Simpler abbrev parsing using less memory

Mark Wielaard
Tue Dec 26 19:38:00 GMT 2017


When we added bounds checking to almost all data reading functions
(commit 7a05347 libdw: Add get_uleb128 and get_sleb128 bounds checking)
we also added extra checks to the abbrev reading. But since we didn't
really have bounds for the "raw" Dwarf_Abbrev reading functions we
just "guessed" the maximum of a uleb128. This wasn't really correct
and not really needed. A struct Dwarf_Abbrev can only be created by
__libdw_getabbrev, which checks the whole abbrev (code, tag, children
and attribute names/forms) is valid already. So whenever we use the
attrp pointer from the Dwarf_Abbrev to read the name/forms we already
know they are in the .debug_abbrev bounds).

[PATCH 1/2] libdw: New get_uleb128_unchecked to use with already
                   checked Dwarf_Abbrev.

So the first patch introduces a get_uleb128_unchecked function that
is used for re-reading such uleb128 values.

The second patch reduces the size of the struct Dwarf_Abbrev by not
storing the attrcnt and by using bitfields for has_children and code.

[PATCH 2/2] libdw: Reduce size of struct Dwarf_Abbrev.

The attrcnt was only used by the dwarf_getattrcnt function. Which
is only used in one testcase. And which seems mostly unnecessary
for real programs. The function now explicitly counts the attrs
instead of using a cached value.

The combined patches very slightly reduces the time for parsing
abbrevs and make the abbrev cache somewhat smaller.

On my machine eu-readelf -N --debug-dump=info
goes down from 1.79 to 1.71 secs. And max rss goes down from 15.296
to 14.684 kbytes.



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