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POHow to modify Erlang assembly? Are any resources available?
 

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  1. POHow to modify Erlang assembly? Are any resources available?
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    <p>I doubt anyone can help with this question because of the following in <a href="http://www.erlang.org/doc/man/compile.html" rel="noreferrer">Erlang's compile documentation</a>:</p> <blockquote> <p>Note that the format of assembler files is not documented, and may change between releases - this option is primarily for internal debugging use.</p> </blockquote> <p>... but just in case, here goes the story's stack trace:</p> <ul> <li>compile:file/2 with ['S'] to generate the assembly code</li> <li>Read .S file and create a key-value data structure with key being the 'function' tuples in the .S file, and value being the function's body, i.e. the assembly instructions which implement the function.</li> <li>Modify data structure by adding the assembly to make an external function call in certain functions.</li> <li>crash...</li> </ul> <p>Unfortunately I just quickly skimmed the .S files generated when compiling a module containing the following function, with and without the first expression in the function commented out:</p> <pre><code>spawn_worker(Which) -&gt; %syner:sync_pt(), case Which of ?NAIVE -&gt; spawn(err1, naive_worker_loop, []) end. </code></pre> <p>When I did that, I thought that the only thing that changed was the tuple: </p> <blockquote> <p>{call_ext,0,{extfunc,syner,sync_pt,0}}.</p> </blockquote> <p>... so I assumed that the only thing necessary to inject a function call in the assembly was to add that tuple... but now that I got to actually injecting the tuple... I'm seeing that the assembly generated has some extra instructions:</p> <p>Without <strong>syner:sync_pt()</strong>:</p> <pre><code>{function, spawn_worker, 1, 4}. {label,3}. {func_info,{atom,err1},{atom,spawn_worker},1}. {label,4}. {test,is_eq_exact,{f,5},[{x,0},{atom,naive}]}. {move,{atom,naive_worker_loop},{x,1}}. {move,nil,{x,2}}. {move,{atom,err1},{x,0}}. {call_ext_only,3,{extfunc,erlang,spawn,3}}. {label,5}. {case_end,{x,0}}. </code></pre> <p>With <strong>syner:sync_pt()</strong>:</p> <pre><code>{function, spawn_worker, 1, 4}. {label,3}. {func_info,{atom,err1},{atom,spawn_worker},1}. {label,4}. {allocate,1,1}. {move,{x,0},{y,0}}. {call_ext,0,{extfunc,syner,sync_pt,0}}. {test,is_eq_exact,{f,5},[{y,0},{atom,naive}]}. {move,{atom,naive_worker_loop},{x,1}}. {move,nil,{x,2}}. {move,{atom,err1},{x,0}}. {call_ext_last,3,{extfunc,erlang,spawn,3},1}. {label,5}. {case_end,{y,0}}. </code></pre> <p>I can't just conclude that adding something like:</p> <pre><code> {allocate,1,1}. {move,{x,0},{y,0}}. {call_ext,0,{extfunc,syner,sync_pt,0}}. </code></pre> <p>to every function I want to inject an external function call into, will do the trick. </p> <ol> <li>because I'm not sure if that assembly code applies to all functions I want to inject into (e.g. is <strong>{allocate,1,1}</strong> always ok)</li> <li>because if you take a closer look at the rest of the assembly, it changes slightly (for e.g. <strong>{call_ext_only,3,{extfunc,erlang,spawn,3}}.</strong> changes to <strong>{call_ext_last,3,{extfunc,erlang,spawn,3},1}.</strong>).</li> </ol> <p>So now the question is, is there any resource out there I can use to understand and manipulate the assembly generated by Erlang's compile:file/2?</p> <p>I'm asking this question just in case. I doubt there is a resource for this since the documentation clearly states that there isn't, but I have nothing to lose I guess. Even if there was, it seems like manipulating the assembly code is going to be trickier than I'd like it to be. Using parse_transform/2 is definitely easier, and I did manage to get something similar to work with it... just trying out different alternatives.</p> <p>Thanks for your time.</p>
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    1. CO(Telling from testing, not from reading sources.) `{allocate,1,1}` allocates one place on the y stack. The x stack is the "normal" stack, the registers. The y stack is an auxiliary stack. `{move,{x,0},{y,0}}` moves the topmost x to the bottommost y (at least think they grow in reverse direction). My test file: http://pastebin.com/R21ZJ29Q. Its result: http://pastebin.com/jULjMCV0.
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    2. COThanks for the help kay. It looks like figuring the assembly out will take me too much time though, so I'm not planning on decrypting it. Cheers.
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