Created attachment 14154 [details]
Test binary

Consider the debug info attached binary. The binary's first FDE in .eh_frame has initial_location 0x1060, and the following instructions:
 
DW_CFA_advance_loc 4    # Move PC by 4
DW_CFA_undefined 16     # Change the rule for R16 to undefined

The linked CIE marks R16 as the return address, and has the following instructions:

DW_CFA_def_cfa 7, 8    # CFA is at R7+8
DW_CFA_offset 16, 1    # Set the rule for R16 to [CFA+1*data_aligment_factor])


The GNU readelf, if executed with --debug-dump=frames-interp, dumps the FDE
as follows:


00000018 0000000000000014 0000001c FDE cie=00000000 pc=0000000000001060..0000000000001086

     LOC           CFA      ra    

0000000000001060 rsp+8    u     
0000000000001064 rsp+8    u

Meanwhile, an alternative parser thinks that at the range [0x1060-0x1064), the rule for RA/R16 should be as inherited from the CIE, and it goes c-8.

I've debugged readelf (the latest master, as of 06/01/22), to that point. There are two passes over the FDE instructions: one starting on dwarf.c:9296, the other starting at dwarf.c:9442. On the first pass, when DW_CFA_undefined is encountered, there is the following case statement:

 

READ_ULEB (reg, start, block_end);
if (frame_need_space (fc, reg) >= 0)
    fc->col_type[reg] = DW_CFA_undefined;
break;

If I understand correctly, the intended purpose of the first pass is to allocate enough memory in the fc->col_type and fc->col_offset arrays, and the logic of this operator's handling was meant to be: if this register was
not mentioned before, allocate space for it, and reset its rule to undefined. HOWEVER, if the register WAS mentioned before (e. g. in the CIE), frame_need_space() returns 0, and the if() body executes anyway, and resets
the rule for the register to undefined, erasing the initial state as specified by the CIE.

 
I think the if statement should go, instead, "if (frame_need_space (fc, reg) > 0)". Same for other register-rule-type operators on the first pass.