr  
2006-07-22 15:00 ---
Here are detailed tests.


# target i686-pc-cygwin 
# gas 2.16.91 20050610 on host Windows XP SP2 ##

$ as bug.s -o bug.o
$ objdump -d bug.o

bug.o: file format pe-i386

Disassembly of section .text:

 <.text>:
   0:   eb 01   jmp3 <.weak.mylabel.>
   2:   90  nop

0003 <.weak.mylabel.>:
   3:   90  nop
   4:   90  nop
...

$ objdump -r bug.o

bug.o: file format pe-i386

$ ld bug.o -o bug
ld: warning: cannot find entry symbol _mainCRTStartup; defaulting to 00401000
$ objdump -d bug

bug: file format pei-i386

Disassembly of section .text:

00401000 <__RUNTIME_PSEUDO_RELOC_LIST_END__>:
  401000:   eb 01   jmp401003 <.weak.mylabel.>
  401002:   90  nop

00401003 <.weak.mylabel.>:
  401003:   90  nop
  401004:   90  nop
...

$ objdump -r bug

bug: file format pei-i386


Everything is perfect here.
There is a short relative jump and no relocation.



# target i386-pc-linux #
# gas 2.17 on host Debian Sarge (stable) ###

$ as bug.s -o bug.o
$ objdump -d bug.o

bug.o: file format elf32-i386

Disassembly of section .text:

 :
   0:   e9 fc ff ff ff  jmp1 
   5:   90  nop

0006 :
   6:   90  nop

$ objdump -r bug.o

bug.o: file format elf32-i386

RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE  VALUE
0001 R_386_PC32mylabel

$ ld bug.o -o bug
ld: warning: cannot find entry symbol _start; defaulting to 08048074
$ objdump -d bug

bug: file format elf32-i386

Disassembly of section .text:

08048074 :
 8048074:   e9 01 00 00 00  jmp804807a 
 8048079:   90  nop

0804807a :
 804807a:   90  nop

$ objdump -r bug

bug: file format elf32-i386


In this case, gas generates a long relative jump AND a 32-bit relocation.
I didn't see that detail on my first post.
In the object file, the value of the jump looks quite strange. It is -4, in 
fact the address to relocate relative to the instruction just after the jump. 
Maybe this behavior is required by the elf object format ?
Anyway, the relocation entry is here and right.
After linking, the relative jump has the correct value and the executable has 
no more relocation.
Even though it would be more optimal with a short jump and without a 
relocation, the result is perfectly correct.



# target m68k-unknown-linux 
# gas 2.17 on host Debian Sarge (stable) ###

$ as bug.s -o bug.o
$ objdump -d bug.o

bug.o: file format elf32-m68k

Disassembly of section .text:

 :
   0:   4ef9    jmp 0 
   6:   4e71nop

0008 :
   8:   4e71nop

$ objdump -r bug.o

bug.o: file format elf32-m68k

RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE  VALUE
0002 R_68K_32  mylabel

$ ld bug.o -o bug
ld: warning: cannot find entry symbol _start; defaulting to 8074
$ objdump -d bug

bug: file format elf32-m68k

Disassembly of section .text:

8074 :
8074:   4ef9 8000 007c  jmp 807c 
807a:   4e71nop

807c :
807c:   4e71nop

$ objdump -r bug

bug: file format elf32-m68k


Here, gas generates an absolute long jump and a 32-bit relocation.
The value of the jump and the relocation are correct.
The linked executable contains the right jump value.
There is no relocation in the final executable, I suppose that it's normal for 
that target.



# target m68k-unknown-netbsd ###
# gas 2.17 on host Debian Sarge (stable) ###

$ as bug.s -o bug.o
$ objdump -d bug.o

bug.o: file format a.out-m68k-netbsd

Disassembly of section .text:

 :
   0:   4efa 000a   jmp %pc@(c )
   4:   4e71nop

0006 :
   6:   4e71nop

$ obj.netbsd/binutils/objdump -r bug.o

bug.o: file format a.out-m68k-netbsd

RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE  VALUE
0002 DISP16.text

$ ld bug.o -o bug
$ objdump -d bug

bug: file format a.out-m68k-netbsd

Disassembly of section .text:

2020 :
2020:   4efa 000a   jmp %pc@(202c <__etext+0x4>)
2024:   4e71nop

2026 :
2026:   4e71nop

2028 <__etext>:
...

$ objdump -r bug

bug: file format a.out-m68k-netbsd


This