rseq/selftests: x86: use ud1 instruction as RSEQ_SIG opcode

Use ud1 as the guard instruction for the restartable sequence abort
handler. Its benefit compared to nopl is to trap execution if the
program ends up trying to execute it by mistake, which makes debugging
easier.

The 4-byte signature per se is unchanged (it is the instruction
operand). Only the opcode is changed from nopl to ud1.

commit 24fa5d1efe98 "rseq/selftests: x86: use ud1 instruction as RSEQ_SIG
opcode" in the Linux kernel.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/include/rseq/rseq-x86.h b/include/rseq/rseq-x86.h
index 77b768e5e889e21953179831e8c36d040c8a8b78..967633325267d6aca06b64581d29ada9e7c628d6 100644 (file)
--- a/include/rseq/rseq-x86.h
+++ b/include/rseq/rseq-x86.h
@@ -7,6 +7,13 @@
 
 #include <stdint.h>
 
+/*
+ * RSEQ_SIG is used with the following reserved undefined instructions, which
+ * trap in user-space:
+ *
+ * x86-32:    0f b9 3d 53 30 05 53      ud1    0x53053053,%edi
+ * x86-64:    0f b9 3d 53 30 05 53      ud1    0x53053053(%rip),%edi
+ */
 #define RSEQ_SIG       0x53053053
 
 /*
@@ -88,8 +95,8 @@ do {                                                                  \
 
 #define RSEQ_ASM_DEFINE_ABORT(label, teardown, abort_label)            \
                ".pushsection __rseq_failure, \"ax\"\n\t"               \
-               /* Disassembler-friendly signature: nopl <sig>(%rip). */\
-               ".byte 0x0f, 0x1f, 0x05\n\t"                            \
+               /* Disassembler-friendly signature: ud1 <sig>(%rip),%edi. */ \
+               ".byte 0x0f, 0xb9, 0x3d\n\t"                            \
                ".long " __rseq_str(RSEQ_SIG) "\n\t"                    \
                __rseq_str(label) ":\n\t"                               \
                teardown                                                \
@@ -609,8 +616,8 @@ do {                                                                        \
 
 #define RSEQ_ASM_DEFINE_ABORT(label, teardown, abort_label)            \
                ".pushsection __rseq_failure, \"ax\"\n\t"               \
-               /* Disassembler-friendly signature: nopl <sig>. */      \
-               ".byte 0x0f, 0x1f, 0x05\n\t"                            \
+               /* Disassembler-friendly signature: ud1 <sig>,%edi. */  \
+               ".byte 0x0f, 0xb9, 0x3d\n\t"                            \
                ".long " __rseq_str(RSEQ_SIG) "\n\t"                    \
                __rseq_str(label) ":\n\t"                               \
                teardown                                                \