If the --shuffle-sections is given, mold now randomize the output by
shuffling input sections randomly. This feature is compatible with lld's
--shuffle-sections=SEED option introduced in
https://reviews.llvm.org/D74791.
This feature is useful when you want to equalize the conditions of
benchmarks. That is, some particular memory layout can produce a very
good benchmark number due to hardware-level cache hit rate or something
like that. Therefore, even if you get a good benchmark number after
changing code, there's a chance that that's caused by the layout change
and not by the new code itself. With --shuffle-sections, you can isolate
that.
The other use case I can think of is to enhance security. If you build
your program as a position-independent executable, the kernel
automatically enables ASLR (Address Space Layout Randomization), but ASLR
only shift the entire program image in memory by some random offset;
Relative offsets between sections remain the same. If you compile programs
from source, by using --shuffle-sections, you can make the offsets
unpredictable to attackers.
The LTO plugin API support is still in progress, but with this change,
mold can link itself with `-flto` with both GCC and Clang.
Since mold now supports LTO natively, I removed the fallback mechanism
to ld.bfd or ld.lld that I implemented in
a5029d19a8.
Fixes https://github.com/rui314/mold/issues/181
`-icf=all` couldn't handle very small programs that has no sections
to process in the main loop. This commit fixes that crash bug.
Fixes https://github.com/rui314/mold/issues/332
GCC creates symbols in comdat groups as STB_GNU_UNIQUE instead of STB_WEAK
if it was configured to do so at build time or the -fgnu-unique option was
given. If mold is given two object files with and without STB_GNU_UNIQUE,
it could end up selecting a sybmol that is in a de-duplicated comdat group.
This is arguably just an ABI incompatibility. Two comdat groups must
contain the same contents if their identifiers are the same. But we
can't handle it as an error because it is not uncommon to link object
files compiled using Clang (or GCC without -fgnu-unique) to static
libraries built with GCC that produces STB_GNU_UNIQUE symbols.
This patch gives the same priority to STB_GNU_UNIQUE as STB_WEAK so that
mold won't select symbols in discarded comdat groups.
Frankly, the situation around STB_GNU_UNIQUE is a mess. That GNU extension
shouldn't have been added to the GNU toolchain in the first place.
It looks like GCC shipped with Linux distros are nowadays do not produce
STB_GNU_UNIQUE symbols by default, but we still need to handle them.
Fixes https://github.com/rui314/mold/issues/324
Previously, if a protected/hidden undef symbol is resolved to a DSO
symbol, mold didn't report a symbol undefined error.
Fixes https://github.com/rui314/mold/issues/329
This commit adds the `--emit-relocs` option to mold. If the flag is
given, mold copies relocation sections from input files to an output
file. This option is used by some binary post-processing tools such
as Facebook's Bolt.
It looks like the relocations emitted by `--emit-relocs` are correct,
but I didn't do extensive testing. If you find any problem, please
let me know.
Fixes https://github.com/rui314/mold/issues/177
`--repro` is a hidden command flag for debugging.
Previously, if the flag was given, mold would create a `.repro`
section in an output file with all input files as its contents.
The flaw of the design is that when mold fails to create an output
file, no .repro section would be created.
So I changed the behavior in this commit. Now, the tar file is
created as an independent file.
PT_GNU_RELRO works on page granularity. We always align the begining
of a RELRO segment to a page boundary, but the end was not. Since the
runtime conservatively align _down_ it to a page boundary, the last
page weren't be marked as read-only.
This patch makes the size of a RELRO always a multiple of the page size.
To process version scriots, we have to match glob patterns against
symbol strings. Sometimes, we have hundreds or thousands of glob
patterns and have to match them against millions of mangled long
C++ symbol names. This step can be very slow.
In this patch, I implemented the Aho-Corasick algorithm to match glob
patterns to symbol strings as quickly as possible. For the details
of the algorithm, see https://en.wikipedia.org/wiki/Glob_(programming).
This patch improves mold's performance for programs that uses large
version scripts. For example, linking libQt6Gui.so.6.3.0 reduced from
1.10s to 0.05s with this patch.
This patch also changes how symbol versions are applied if two or more
version patterns match to a single symbol string. Previously, the last
one in a script file took precedence. Now, the first one takes
precedence. I believe the new behavior is compatible with GNU ld.
Fixes https://github.com/rui314/mold/issues/156
Fixed https://github.com/rui314/mold/issues/287
Previously, we didn't handle version scripts like this correctly:
ver {
global: *;
local: foo*;
}
We didn't handle `local:` part correctly except for `*`.
Fixes https://github.com/rui314/mold/issues/277
.relr.dyn is a new section that has been implemented in other linkers
recently. That section contains only the RELATIVE-type dynamic
relocations (i.e. base relocations). Compared to the regular
.rela.dyn, a .relr.dyn's size is typically less than 1/10 because the
section is compressed.
Since PIEs (position-independent executables) tend to contain lots of
RELATIVE-type relocations and PIEs are now the default on many Linux
distributions for security reasons, .relr.dyn is more effective than
it was. It can reduce binary size by a few percent or more.
Note that the runtime support is catching up, so binaries built with
`-pack-dyn-relocs=relr` may not work on your system unless you are
running a very recent version of Linux.
gdb uses this field only if it is an executable. Having an extra
dynamic entry is usually harmless, but unfortunately, RPM's `rpmdeps`
tool uses the presence of this entry to distinguish executables from
shared object files. So, it is better not to create a DT_DEBUG entry
for shared object files.
Fixes https://github.com/rui314/mold/issues/254
That substring was introduced in de7ba9014f
to appease Gentoo's dev-libs/jansson-2.13.1-r1 package. But I think I
shouldn't done that from the beginning to avoid the sitaution of the
"User-Agent" string of the web browser, which everybody claims they
are Mozilla, AppleWebKit, Chrome and Safari simultaneously.
Previously, the `bnd jmp` instruction in a IBTPLT entry jumps to the
middle of an instruction due to an incorrect GOTPLT value. That happened
to work because a value that was there after the jump can be interpreted
as an `add` instruction and the control falled through the next `pushq`.
Previously, mold ignored common symbols in an archive when resolving
symbols. As a result, even if an undefined symbol could be resolved
using a common symbol in an archive, mold would end up with an undefined
symbol error.
This commit changes the behavior. Now, an undefined symbol is resolved
to a common symbol in an archive if exists.
Fixes https://github.com/rui314/mold/issues/256
