commit b0c4ddf9196738d9c8a162521b61bcefb7aa4c0a
author Tess Strickland <sstrickl@google.com> Thu Sep 29 08:32:47 2022 +0000
committer Commit Queue <dart-scoped@luci-project-accounts.iam.gserviceaccount.com> Thu Sep 29 08:32:47 2022 +0000
tree 9ff82fb0ea0818f98c1e889ca68f0ec1cb90a6a8
parent a2466e26c53331511b73258c2c890c61484ae8c8

[pkg/dart2native] Avoid overwriting section contents in MachO files.

To create a Dart standalone executable on MacOS, we modify the
dartaotruntime executable to add the snapshot contents, and the
VM looks into the executable on disk to find the snapshot to load.

Previously, we did this by adding a new 64-bit segment load command
with a single section, where the section's file offset and size
describes the inserted snapshot. This meant the Mach-O header size increased by 152 bytes.

Originally, this wasn't an issue as there was plenty of padding, but
later clang updates removed most of this padding, and so writing the
new header actually overwrote the initial contents of the first section
in the file, which happens to be the __text section. In addition, since
the first section's offset was now declared to be within the header,
utilities that strictly validated the Mach-O format, like codesign,
would report errors.

This CL changes it so that we actually reserve space in the
dartaotruntime header using the -add_empty_section flag to the linker.
In addition, we change from using a segment load command to using a
(40 byte) note load command. This is because a segment load command
specifies that the contents should be loaded in memory, but we don't
use that loaded version. Instead, the VM reloads it from the executable
on disk so it can appropriately mmap the different parts of the
snapshot. A note section instead just declares a section of the
executable as arbitrary data that the owner can read from the file
and use as desired, which is semantically closer to our current usage.

This CL also adds a test to pkg/dartdev/test/commands/compile_test to
ensure that corrupting a random part of the snapshot in the executable
causes signature verification to fail.

This CL also reverts CL 256208, thus relanding the clang changes
starting from June that originally raised awareness of the issue by
greatly reduced the amount of padding after the load commands.

TEST=pkg/dartdev/test/commands/compile_test

Bug: https://github.com/dart-lang/sdk/issues/49783
Change-Id: Iee554d87b0eabaecd7a534ca4e4facfefbce6385
Cq-Include-Trybots: luci.dart.try:analyzer-mac-release-try,dart-sdk-mac-arm64-try,dart-sdk-mac-try,pkg-mac-release-arm64-try,pkg-mac-release-try,vm-kernel-precomp-mac-product-x64-try,vm-kernel-precomp-nnbd-mac-release-arm64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/260108
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>