The original heuristic of "a library being in `s_global_objects` means
that it was fully initialized already" doesn't hold up anymore since we
changed the loading order. This was causing us to skip parts of the
initialization of dependency libraries when running dlopen (since it was
the only user of that setting).
Instead, set a flag after we run stage 4 (which is the "run the global
initializers" stage) and check that flag when determining unfinished
dependencies. This entirely replaces the `skip_global_objects` logic.
This keeps us from needlessly allocating storage via `malloc` as part
of the `Vector`s that early, which we might conflict on while reserving
memory for the main executable.
We previously trusted the `map` part in `map_library` too much, and
assumed that this would already lock in the binary at its final place.
However, the `map()` function of the loader was only called in
`load_main_library`, which ran only right before jumping to the
entrypoint.
Make our binary loading a bit more stable by actually mapping the binary
right after we read its information, and only do the linking right
before jumping to the entrypoint.
Using the main executable basename produces the wrong $ORIGIN processing
for libraries that are secondary dependencies of the main executable,
or dependencies of an object loaded via dlopen.
.text sections of objects that contain textrels have to be writable
during the relocation procedure. Because of this, we would segfault if
we tried to execute IFUNC resolvers defined in them. Let's print a
meaningful error message instead.
Additionally, a warning is now printed when we load objects with
textrels, as in the future, additional security mitigations might
interfere with them being loaded.
IFUNC is a GNU extension to the ELF standard that allows a function to
have multiple implementations. A resolver function has to be called at
load time to choose the right one to use. The PLT will contain the entry
to the resolved function, so branching and more indirect jumps can be
avoided at run-time.
This mechanism is usually used when a routine can be made faster using
CPU features that are available in only some models, and a fallback
implementation has to exist for others.
We will use this feature to have two separate memset implementations for
CPUs with and without ERMS (Enhanced REP MOVSB/STOSB) support.
IFUNC resolvers depend on the resolved function's address having been
relocated by the time they are called. This means that relative
relocations have to be done first.
The linker is kind enough to put R_*_RELATIVE before R_*_IRELATIVE in
.rel.dyn, but .relr.dyn contains relative relocations too.
This check is here to make sure we only try to load serenity binaries.
However, with -fprofile-instr-generate -fcoverage-mapping, clang
sets the EI_OSABI field to 3, for GNU. The instrumentation uses a lot of
retained COMDAT sections for coverage instrumentation that get the
SHF_GNU_RETAINED section header flag set on them, forcing llvm to set
the ABI to GNU.
I noticed that we were populating this StringBuilder and then throwing
away the result while profiling `true` with UserSpace emulator.
Before:
courage:~ $ time -n 1000 true
Timing report: 3454 ms
==============
Command: true
Average time: 3.45 ms (median: 3, stddev: 3.42, min: 0, max:11)
Excluding first: 3.45 ms (median: 3, stddev: 3.42, min: 0, max:11)
After:
courage:~ $ time -n 1000 true
Timing report: 3308 ms
==============
Command: true
Average time: 3.30 ms (median: 3, stddev: 3.28, min: 0, max:12)
Excluding first: 3.30 ms (median: 3, stddev: 3.29, min: 0, max:12)
The DT_RELR relocation is a relatively new relocation encoding designed
to achieve space-efficient relative relocations in PIE programs.
The description of the format is available here:
https://groups.google.com/g/generic-abi/c/bX460iggiKg/m/Pi9aSwwABgAJ
It works by using a bitmap to store the offsets which need to be
relocated. Even entries are *address* entries: they contain an address
(relative to the base of the executable) which needs to be relocated.
Subsequent even entries are *bitmap* entries: "1" bits encode offsets
(in word size increments) relative to the last address entry which need
to be relocated.
This is in contrast to the REL/RELA format, where each entry takes up
2/3 machine words. Certain kinds of relocations store useful data in
that space (like the name of the referenced symbol), so not everything
can be encoded in this format. But as position-independent executables
and shared libraries tend to have a lot of relative relocations, a
specialized encoding for them absolutely makes sense.
The authors of the format suggest an overall 5-20% reduction in the file
size of various programs. Due to our extensive use of dynamic linking
and us not stripping debug info, relative relocations don't make up such
a large portion of the binary's size, so the measurements will tend to
skew to the lower side of the spectrum.
The following measurements were made with the x86-64 Clang toolchain:
- The kernel contains 290989 relocations. Enabling RELR decreased its
size from 30 MiB to 23 MiB.
- LibUnicodeData contains 190262 relocations, almost all of them
relative. Its file size changed from 17 MiB to 13 MiB.
- /bin/WebContent contains 1300 relocations, 66% of which are relative
relocations. With RELR, its size changed from 832 KiB to 812 KiB.
This change was inspired by the following blog post:
https://maskray.me/blog/2021-10-31-relative-relocations-and-relr
Apologies for the enormous commit, but I don't see a way to split this
up nicely. In the vast majority of cases it's a simple change. A few
extra places can use TRY instead of manual error checking though. :^)
This change lays the foundation for making the require_promise return
an error hand handling the process abort outside of the syscall
implementations, to avoid cases where we would leak resources.
It also has the advantage that it makes removes a gs pointer read
to look up the current thread, then process for every syscall. We
can instead go through the Process this pointer in most cases.
When the member is initialized in the constructor body, but also has a
default constructor, you pay for default construction to just throw it
away. In this case a StringView is light weight to initialize, but we
might as well fix all cases we find.
Found by PVS-Studio: https://pvs-studio.com/en/docs/warnings/v818/
Consider the situation where two shared libraries libA and libB, both
depending (as in having a NEEDED dtag) on libC. libA is first
dlopen()-ed, which produces libC to be mapped and linked. When libB is
dlopen()-ed the DynamicLinker would re-map and re-link libC though,
causing any previous references to its old location to be invalid. And
if libA's PLT has been patched to point to libC's symbols, then any
further invocations to libA will cause the code to jump to a virtual
address that isn't mapped anymore, therefore causing a crash. This
situation was reported in #10014, although the setup was more convolved
in the ticket.
This commit fixes the issue by distinguishing between a main program
loading being performed by Loader.so, and a dlopen() call. The main
difference between these two cases is that in the former the
s_globals_objects maps is always empty, while in the latter it might
already contain dependencies for the library being dlopen()-ed. Hence,
when collecting dependencies to map and link, dlopen() should skip those
that are present in the global map to avoid the issue described above.
With this patch the original issue seen in #10014 is gone, with all
python3 modules (so far) loading correctly.
A unit test reproducing a simplified issue is also included in this
commit. The unit test includes the building of two dynamic libraries A
and B with both depending on libline.so (and B also depending on A); the
test then dlopen()s libA, invokes one its function, then does the same
with libB.
These are found in some libraries, and LibELF doesn't know how to handle
them, not even their name. Adding these definitions should at least help
readelf display information correctly, but more work is needed to
actually implement them.
A copy of the same mapping was found both in LibELF and in the readelf
utility, which uses LibELF; keeping them both is redundant and removing
the duplicate saves (a bit of) space.
