ringlogger: hook into global panic writer

This is a grotesque hack, and hopefully upstream Go will provide a nicer
way of doing this, but already it seems quite adept at catching panics.
See https://github.com/golang/go/issues/42888 for more info.

This requires us to rewrite the ringlogger path to avoid all
allocations.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

1 files changed, 35 insertions, 0 deletions

diff --git a/ringlogger/global.go b/ringlogger/global.go
index 0d99e5a0..6f4458ca 100644
--- a/ringlogger/global.go
+++ b/ringlogger/global.go
@@ -8,6 +8,7 @@ package ringlogger
 import (
 	"log"
 	"path/filepath"
+	"unsafe"
 
 	"golang.zx2c4.com/wireguard/windows/conf"
 )
@@ -28,5 +29,39 @@ func InitGlobalLogger(tag string) error {
 	}
 	log.SetOutput(Global)
 	log.SetFlags(0)
+	overrideWrite = globalWrite
 	return nil
 }
+
+//go:linkname overrideWrite runtime.overrideWrite
+var overrideWrite func(fd uintptr, p unsafe.Pointer, n int32) int32
+
+var globalBuffer [maxLogLineLength - 1 - maxTagLength - 3]byte
+var globalBufferLocation int
+
+//go:nosplit
+func globalWrite(fd uintptr, p unsafe.Pointer, n int32) int32 {
+	b := (*[1 << 30]byte)(p)[:n]
+	for len(b) > 0 {
+		amountAvailable := len(globalBuffer) - globalBufferLocation
+		amountToCopy := len(b)
+		if amountToCopy > amountAvailable {
+			amountToCopy = amountAvailable
+		}
+		copy(globalBuffer[globalBufferLocation:], b[:amountToCopy])
+		b = b[amountToCopy:]
+		globalBufferLocation += amountToCopy
+		foundNl := false
+		for i := globalBufferLocation - amountToCopy; i < globalBufferLocation; i++ {
+			if globalBuffer[i] == '\n' {
+				foundNl = true
+				break
+			}
+		}
+		if foundNl || len(b) > 0 {
+			Global.Write(globalBuffer[:globalBufferLocation])
+			globalBufferLocation = 0
+		}
+	}
+	return n
+}