Debug with GDB on macOS 11

The big reason to do this is that LLDB has no ability to "follow-fork-mode child", in other words, a multi-process target that doesn't have a single-process mode (or, a bug that only manifests when in multi-process mode) is going to be difficult or impossible to debug, especially if you have to run the target over and over in order to make the bug manifest. If you have a repeatable bug, no big deal, break on the fork from the parent process and attach to the child in a second lldb instance. Otherwise, read on.

Install GDB

Don't make the mistake of thinking you can just brew install gdb. Currently this is version 10.2 and it's mostly broken, with at least two annoying bugs as of April 29th 2021, but the big one is https://sourceware.org/bugzilla/show_bug.cgi?id=24069

$ xcode-select install  # install the XCode command-line tools
$ brew install --force --build-from-source domq/gdb/gdb  # a patched version based on 10.1

It's now installed at /usr/local/bin/gdb.

Create and Install a Self-Signed Code-Signing Certificate

Start the Keychain Access application, found in /Applications/Utilities.
From the Keychains list on the left, right-click on the System item and select Unlock Keychain "System".
Choose Keychain Access > Certificate Assistant > Create a Certificate... from the menu.
Choose a name (e.g. gdb-cert), set Identity Type to Self Signed Root, set Certificate Type to Code Signing and select the Let me override defaults. Click several times on Continue until you get to the Specify a Location For The Certificate screen, then set Keychain to System.
If for some reason you create the certificate in the System keychain directly, create it in the Login keychain, then export it. You can then import it into the System keychain.
Finally, using the context menu for the certificate, select Get Info, open the Trust item, and set Code Signing to Always Trust.
From the Keychains list on the left, right-click on the System item and select Lock Keychain "System".
You must quit the Keychain Access application and restart the system, in order to use the certificate.

Create the entitlements plist file (for macOS 10.14 and newer)

We'll call it gdb-entitlement.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
"http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>com.apple.security.cs.allow-jit</key>
    <true/>
    <key>com.apple.security.cs.allow-unsigned-executable-memory</key>
    <true/>
    <key>com.apple.security.cs.allow-dyld-environment-variables</key>
    <true/>
    <key>com.apple.security.cs.disable-library-validation</key>
    <true/>
    <key>com.apple.security.cs.disable-executable-page-protection</key>
    <true/>
    <key>com.apple.security.cs.debugger</key>
    <true/>
    <key>com.apple.security.get-task-allow</key>
    <true/>
</dict>
</plist>

Codesign and entitle the GDB executable

$ codesign --entitlements gdb-entitlement.xml -fs gdb-cert $(which gdb)

This seems to work even though that's Homebrew's link to the actual file on disk:

% ls -la $(which gdb)
lrwxr-xr-x  1 mmyers  admin  26 Apr 28 16:16 /usr/local/bin/gdb -> ../Cellar/gdb/10.2/bin/gdb
% codesign -vvv /usr/local/Cellar/gdb/10.2/bin/gdb
/usr/local/Cellar/gdb/10.2/bin/gdb: valid on disk
/usr/local/Cellar/gdb/10.2/bin/gdb: satisfies its Designated Requirement

Finally, Run GDB

$ gdb hello_world
(gdb) break main
(gdb) run

Troubleshooting

GDB will not understand "fat" executables. You can "lipo -thin x86_64" them.
If you tried debugging with gdb, but you get a "No symbol table is loaded" error, you might need to compile programs with the -ggdb option in gcc. I didn't have this issue personally.
If after hitting run in gdb, you get "Starting program: /path/to/your/executable [args] [New Thread 0x2303 of process 795]" followed by a blank line which does not respond to anything, then you have hit GDB bug 24069. Check that you built the patched version from source.

shi27feng/Working GDB on macOS 11.md