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view docs/internals.txt @ 342:12e2453f8417
Fix potential memory access after free in lw_expr_simplify_go()
Thanks to Erik Gavriluk (erik@bombfactory.com) for spotting a missing "else"
when attempting to resolve a "special" reference. This could have led to
access to memory that had already been freed if a "special" resolved to an
expression that also referenced the same special (a circular reference).
Whether that can actually happen is questionable but the check must have
been added for a reason so it follows that it should be done correctly.
This also brings the code in line with similar code resolving symbol
references which makes a similar check.
| author | William Astle <lost@l-w.ca> |
|---|---|
| date | Sat, 14 Mar 2015 14:06:13 -0600 |
| parents | 2c24602be78f |
| children |
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LWASM Internals =============== LWASM is a table-driven assembler that notionally uses two passes. However, it implements its assembly in several passes as follows. Pass 1 ------ This pass reads the entire source code and parses each line into an internal representation. Macros, file inclusions, and conditional assembly instructions are resolved at this point as well. Instructions with known sizes will have their sizes resolved at this point. Pass 2 ------ Check all exported symbols for validity and set them as imports if the assembler state says so. Also resolve all symbol references in all expressions to be direct references either to the symbol table or to the import list. Pass 3 ------ This pass resolves all instruction sizes that can be resolved without forcing any instruction sizes. This pass will run repeatedly until no no new resolution occurs. Pass 4 ------ Work through all un-resolved instructions and force sizes. After each size is forced, try re-resolving all other instructions. This is done starting at the beginning of the source and working forward. If any instruction does not resolve when forced, an error will be thrown. Pass 5 ------ Constantize all line addresses and throw errors if any cannot be. This pass will repeat until no further lines addresses are reduced to constants at which time all lines will be checked for constant-ness. Pass 6 ------ Finalize all expressions related to instructions. Carp about any that cannot be reduced to a usable form. That means, for the non-object target all expressions must resolve to a constant. For the object form, all expressions must resolve to symbol references and constants. Those symbol references may be internal or external. Pass 7 ------ Emit object code for each line for later output.