Dwarf DW_AT_location objdump and dwarfdump inconsistent

I am playing around with CPython and trying to understand how a debugger works. Specifically, I am trying to get the location of the last PyFrameObject so that I can traverse that and get the Python backtrace.

In the file ceval.c, line 689 has the definition of the function:

PyObject * PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)

What I am interested in getting is the location of f on the stack. When dumping the binary with dwarfdump I get that f is at $rbp-824, but if I dump the binary with objdump I get that the location is $rbp-808 - a discrepancy of 16. Also, when debugging with GDB, I get that the correct answer is $rbp-808 like objdump gives me. Why the discrepancy, and why is dwarfdump incorrect? What am I not understanding?

How to technically recreate the problem: Download python-2.7.17.tgz from Python website. Extract.

I compiled python-2.7.17 from source with debug symbols (./configure --enable-pydebug && make). Run the following commands on the resulting python binary:

dwarfdump Python-2.7.17/python has the following output:

                        DW_AT_name                  f           
                        DW_AT_decl_file             0x00000001 /home/meir/code/python/Python-2.7.17/Python/ceval.c
                        DW_AT_decl_line             0x000002b1                         
                        DW_AT_type                  <0x00002916>
                        DW_AT_location              len 0x0003: 91c879: DW_OP_fbreg -824

I know this is the correct f because the line the variable is declared on is 689 (0x2b1). As you can see the location is:

DW_AT_location len 0x0003: 91c879: DW_OP_fbreg -824: Meaning $rbp-824.

Running the command objdump -S Python-2.7.17/python has the following output:

PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
{
   f7577:       55                      push   %rbp
   f7578:       48 89 e5                mov    %rsp,%rbp
   f757b:       41 57                   push   %r15
   f757d:       41 56                   push   %r14
   f757f:       41 55                   push   %r13
   f7581:       41 54                   push   %r12
   f7583:       53                      push   %rbx
   f7584:       48 81 ec 38 03 00 00    sub    $0x338,%rsp
   f758b:       48 89 bd d8 fc ff ff    mov    %rdi,-0x328(%rbp)
   f7592:       89 b5 d4 fc ff ff       mov    %esi,-0x32c(%rbp)
   f7598:       64 48 8b 04 25 28 00    mov    %fs:0x28,%rax
   f759f:       00 00 
   f75a1:       48 89 45 c8             mov    %rax,-0x38(%rbp)
   f75a5:       31 c0                   xor    %eax,%eax

Debugging this output will show you that the relevant line is: f758b: 48 89 bd d8 fc ff ff mov %rdi,-0x328(%rbp) where you can clearly see that f is being loaded from -0x328(%rbp) which is $rbp-808. Also, GDB supports this finding.

So again, the question is, what am I missing and why the 16 byte discrepency between dwarfdump and reality?

Thanks

Edit: The dwarfdump including the function above is:

< 1><0x00004519>    DW_TAG_subprogram
                      DW_AT_external              yes(1)
                      DW_AT_name                  PyEval_EvalFrameEx
                      DW_AT_decl_file             0x00000001 /home/meir/code/python/Python-2.7.17/Python/ceval.c
                      DW_AT_decl_line             0x000002b1
                      DW_AT_prototyped            yes(1)
                      DW_AT_type                  <0x00000817>
                      DW_AT_low_pc                0x000f7577
                      DW_AT_high_pc               <offset-from-lowpc>53969
                      DW_AT_frame_base            len 0x0001: 9c: DW_OP_call_frame_cfa
                      DW_AT_GNU_all_tail_call_sites yes(1)
                      DW_AT_sibling               <0x00005bbe>
< 2><0x0000453b>      DW_TAG_formal_parameter
                        DW_AT_name                  f
                        DW_AT_decl_file             0x00000001 /home/meir/code/python/Python-2.7.17/Python/ceval.c
                        DW_AT_decl_line             0x000002b1
                        DW_AT_type                  <0x00002916>
                        DW_AT_location              len 0x0003: 91c879: DW_OP_fbreg -824

According to the answer below, DW_OP_fbreg is offset from the frame base - in my case DW_OP_call_frame_cfa. I am having trouble identifying the frame base. My registers are as following:

(gdb) info registers
rax            0xfffffffffffffdfe       -514
rbx            0x7f6a4887d040   140094460121152
rcx            0x7f6a48e83ff7   140094466441207
rdx            0x0      0
rsi            0x0      0
rdi            0x0      0
rbp            0x7ffd24bcef00   0x7ffd24bcef00
rsp            0x7ffd24bceba0   0x7ffd24bceba0
r8             0x7ffd24bcea50   140725219813968
r9             0x0      0
r10            0x0      0
r11            0x246    582
r12            0x7f6a48870df0   140094460071408
r13            0x7f6a48874b58   140094460087128
r14            0x1      1
r15            0x7f6a48873794   140094460082068
rip            0x5559834e99c0   0x5559834e99c0 <PyEval_EvalFrameEx+46153>
eflags         0x246    [ PF ZF IF ]
cs             0x33     51
ss             0x2b     43
ds             0x0      0
es             0x0      0
fs             0x0      0
gs             0x0      0

As stated above, I already know that %rbp-808 works. What is the correct way to do it with the registers that I have?

Edit: I finally understood the answer. I needed to unwind one more function, and find the place my function was called. There, the variable I was looking for really was in $rsp and $rsp-824 was correct

DW_OP_fbreg -824: Meaning $rbp-824

It does not mean that. It means, offset -824 from frame base (virtual) register, which is not necessarily (nor usually) equal to $rbp.

You need to look for DW_AT_frame_base to know what the frame base in the current function is.

Most likely it's defined as DW_OP_call_frame_cfa, which is the value of $RSP just before current function was called, and is equal to $RBP-16 (8 bytes for return address saved by the CALL instruction, and 8 bytes for previous $RBP saved by the first instruction of your function).