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Fangrui Song 5391f158c2 [ELF] Add -z separate-code and pad the last page of last PF_X PT_LOAD with traps only if -z separate-code is specified 
This patch

1) adds -z separate-code and -z noseparate-code (default).
2) changes the condition that the last page of last PF_X PT_LOAD is
 padded with trap instructions.
 Current condition (after D33630): if there is no `SECTIONS` commands.
 After this change: if -z separate-code is specified.

-z separate-code was introduced to ld.bfd in 2018, to place the text
segment in its own pages. There is no overlap in pages between an
executable segment and a non-executable segment:

1) RX cannot load initial contents from R or RW(or non-SHF_ALLOC).
2) R and RW(or non-SHF_ALLOC) cannot load initial contents from RX.

lld's current status:

- Between R and RX: in `Writer<ELFT>::fixSectionAlignments()`, the start of a
  segment is always aligned to maxPageSize, so the initial contents loaded by R
  and RX do not overlap. I plan to allow overlaps in D64906 if -z noseparate-code
  is in effect.
- Between RX and RW(or non-SHF_ALLOC if RW doesn't exist):
  we currently unconditionally pad the last page to commonPageSize
  (defaults to 4096 on all targets we support).
  This patch will make it effective only if -z separate-code is specified.

-z separate-code is a dubious feature that intends to reduce the number
of ROP gadgets (which is actually ineffective because attackers can find
plenty of gadgets in the text segment, no need to find gadgets in
non-code regions).

With the overlapping PT_LOAD technique D64906, -z noseparate-code
removes two more alignments at segment boundaries than -z separate-code.
This saves at most defaultCommonPageSize*2 bytes, which are significant
on targets with large defaultCommonPageSize (AArch64/MIPS/PPC: 65536).

Issues/feedback on alignment at segment boundaries to help understand
the implication:

* binutils PR24490 (the situation on ld.bfd is worse because they have
  two R-- on both sides of R-E so more alignments.)

* In binutils, the 2018-02-27 commit "ld: Add --enable-separate-code" made -z separate-code the default on Linux.
  d969dea983
  In musl-cross-make, binutils is configured with --disable-separate-code
  to address size regressions caused by -z separate-code. (lld actually has the same
  issue, which I plan to fix in a future patch. The ld.bfd x86 status is
  worse because they default to max-page-size=0x200000).

* https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237676 people want
  smaller code size. This patch will remove one alignment boundary.

* Stef O'Rear: I'm opposed to any kind of page alignment at the
  text/rodata line (having a partial page of text aliased as rodata and
  vice versa has no demonstrable harm, and I actually care about small
  systems).

So, make -z noseparate-code the default.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D64903

llvm-svn: 367537
 		2019-08-01 09:58:25 +00:00
..
COFF [lld-link] diagnose undefined symbols before LTO when possible 2019-07-26 17:56:45 +00:00
Common [ELF] With --vs-diagnostics, print a separate message for each location of a duplicate symbol. 2019-08-01 09:58:03 +00:00
ELF [ELF] Add -z separate-code and pad the last page of last PF_X PT_LOAD with traps only if -z separate-code is specified 2019-08-01 09:58:25 +00:00
MinGW [Coding style change][lld] Rename variables for non-ELF ports 2019-07-11 05:40:30 +00:00
cmake/modules [CMake] Properly conditionalize installation of lld libraries 2018-03-09 13:09:36 +00:00
docs Bump the trunk version to 10.0.0svn 2019-07-18 11:51:05 +00:00
include/lld [ELF] With --vs-diagnostics, print a separate message for each location of a duplicate symbol. 2019-08-01 09:58:03 +00:00
lib [Coding style change][lld] Rename variables for non-ELF ports 2019-07-11 05:40:30 +00:00
test [ELF] Add -z separate-code and pad the last page of last PF_X PT_LOAD with traps only if -z separate-code is specified 2019-08-01 09:58:25 +00:00
tools/lld Fix build breakage on Darwin. 2019-07-11 06:12:18 +00:00
unittests Update the file headers across all of the LLVM projects in the monorepo 2019-01-19 08:50:56 +00:00
utils Python 2/3 compatibility 2019-03-20 07:42:13 +00:00
wasm [WebAssembly] Set __tls_align to 1 when there is no TLS 2019-07-24 21:48:14 +00:00
.arcconfig [lld] Set up .arcconfig to point to new Diffusion LLD repository 2017-12-06 20:56:23 +00:00
.clang-format
…
.gitignore
…
CMakeLists.txt [CMake] Don't set Python_ADDITIONAL_VERSIONS 2019-07-18 15:17:42 +00:00
CODE_OWNERS.TXT
…
LICENSE.TXT Fix typos throughout the license files that somehow I and my reviewers 2019-01-21 09:52:34 +00:00
README.md Note that the test .tar.xz has been updated. 2017-12-22 19:37:32 +00:00

README.md

LLVM Linker (lld)

This directory and its subdirectories contain source code for the LLVM Linker, a modular cross platform linker which is built as part of the LLVM compiler infrastructure project.

lld is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt.

Benchmarking

In order to make sure various developers can evaluate patches over the same tests, we create a collection of self contained programs.

It is hosted at https://s3-us-west-2.amazonaws.com/linker-tests/lld-speed-test.tar.xz

The current sha256 is 10eec685463d5a8bbf08d77f4ca96282161d396c65bd97dc99dbde644a31610f.