Index: clang/lib/Sema/SemaDeclAttr.cpp
--- clang/lib/Sema/SemaDeclAttr.cpp.orig
+++ clang/lib/Sema/SemaDeclAttr.cpp
@@ -3394,6 +3394,7 @@ static FormatAttrKind getFormatAttrKind(StringRef Form
       .Case("freebsd_kprintf", SupportedFormat) // FreeBSD.
       .Case("os_trace", SupportedFormat)
       .Case("os_log", SupportedFormat)
+      .Case("syslog", SupportedFormat)
 
       .Cases("gcc_diag", "gcc_cdiag", "gcc_cxxdiag", "gcc_tdiag", IgnoredFormat)
       .Default(InvalidFormat);
