From c45000de73957993a7a29f46d4571c806b07b4ae Mon Sep 17 00:00:00 2001
From: Joel Reymont <18791+joelreymont@users.noreply.github.com>
Date: Wed, 19 Nov 2025 05:29:36 +0200
Subject: [PATCH] Fix integer underflow in Unit.resizeHeader()
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When a DWARF unit has no previous unit (i.e., it's the first unit
in a section), the code incorrectly calculated `available_len = 0`,
even though there was actually `unit.off` bytes of available space
before the unit.

```zig
const available_len = if (unit.prev.unwrap()) |prev_unit| prev_excess: {
    const prev_unit_ptr = sec.getUnit(prev_unit);
    break :prev_excess unit.off - prev_unit_ptr.off - prev_unit_ptr.len;
} else 0;  // ← BUG: Should be unit.off, not 0
```
---
 src/link/Dwarf.zig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/link/Dwarf.zig b/src/link/Dwarf.zig
index e8fea3c988c7..b4606611f87d 100644
--- a/src/link/Dwarf.zig
+++ b/src/link/Dwarf.zig
@@ -635,7 +635,7 @@ const Unit = struct {
         const available_len = if (unit.prev.unwrap()) |prev_unit| prev_excess: {
             const prev_unit_ptr = sec.getUnit(prev_unit);
             break :prev_excess unit.off - prev_unit_ptr.off - prev_unit_ptr.len;
-        } else 0;
+        } else unit.off;
         if (available_len + unit.header_len < len)
             try unit.resize(sec, dwarf, len - unit.header_len, unit.len - unit.header_len + len);
         if (unit.header_len > len) {