GDB shows broken instruction address offset

Question

$ gdb --version
GNU gdb (Ubuntu 8.1-0ubuntu3) 8.1.0.20180409-git

I'm debugging with gdb and faced some weird address offset. I have a shared library written in C++ with a function foo which disassembles as follows (the first fragment):

Dump of assembler code for function foo:
   0x00007ffff5c82d70 <+0>:     push   rbp
   0x00007ffff5c82d71 <+1>:     mov    rbp,rsp
   0x00007ffff5c82d74 <+4>:     push   r15
   0x00007ffff5c82d76 <+6>:     push   r14
   0x00007ffff5c82d78 <+8>:     push   r13
   0x00007ffff5c82d7a <+10>:    push   r12
   0x00007ffff5c82d7c <+12>:    push   rbx
   0x00007ffff5c82d7d <+13>:    sub    rsp,0x18
   0x00007ffff5c82d81 <+17>:    test   rdi,rdi
   0x00007ffff5c82d84 <+20>:    mov    DWORD PTR [rbp-0x34],r8d
   0x00007ffff5c82d88 <+24>:    je     0x7ffff5c82e58 <foo+232>
   0x00007ffff5c82d8e <+30>:    mov    r15,rsi
   0x00007ffff5c82d91 <+33>:    mov    r14,rdx
   0x00007ffff5c82d94 <+36>:    mov    r13d,ecx
   0x00007ffff5c82d97 <+39>:    mov    rbx,rdi
   0x00007ffff5c82d9a <+42>:    jmp    0x7ffff5c82ded <foo+125>
   0x00007ffff5c82d9c <+44>:    nop    DWORD PTR [rax+0x0]
   # Where do these negative offsets come from?
   0x00007ffff5c82da0 <+-1632>: mov    r9d,DWORD PTR [r12]
   0x00007ffff5c82da4 <+-1628>: lea    rcx,[rip+0xcadbe2]        
   0x00007ffff5c82dab <+-1621>: lea    rdx,[rip+0xcadde6]        
   0x00007ffff5c82db2 <+-1614>: lea    rdi,[rip+0xcadb67]   
   #...
   # negative offset of some unrelated function `bar`
   0x00007ffff5c82e19 <+-1511>: js     0x7ffff5c82dde <bar+4294965726>

Such negative offsets point are interpreted as belonging to some unrelated function (I did not find anything that could match these function up) bar.

Such things causes confusion when using GUI gdb frontend.

Any idea why gdb may show such negative instruction offset? Is it a GDB bug? If so, is there any workaround?

I tried to find out what is the objdump output regarding this function and did not notice anything strange (the same fragment as in the gdb example):

$ objdump -M intel -d libsomelib.so | grep -A 100 -i "0000000000e78d70"                                                        
0000000000e78d70 <foo>:                                                                                   
  e78d70:       55                      push   rbp                                                                                                                                                                 
  e78d71:       48 89 e5                mov    rbp,rsp                                                                                                                                                             
  e78d74:       41 57                   push   r15                                                                                                                                                                 
  e78d76:       41 56                   push   r14                                                                                                                                                                 
  e78d78:       41 55                   push   r13                                                                                                                                                                 
  e78d7a:       41 54                   push   r12                                                                                                                                                                 
  e78d7c:       53                      push   rbx                                                                                                                                                                 
  e78d7d:       48 83 ec 18             sub    rsp,0x18                                                                                                                                                            
  e78d81:       48 85 ff                test   rdi,rdi                                                                                                                                                             
  e78d84:       44 89 45 cc             mov    DWORD PTR [rbp-0x34],r8d                                                                                                                                            
  e78d88:       0f 84 ca 00 00 00       je     e78e58 <foo+0xe8>                                          
  e78d8e:       49 89 f7                mov    r15,rsi                                                                                                                                                             
  e78d91:       49 89 d6                mov    r14,rdx                                                                                                                                                             
  e78d94:       41 89 cd                mov    r13d,ecx                                                                                                                                                            
  e78d97:       48 89 fb                mov    rbx,rdi                                                                                                                                                             
  e78d9a:       eb 51                   jmp    e78ded <foo+0x7d>                                          
  e78d9c:       0f 1f 40 00             nop    DWORD PTR [rax+0x0]                                                                                                                                                 
  e78da0:       45 8b 0c 24             mov    r9d,DWORD PTR [r12]                                                                                                                                                 
  e78da4:       48 8d 0d e2 db ca 00    lea    rcx,[rip+0xcadbe2]                                                           
  e78dab:       48 8d 15 e6 dd ca 00    lea    rdx,[rip+0xcadde6]                                                           
  e78db2:       48 8d 3d 67 db ca 00    lea    rdi,[rip+0xcadb67]                                                        
  //...
  //just fine
  e78e19:       78 c3                   js     e78dde <foo+0x6e> 

The clear difference between the output of the gdb and objdump can be seen in the final line.

GDB:

0x00007ffff5c82e19 <+-1511>: js     0x7ffff5c82dde <bar+4294965726>

objdump:

e78e19:       78 c3                   js     e78dde <foo+0x6e>

Answer 1

Any idea why gdb may show such negative instruction offset?

Yes: in <bar+4294965726>, GDB interprets offset as signed int: 4294965726 == -1570

Is it a GDB bug?

Yes.

Unfortunately it's not clear how to reproduce this, or why GDB decided to show offset from bar instead of showing offset from foo. And without that, a proper bug report would be hard to create.