From 8950831d3c4af4dd169e0a404e25e8aa9b045caa Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Fri, 26 Sep 2025 12:00:41 +0100
Subject: [PATCH] Dwarf.Unwind: handle macOS deviation from standard

Apparently the `__eh_frame` in Mach-O binaries doesn't include the
terminator entry, but in all other respects it acts like `.eh_frame`
rather than `.debug_frame`. I have no idea.
---
 lib/std/debug/Dwarf/Unwind.zig          | 14 ++++++++++----
 lib/std/debug/SelfInfo/DarwinModule.zig |  2 +-
 lib/std/debug/SelfInfo/ElfModule.zig    |  2 +-
 3 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/lib/std/debug/Dwarf/Unwind.zig b/lib/std/debug/Dwarf/Unwind.zig
index e251a9175d..90e531b349 100644
--- a/lib/std/debug/Dwarf/Unwind.zig
+++ b/lib/std/debug/Dwarf/Unwind.zig
@@ -475,10 +475,15 @@ pub fn prepare(
     addr_size_bytes: u8,
     endian: Endian,
     need_lookup: bool,
+    /// The `__eh_frame` section in Mach-O binaries deviates from the standard `.eh_frame` section
+    /// in one way which this function needs to be aware of.
+    is_macho: bool,
 ) !void {
     if (unwind.cie_list.len > 0 and (!need_lookup or unwind.lookup != null)) return;
     unwind.cie_list.clearRetainingCapacity();
 
+    if (is_macho) assert(unwind.lookup == null or unwind.lookup.? != .eh_frame_hdr);
+
     const section = unwind.frame_section;
 
     var r: Reader = .fixed(section.bytes);
@@ -519,10 +524,11 @@ pub fn prepare(
             .terminator => break true,
         }
     } else false;
-    switch (section.id) {
-        .eh_frame => if (!saw_terminator) return bad(), // `.eh_frame` indicates the end of the CIE/FDE list with a sentinel entry
-        .debug_frame => if (saw_terminator) return bad(), // `.debug_frame` uses the section bounds and does not specify a sentinel entry
-    }
+    const expect_terminator = switch (section.id) {
+        .eh_frame => !is_macho, // `.eh_frame` indicates the end of the CIE/FDE list with a sentinel entry, though macOS omits this
+        .debug_frame => false, // `.debug_frame` uses the section bounds and does not specify a sentinel entry
+    };
+    if (saw_terminator != expect_terminator) return bad();
 
     std.mem.sortUnstable(SortedFdeEntry, fde_list.items, {}, struct {
         fn lessThan(ctx: void, a: SortedFdeEntry, b: SortedFdeEntry) bool {
diff --git a/lib/std/debug/SelfInfo/DarwinModule.zig b/lib/std/debug/SelfInfo/DarwinModule.zig
index caf2176f75..71e43a9a74 100644
--- a/lib/std/debug/SelfInfo/DarwinModule.zig
+++ b/lib/std/debug/SelfInfo/DarwinModule.zig
@@ -59,7 +59,7 @@ fn loadUnwindInfo(module: *const DarwinModule, gpa: Allocator, out: *DebugInfo)
     var dwarf: Dwarf.Unwind = .initSection(.eh_frame, @intFromPtr(eh_frame.ptr) - vmaddr_slide, eh_frame);
     errdefer dwarf.deinit(gpa);
     // We don't need lookups, so this call is just for scanning CIEs.
-    dwarf.prepare(gpa, @sizeOf(usize), native_endian, false) catch |err| switch (err) {
+    dwarf.prepare(gpa, @sizeOf(usize), native_endian, false, true) catch |err| switch (err) {
         error.ReadFailed => unreachable, // it's all fixed buffers
         error.InvalidDebugInfo,
         error.MissingDebugInfo,
diff --git a/lib/std/debug/SelfInfo/ElfModule.zig b/lib/std/debug/SelfInfo/ElfModule.zig
index eead810a86..7ce24e2e2a 100644
--- a/lib/std/debug/SelfInfo/ElfModule.zig
+++ b/lib/std/debug/SelfInfo/ElfModule.zig
@@ -228,7 +228,7 @@ pub fn getSymbolAtAddress(module: *const ElfModule, gpa: Allocator, di: *DebugIn
     };
 }
 fn prepareUnwindLookup(unwind: *Dwarf.Unwind, gpa: Allocator) Error!void {
-    unwind.prepare(gpa, @sizeOf(usize), native_endian, true) catch |err| switch (err) {
+    unwind.prepare(gpa, @sizeOf(usize), native_endian, true, false) catch |err| switch (err) {
         error.ReadFailed => unreachable, // it's all fixed buffers
         error.InvalidDebugInfo,
         error.MissingDebugInfo,