gdb: fix crash when GDB can't read an objfile

If a user starts an inferior composed of objfiles that GDB is unable to
read, there is an error thrown in find_sym_fns, printing the famous "I'm
sorry, Dave, I can't do that" and the objfile stops being read. However,
the objfile will already have been linked to the program space, and
future interactions with the objfile will assume that it is readable.

Relevant to this commit, if GDB tries to find out the section that
contains a PC, and this section happens to land in the unreadable
objfile, GDB will try to create a section mapping, eventually calling
update_section_map. Since that function uses bfd to calculate the
sections, it'll think there are sections to be ordered, but when trying
to access the objfile::section_offsets, it'll be indexing a size 0
std::vector, which will end up segfaulting.

Currently, it isn't easy to trigger this crash, but the upcoming
possibility to disable support for some file formats would make the
crash very easy to reproduce, by attempting to debug an unsupported
inferior and using "break *<instruction>" command, or simply connecting
to a gdbserver loaded with an unsupported inferior.

The struct objfile_up seems to have been created to catch these kinds of
errors and unlink the partially-read objfile from the program space, as
the objfile isn't useful to GDB anymore, but it seems to have been added
before find_sym_fns would throw errors for unreadable objfiles, as the
instance in syms_from_objfile_1 (that could save GDB from this crash) is
declared well after find_sym_fns, too late to guard us. This commit
moves the declaration up to the top of the function, so it works as
intended.

Further discussion on the mailing list also agreed that the name
"objfile_up" implies some level of ownership of the pointer, which this
struct doesn't have. So this commit renames the struct to
scoped_objfile_unlinker, which is more descriptive of what the struct is
actually meant to do.

The final change this commit does is add an assertion to
objfile::section_offset and objfile::set_section_offset, which ensures
that the section_offsets vector is large enough to return the desired
offset. This ensures that we won't misteriously segfault or worse,
continue going with garbage data.

Reported-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>

0 files changed, 0 insertions, 0 deletions