diff --git a/libc/arch-x86_64/bionic/setjmp.S b/libc/arch-x86_64/bionic/setjmp.S
index 56ebb07..92bd5a9 100644
--- a/libc/arch-x86_64/bionic/setjmp.S
+++ b/libc/arch-x86_64/bionic/setjmp.S
@@ -35,8 +35,22 @@
 
 #include <private/bionic_asm.h>
 
-// These are only the callee-saved registers. Code calling setjmp
-// will expect the rest to be clobbered anyway.
+
+// The internal structure of a jmp_buf is totally private.
+// Current layout (changes from release to release):
+//
+// word   name            description
+// 0      rbx             registers
+// 1      rbp
+// 2      r12
+// 3      r13
+// 4      r14
+// 5      r15
+// 6      rsp
+// 7      pc
+// 8      sigflag/cookie  setjmp cookie in top 31 bits, signal mask flag in low bit
+// 9      sigmask         signal mask (includes rt signals as well)
+// 10     reserved
 
 #define _JB_RBX 0
 #define _JB_RBP 1
@@ -48,7 +62,6 @@
 #define _JB_PC 7
 #define _JB_SIGFLAG 8
 #define _JB_SIGMASK 9
-#define _JB_SIGMASK_RT 10 // sigprocmask will write here too.
 
 #define MANGLE_REGISTERS 1
 .macro m_mangle_registers reg
diff --git a/tests/signal_test.cpp b/tests/signal_test.cpp
index 32308aa..c5128ea 100644
--- a/tests/signal_test.cpp
+++ b/tests/signal_test.cpp
@@ -411,4 +411,14 @@
                                             << sent.si_code << ", received " << received.si_code
                                             << error_msg;
 }
+
+#if defined(__arm__) || defined(__aarch64__) || defined(__i386__) || defined(__x86_64__)
+TEST(signal, sigset_size) {
+  // The setjmp implementations for ARM, AArch64, x86, and x86_64 assume that sigset_t can fit in a
+  // long. This is true because ARM and x86 have broken rt signal support, and AArch64 and x86_64
+  // both have a SIGRTMAX defined as 64.
+  static_assert(sizeof(sigset_t) <= sizeof(long), "sigset_t doesn't fit in a long");
+}
+
+#endif
 #endif