gdb/dwarf: fix reading subprogram with DW_AT_specification (PR gdb/26693)

gdb/dwarf: fix reading subprogram with DW_AT_specification (PR gdb/26693)

Fix a regression introduced by commit 7188ed02d2a7 ("Replace
dwarf2_per_cu_data::cu backlink with per-objfile map").

This patch targets both master and gdb-10-branch, since this is a
regression from GDB 9.

Analysis
--------

The DWARF generated by the included test case looks like:

    0x0000000b: DW_TAG_compile_unit
                  DW_AT_language [DW_FORM_sdata]    (4)

    0x0000000d:   DW_TAG_base_type
                    DW_AT_name [DW_FORM_string]     ("int")
                    DW_AT_byte_size [DW_FORM_data1] (0x04)
                    DW_AT_encoding [DW_FORM_sdata]  (5)

    0x00000014:   DW_TAG_subprogram
                    DW_AT_name [DW_FORM_string]     ("apply")

    0x0000001b:   DW_TAG_subprogram
                    DW_AT_specification [DW_FORM_ref4]      (0x00000014 "apply")
                    DW_AT_low_pc [DW_FORM_addr]     (0x0000000000001234)
                    DW_AT_high_pc [DW_FORM_data8]   (0x0000000000000020)

    0x00000030:     DW_TAG_template_type_parameter
                      DW_AT_name [DW_FORM_string]   ("T")
                      DW_AT_type [DW_FORM_ref4]     (0x0000000d "int")

    0x00000037:     NULL

    0x00000038:   NULL

Simply loading the file in GDB makes it crash:

    $ ./gdb -nx --data-directory=data-directory testsuite/outputs/gdb.dwarf2/pr26693/pr26693
    [1]    15188 abort (core dumped)  ./gdb -nx --data-directory=data-directory

The crash happens here, where htab (a dwarf2_cu::die_hash field) is
unexpectedly NULL while generating partial symbols:

    #0  0x000055555fa28188 in htab_find_with_hash (htab=0x0, element=0x7fffffffbfa0, hash=27) at /home/simark/src/binutils-gdb/libiberty/hashtab.c:591
    #1  0x000055555cb4eb2e in follow_die_offset (sect_off=(unknown: 27), offset_in_dwz=0, ref_cu=0x7fffffffc110) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:22951
    #2  0x000055555cb4edfb in follow_die_ref (src_die=0x0, attr=0x7fffffffc130, ref_cu=0x7fffffffc110) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:22968
    #3  0x000055555caa48c5 in partial_die_full_name (pdi=0x621000157e70, cu=0x615000023f80) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8441
    #4  0x000055555caa4d79 in add_partial_symbol (pdi=0x621000157e70, cu=0x615000023f80) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8469
    #5  0x000055555caa7d8c in add_partial_subprogram (pdi=0x621000157e70, lowpc=0x7fffffffc5c0, highpc=0x7fffffffc5e0, set_addrmap=1, cu=0x615000023f80) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8737
    #6  0x000055555caa265c in scan_partial_symbols (first_die=0x621000157e00, lowpc=0x7fffffffc5c0, highpc=0x7fffffffc5e0, set_addrmap=1, cu=0x615000023f80) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8230
    #7  0x000055555ca98e3f in process_psymtab_comp_unit_reader (reader=0x7fffffffc6b0, info_ptr=0x60600009650d "\003int", comp_unit_die=0x621000157d10, pretend_language=language_minimal) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:7614
    #8  0x000055555ca9aa2c in process_psymtab_comp_unit (this_cu=0x621000155510, per_objfile=0x613000009f80, want_partial_unit=false, pretend_language=language_minimal) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:7712
    #9  0x000055555caa051a in dwarf2_build_psymtabs_hard (per_objfile=0x613000009f80) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8073

The special thing about this DWARF is that the subprogram at 0x1b is a
template specialization described with DW_AT_specification, and has no
DW_AT_name in itself.  To compute the name of this subprogram,
partial_die_full_name needs to load the full DIE for this partial DIE.
The name is generated from the templated function name and the actual
tempalate parameter values of the specialization.

