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kallsyms: add reliable symbol size info
The existing mechanisms in get_symbol_pos to determine the end of a symbol is an inaccurate heuristic. By passing nm -S output into scripts/kallsyms.c and writing the symbol sizes to a new .kallsyms_sizes section, we can get accurate sizes and sort the symbols accordingly, reliably sorting zero-size symbols first (on the grounds that they are usually e.g. section markers, and other symbols at the same address are conceptually contained within them and should be sorted after them), then larger symbols before smaller ones (so that overlapping symbols print the containing symbol first, before its containees). We can also use this to improve aliased symbol detection. Emit the size info as an extra column in /proc/kallmodsyms (since its format is not yet set in stone), and export it to iterator consumers. The notable downside of this is that the new .kallsyms_sizes is pretty big: a PTR per symbol, so vmlinux.o grows by almost a megabyte, though it compresses pretty well, so bzImage grows by only a megabyte. I'm not sure how to reduce this (perhaps using an array with elements sized to be no larger than needed for the contents, so that almost always two-byte entries would do? except that in my test kernel two symbols are bigger than this: sme_workarea, at 400K, and __log_buf, at 100K: the latter seems often likely to be larger than 64K). A simple scheme to reduce this would be to split the sizes array into several arrays with differently-sized elements, and run-length-compress away the zero bytes -- but that's not implemented yet, and might never be if people think the whole idea of this is pointless. In the absence of a way to shrink things, this should probably be hidden behind a new config symbol if exposed at all, and kallmodsyms just shows zero sizes if it's configured out (but this is enough of an RFC that that's not yet done: possibly the benefits of this are too marginal to be worth it, even if they do let kall(mod)syms consumers distinguish symbols from padding, which was previously impossible). Signed-off-by: Nick Alcock <nick.alcock@oracle.com> Signed-off-by: Eugene Loh <eugene.loh@oracle.com>
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