AT_SECURE is set in the auxiliary vector when we execute setuid/setgid
programs.
In those cases, we do not want to read environment variables that
influence the logic of the dynamic loader, as they can be controlled
by the user.
Commit a3a9016701 removed the PT_INTERP header
from Loader.so which cleaned up some kernel code in execve. Unfortunately
it prevents Loader.so from being run as an executable
Loader.so now just performs the initial self relocations and static
LibC initialisation before handing over to ELF::DynamicLinker::linker_main
to handle the rest of the process.
As a trade-off, ELF::DynamicLinker needs to be explicitly excluded from
Lagom unless we really want to try writing a cross platform dynamic loader
Loader.so is an actual executable, as well as the interpreter for dynamic
libraries. Currently launching Loader.so as a standalone executable results
in an obsucre crash as it tries to load itself over itself.
Now we at least print a helpful message saying that you're doing the wrong
thing and exit gracefully. In future we may wish to allow users to specify
additional options to learn more about what's going on during dynamic
linking, such as ld-linux.so.2 on Linux.
This commit gets rid of ELF::Loader entirely since its very ambiguous
purpose was actually to load executables for the kernel, and that is
now handled by the kernel itself.
This patch includes some drive-by cleanup in LibDebug and CrashDaemon
enabled by the fact that we no longer need to keep the ref-counted
ELF::Loader around.
Problem:
- `(void)` simply casts the expression to void. This is understood to
indicate that it is ignored, but this is really a compiler trick to
get the compiler to not generate a warning.
Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.
Note:
- Functions taking a `(void)` argument list have also been changed to
`()` because this is not needed and shows up in the same grep
command.
When loading dynamic objects, the emulator loads the interpreter,
generates an auxiliary vector and starts executing the loader.
Additionally, this commits also makes the MallocTracer and backtrace
symbolication work for dynamically loaded programs.
The dynamic loader exists as /usr/lib/Loader.so and is loaded by the
kernel when ET_DYN programs are executed.
The dynamic loader is responsible for loading the dependencies of the
main program, allocating TLS storage, preparing all loaded objects for
execution and finally jumping to the entry of the main program.