To load the full DIE, partial_die_full_name creates a dummy DWARF
attribute of form DW_FORM_ref_addr that points to our subprogram's DIE,
and calls follow_die_ref on it.  This eventually causes
load_full_comp_unit to be called for the exact same CU we are currently
making partial symbols for:

    #0  load_full_comp_unit (this_cu=0x621000155510, per_objfile=0x613000009f80, skip_partial=false, pretend_language=language_minimal) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:9238
    #1  0x000055555cb4e943 in follow_die_offset (sect_off=(unknown: 27), offset_in_dwz=0, ref_cu=0x7fffffffc110) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:22942
    #2  0x000055555cb4edfb in follow_die_ref (src_die=0x0, attr=0x7fffffffc130, ref_cu=0x7fffffffc110) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:22968
    #3  0x000055555caa48c5 in partial_die_full_name (pdi=0x621000157e70, cu=0x615000023f80) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8441
    #4  0x000055555caa4d79 in add_partial_symbol (pdi=0x621000157e70, cu=0x615000023f80) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8469
    #5  0x000055555caa7d8c in add_partial_subprogram (pdi=0x621000157e70, lowpc=0x7fffffffc5c0, highpc=0x7fffffffc5e0, set_addrmap=1, cu=0x615000023f80) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8737
    #6  0x000055555caa265c in scan_partial_symbols (first_die=0x621000157e00, lowpc=0x7fffffffc5c0, highpc=0x7fffffffc5e0, set_addrmap=1, cu=0x615000023f80) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8230
    #7  0x000055555ca98e3f in process_psymtab_comp_unit_reader (reader=0x7fffffffc6b0, info_ptr=0x60600009650d "\003int", comp_unit_die=0x621000157d10, pretend_language=language_minimal) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:7614
    #8  0x000055555ca9aa2c in process_psymtab_comp_unit (this_cu=0x621000155510, per_objfile=0x613000009f80, want_partial_unit=false, pretend_language=language_minimal) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:7712
    #9  0x000055555caa051a in dwarf2_build_psymtabs_hard (per_objfile=0x613000009f80) at /home/simark/src/binutils-gdb/gdb/dwarf2/read.c:8073

load_full_comp_unit creates a cutu_reader for the CU.  Since a dwarf2_cu
object already exists for the CU, load_full_comp_unit is expected to
find it and pass it to cutu_reader, so that cutu_reader doesn't create
a new dwarf2_cu for the CU.

And this is the difference between before and after the regression.
Before commit 7188ed02d2a7, the dwarf2_per_cu_data -> dwarf2_cu link was
a simple pointer in dwarf2_per_cu_data.  This pointer was set up when
starting the read the partial symbols.  So it was already available at
that point where load_full_comp_unit gets called.  Post-7188ed02d2a7,
this link is per-objfile, kept in the dwarf2_per_objfile::m_dwarf2_cus
hash map.  The entry is only put in the hash map once the partial
symbols have been successfully read, when cutu_reader::keep is called.
Therefore, it is _not_ set at the point load_full_comp_unit is called.

As a consequence, a new dwarf2_cu object gets created and initialized by
load_full_comp_unit (including initializing that dwarf2_cu::die_hash
field).  Meanwhile, the dwarf2_cu object created and used by the callers
up the stack does not get initialized for full symbol reading, and the
dwarf2_cu::die_hash field stays unexpectedly NULL.

Solution
--------

Since the caller of load_full_comp_unit knows about the existing
dwarf2_cu object for the CU we are reading (the one load_full_comp_unit
is expected to find), we can simply make it pass it down, instead of
having load_full_comp_unit look up the per-objfile map.

load_full_comp_unit therefore gets a new `existing_cu` parameter.  All
other callers get updated to pass `per_objfile->get_cu (per_cu)`, so the
behavior shouldn't change for them, compared to the current HEAD.

A test is added, which is the bare minimum to reproduce the issue.

Notes
-----

The original problem was reproduced by downloading

    https://github.com/oneapi-src/oneTBB/releases/download/v2020.3/tbb-2020.3-lin.tgz

and loading libtbb.so in GDB.  This code was compiled with the Intel
C/C++ compiler.  I was not able to reproduce the issue using GCC, I
think because GCC puts a DW_AT_name in the specialized subprogram, so
there's no need for partial_die_full_name to load the full DIE of the
subprogram, and the faulty code doesn't execute.

gdb/ChangeLog:

	PR gdb/26693
	* dwarf2/read.c (load_full_comp_unit): Add existing_cu
	parameter.
	(load_cu): Pass existing CU.
	(process_imported_unit_die): Likewise.
	(follow_die_offset): Likewise.

gdb/testsuite/ChangeLog:

	PR gdb/26693
	* gdb.dwarf2/template-specification-full-name.exp: New test.

Change-Id: I57c8042f96c45f15797a3848e4d384181c56bb44