James Molloy wrote: > Andrew Haley wrote: >> James Molloy wrote: >> >>> Hi, >>> >>> I'm attempting to write a cross-platform debugger for a hobby kernel, >>> and as part of which I use DWARF-2's CFI stack unwinding functionality. >>> >>> This works perfectly, however I would also like to obtain the parameters >>> given to each function call on the stack. I can do this easily in x86 as >>> all the parameters are passed via stack, however on x64 and MIPS I'm >>> having difficulty, as the first X parameters are passed via register. >>> >>> The DWARF-2 specification makes allowance for this - the CFI system is >>> capable of determining the value of any and every register at the start >>> of any stack frame - but I have noticed that GCC doesn't record >>> unwinding rules for many registers (seemingly any register not essential >>> to the finding of the CFA or return address). >>> >>> Is there any flag available to enable recording of every register, or, >>> even better, certain registers, for every stack frame? I've grepped the >>> manual to no avail. >> >> Surely this is in the debuginfo, not the unwinder data. Aren't >> you looking in the wrong place? > Although it is indeed possible to pull this information out of the > debuginfo, the debuginfo section is *massive* and gives far more > information than I need or want - it describes the entire low level > structure of the program - all I want is to be able to find where > parameters are! So what is the problem with using the debuginfo in a debugger? > The .debug_frames section is far smaller by comparison and seeing as I > already use it for stack unwinding I felt that using it along with an > idea of the default ABI for a given architecture would provide a useful > and compact way of determining function parameters and helping my > developers. > > The DWARF definition version 3.0 section 6.4 describes editing > "activations" - this section seems to imply that the full register set > should be available for editing for any unwound stack frame - not just > select registers. > > Has this been implemented in GCC? Or was there an assumption that most > debuggers would use the .debug_info information anyway so it was > redundant? Well, yes. The unwinder data contains what you need for unwinding; the debug data contains what you need for debugging. It's important to realize that the unwinder data is as small as possible, containing only what's needed to unwind the stack. Andrew.
