Re: [RFC PATCH 1/1] Revert "rseq/selftests: arm: use udf instruction for RSEQ_SIG"

This reverts commit 2b845d4b4acd9422bbb668989db8dc36dfc8f438.

That commit introduces build issues for programs compiled in Thumb mode.
Rather than try to be clever and emit a valid trap instruction on arm32,
which requires special care about big/little endian handling on that
architecture, just emit plain data. Data in the instruction stream is
technically expected on arm32: this is how literal pools are
implemented. Reverting to the prior behavior does exactly that.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Thomas Gleixner <redacted>
CC: Joel Fernandes <redacted>
CC: Catalin Marinas <catalin.marinas@arm.com>
CC: Dave Watson <redacted>
CC: Will Deacon <redacted>
CC: Shuah Khan <shuah@kernel.org>
CC: Andi Kleen <redacted>
CC: linux-kselftest@vger.kernel.org
CC: "H . Peter Anvin" <hpa@zytor.com>
CC: Chris Lameter <redacted>
CC: Russell King <redacted>
CC: Michael Kerrisk <redacted>
CC: "Paul E . McKenney" <redacted>
CC: Paul Turner <redacted>
CC: Boqun Feng <redacted>
CC: Josh Triplett <josh@joshtriplett.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ben Maurer <redacted>
CC: linux-api@vger.kernel.org
CC: Andy Lutomirski <luto@amacapital.net>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Carlos O'Donell <redacted>
CC: Florian Weimer <redacted>
---
 tools/testing/selftests/rseq/rseq-arm.h | 52 ++-------------------------------
 1 file changed, 2 insertions(+), 50 deletions(-)

diff --git a/tools/testing/selftests/rseq/rseq-arm.h b/tools/testing/selftests/rseq/rseq-arm.h
index 84f28f147fb6..5f262c54364f 100644
--- a/tools/testing/selftests/rseq/rseq-arm.h
+++ b/tools/testing/selftests/rseq/rseq-arm.h

@@ -5,54 +5,7 @@
  * (C) Copyright 2016-2018 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  */
 
-/*
- * RSEQ_SIG uses the udf A32 instruction with an uncommon immediate operand
- * value 0x5de3. This traps if user-space reaches this instruction by mistake,
- * and the uncommon operand ensures the kernel does not move the instruction
- * pointer to attacker-controlled code on rseq abort.
- *
- * The instruction pattern in the A32 instruction set is:
- *
- * e7f5def3    udf    #24035    ; 0x5de3
- *
- * This translates to the following instruction pattern in the T16 instruction
- * set:
- *
- * little endian:
- * def3        udf    #243      ; 0xf3
- * e7f5        b.n    <7f5>
- *
- * pre-ARMv6 big endian code:
- * e7f5        b.n    <7f5>
- * def3        udf    #243      ; 0xf3
- *
- * ARMv6+ -mbig-endian generates mixed endianness code vs data: little-endian
- * code and big-endian data. Ensure the RSEQ_SIG data signature matches code
- * endianness. Prior to ARMv6, -mbig-endian generates big-endian code and data
- * (which match), so there is no need to reverse the endianness of the data
- * representation of the signature. However, the choice between BE32 and BE8
- * is done by the linker, so we cannot know whether code and data endianness
- * will be mixed before the linker is invoked.
- */
-
-#define RSEQ_SIG_CODE	0xe7f5def3
-
-#ifndef __ASSEMBLER__
-
-#define RSEQ_SIG_DATA							\
-	({								\
-		int sig;						\
-		asm volatile ("b 2f\n\t"				\
-			      "1: .inst " __rseq_str(RSEQ_SIG_CODE) "\n\t" \
-			      "2:\n\t"					\
-			      "ldr %[sig], 1b\n\t"			\
-			      : [sig] "=r" (sig));			\
-		sig;							\
-	})
-
-#define RSEQ_SIG	RSEQ_SIG_DATA
-
-#endif
+#define RSEQ_SIG	0x53053053