[PATCH v2 5/7] rseq: selftests: Add non-glibc compatibility fixes

[Aqib Faruqui <aqibaf@amazon.com>]

The rseq selftests rely on features provided by glibc that may not be
available in non-glibc C libraries:

1. The __GNU_PREREQ macro and glibc's thread pointer implementation are
   not available in non-glibc libraries
2. The __NR_rseq syscall number may not be defined in non-glibc headers

Add a fallback thread pointer implementation for non-glibc systems using
the pre-existing inline assembly to access thread-local storage directly
via %fs/%gs registers. Also provide a fallback definition for __NR_rseq
when not already defined by the C library headers: 527 for alpha and 293
for other architectures.

Signed-off-by: Aqib Faruqui <aqibaf@amazon.com>
---
 .../selftests/rseq/rseq-x86-thread-pointer.h       | 14 ++++++++++++++
 tools/testing/selftests/rseq/rseq.c                |  8 ++++++++
 2 files changed, 22 insertions(+)

diff --git a/tools/testing/selftests/rseq/rseq-x86-thread-pointer.h b/tools/testing/selftests/rseq/rseq-x86-thread-pointer.h
index d3133587d..a7c402926 100644
--- a/tools/testing/selftests/rseq/rseq-x86-thread-pointer.h
+++ b/tools/testing/selftests/rseq/rseq-x86-thread-pointer.h
@@ -14,6 +14,7 @@
 extern "C" {
 #endif
 
+#ifdef __GLIBC__
 #if __GNUC_PREREQ (11, 1)
 static inline void *rseq_thread_pointer(void)
 {
@@ -32,6 +33,19 @@ static inline void *rseq_thread_pointer(void)
 	return __result;
 }
 #endif /* !GCC 11 */
+#else
+static inline void *rseq_thread_pointer(void)
+{
+	void *__result;
+
+# ifdef __x86_64__
+	__asm__ ("mov %%fs:0, %0" : "=r" (__result));
+# else
+	__asm__ ("mov %%gs:0, %0" : "=r" (__result));
+# endif
+	return __result;
+}
+#endif /* !__GLIBC__ */
 
 #ifdef __cplusplus
 }
diff --git a/tools/testing/selftests/rseq/rseq.c b/tools/testing/selftests/rseq/rseq.c
index 663a9cef1..1a6f73c98 100644
--- a/tools/testing/selftests/rseq/rseq.c
+++ b/tools/testing/selftests/rseq/rseq.c
@@ -36,6 +36,14 @@
 #include "../kselftest.h"
 #include "rseq.h"
 
+#ifndef __NR_rseq
+#ifdef __alpha__
+#define __NR_rseq 527
+#else
+#define __NR_rseq 293
+#endif
+#endif
+
 /*
  * Define weak versions to play nice with binaries that are statically linked
  * against a libc that doesn't support registering its own rseq.
-- 
2.47.3