Re: [PATCH] mtrace: Fix output with PIE and ASLR [BZ #22716]

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

On 7/22/21 7:42 PM, John Ogness wrote:
> AFAIK you are only interested in @l_map_start of the link map. That is
> the first argument.
> 
>>   		$locs{$1} = $2;
>> +		$rel{$1} = hex($2) - hex($3);
> 
> Subtracting @l_addr from @l_map_start will probably always result in
> 0. Why should @l_addr be interesting for mtrace?
> 

l_addr is the difference between the ELF file and that in memory.  For 
PIC/PIE, the base address in the ELF is set at 0, so l_addr and 
l_map_start is the same.

However for non-pie binaries, the base address in the ELF is non-zero, 
because of which you'll see the same value in l_map_start as in the ELF, 
which makes l_addr 0.

So the (l_addr - l_map_start) is 0 for PIC and PIE, but not for non-pie 
executables.  Adding that relative offset in the binary for addr2line 
will give you the correct symbol in both pic/pie and non-pie cases.

Siddhesh