From: Mathieu Desnoyers mathieu.desnoyers@efficios.com

[ Upstream commit ee8a84c60bcc1f1615bd9cb3edfe501e26cdc85b ]

Using ".arm .inst" for the arm signature introduces build issues for programs compiled in Thumb mode because the assembler stays in the arm mode for the rest of the inline assembly. Revert to using a ".word" to express the signature as data instead.

The choice of signature is a valid trap instruction on arm32 little endian, where both code and data are little endian.

ARMv6+ big endian (BE8) generates mixed endianness code vs data: little-endian code and big-endian data. The data value of the signature needs to have its byte order reversed to generate the trap instruction.

Prior to ARMv6, -mbig-endian generates big-endian code and data (which match), so the endianness of the data representation of the signature should not be reversed. 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. So rather than try to play tricks with the linker, the rseq signature is simply data (not a trap instruction) prior to ARMv6 on big endian. This is why the signature is expressed as data (.word) rather than as instruction (.inst) in assembler.

Because a ".word" is used to emit the signature, it will be interpreted as a literal pool by a disassembler, not as an actual instruction. Considering that the signature is not meant to be executed except in scenarios where the program execution is completely bogus, this should not be an issue.

Signed-off-by: Mathieu Desnoyers mathieu.desnoyers@efficios.com Acked-by: Will Deacon will.deacon@arm.com CC: Peter Zijlstra peterz@infradead.org CC: Thomas Gleixner tglx@linutronix.de CC: Joel Fernandes joelaf@google.com CC: Catalin Marinas catalin.marinas@arm.com CC: Dave Watson davejwatson@fb.com CC: Will Deacon will.deacon@arm.com CC: Shuah Khan shuah@kernel.org CC: Andi Kleen andi@firstfloor.org CC: linux-kselftest@vger.kernel.org CC: "H . Peter Anvin" hpa@zytor.com CC: Chris Lameter cl@linux.com CC: Russell King linux@arm.linux.org.uk CC: Michael Kerrisk mtk.manpages@gmail.com CC: "Paul E . McKenney" paulmck@linux.vnet.ibm.com CC: Paul Turner pjt@google.com CC: Boqun Feng boqun.feng@gmail.com CC: Josh Triplett josh@joshtriplett.org CC: Steven Rostedt rostedt@goodmis.org CC: Ben Maurer bmaurer@fb.com 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 carlos@redhat.com CC: Florian Weimer fweimer@redhat.com Signed-off-by: Shuah Khan skhan@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- tools/testing/selftests/rseq/rseq-arm.h | 61 +++++++++++++------------ 1 file changed, 33 insertions(+), 28 deletions(-)

diff --git a/tools/testing/selftests/rseq/rseq-arm.h b/tools/testing/selftests/rseq/rseq-arm.h index 84f28f147fb6..5943c816c07c 100644 --- a/tools/testing/selftests/rseq/rseq-arm.h +++ b/tools/testing/selftests/rseq/rseq-arm.h @@ -6,6 +6,8 @@ */

/* + * - ARM little endian + * * 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 @@ -22,36 +24,40 @@ * def3 udf #243 ; 0xf3 * e7f5 b.n <7f5> * - * pre-ARMv6 big endian code: - * e7f5 b.n <7f5> - * def3 udf #243 ; 0xf3 + * - ARMv6+ big endian (BE8): * * 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. + * code and big-endian data. The data value of the signature needs to have its + * byte order reversed to generate the trap instruction: + * + * Data: 0xf3def5e7 + * + * Translates to this A32 instruction pattern: + * + * e7f5def3 udf #24035 ; 0x5de3 + * + * Translates to this T16 instruction pattern: + * + * def3 udf #243 ; 0xf3 + * e7f5 b.n <7f5> + * + * - Prior to ARMv6 big endian (BE32): + * + * Prior to ARMv6, -mbig-endian generates big-endian code and data + * (which match), so the endianness of the data representation of the + * signature should not be reversed. 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. So rather + * than try to play tricks with the linker, the rseq signature is simply + * data (not a trap instruction) prior to ARMv6 on big endian. This is + * why the signature is expressed as data (.word) rather than as + * instruction (.inst) in assembler. */

-#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 - +#ifdef __ARMEB__ +#define RSEQ_SIG 0xf3def5e7 /* udf #24035 ; 0x5de3 (ARMv6+) */ +#else +#define RSEQ_SIG 0xe7f5def3 /* udf #24035 ; 0x5de3 */ #endif

#define rseq_smp_mb() __asm__ __volatile__ ("dmb" ::: "memory", "cc") @@ -125,8 +131,7 @@ do { \ __rseq_str(table_label) ":\n\t" \ ".word " __rseq_str(version) ", " __rseq_str(flags) "\n\t" \ ".word " __rseq_str(start_ip) ", 0x0, " __rseq_str(post_commit_offset) ", 0x0, " __rseq_str(abort_ip) ", 0x0\n\t" \ - ".arm\n\t" \ - ".inst " __rseq_str(RSEQ_SIG_CODE) "\n\t" \ + ".word " __rseq_str(RSEQ_SIG) "\n\t" \ __rseq_str(label) ":\n\t" \ teardown \ "b %l[" __rseq_str(abort_label) "]\n\t"