Dodji Seketeli b1b0586dc2 dwarf-reader: Fix bloom filter access in GNU_HASH section ...

The bloom filter of the GNU_HASH section of binaries is made of words
(bitmasks) that are either 32-bits for ELFCLASS32 binaries or 64-bits
for ELFCLASS64 binaries.

By using the GElf_Word type to hold the content of each bitmask we
assume the bitmask to be of a size of at most 'sizeof(Elf64_Word)'.
Unfortunately, the name Elf64_Word is a bit misleading because it's a
32-bits wide type (uint32_t).  That type is thus too short to hold an
entire bitmask for 64-bits binaries.

I won't give too many details here about how the bloom filter works as
it's described in details in the documentation for the GNU_HASH
section at http://www.linker-aliens.org/blogs/ali/entry/gnu_hash_elf_sections/.

Suffice it to say that in practise, we were comparing the least
significant bytes of a 64-bits bloom bitmask to a 32-bits value.
Depending on if we read those least significant bytes on a big or
little endian, we obviously don't get the same result.  Hence the
recent buid breakage at
https://builder.wildebeest.org/buildbot/#builders/14/builds/352 where
the runtestlookupsyms test fails.

This patch thus changes the code of lookup_symbol_from_gnu_hash_tab to
use a 64-bits type to hold the value of the bloom bitmask.  That way,
we never have any information loss.  The function bloom_word_at is
also changed to read the bloom bitmask as a 64-bits value when looking
at an ELFCLASS64 binary and to always return a 64-bits value.

It also adds a test to access the bloom filter of an ELFCLASS32
binary.

	* src/abg-dwarf-reader.cc (bloom_word_at): Properly read an
	element from the bloom bitmasks array of 32-bits values as a
	64-bits value in a portable way.  Always return a 64 bits value.
	(lookup_symbol_from_gnu_hash_tab): Use a 64-bits value to store
	the bloom bitmask.
	* tests/data/test-lookup-syms/test1-32bits.so: New test input,
	compiled as a 32bits binary to test for ELFCLASS32 binaries.
	* tests/data/test-lookup-syms/test1.c.compiling.txt: Documentation
	about how to compile the test1[-21bits].so files.
	* tests/data/Makefile.am: Add the new test material above to
	source distribution.
        * tests/test-lookup-syms.cc (in_out_specs): Add the
	test1-32bits.so test case to this test harness.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>

2020-03-20 13:35:00 +01:00

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