[PATCH v3] selftests/seccomp: add Alpha support to seccomp_bpf

[Magnus Lindholm <linmag7@gmail.com>]

Enable the seccomp BPF selftests on Alpha with a fallback seccomp syscall
number and architecture-specific syscall register accessors.

Use the NT_PRSTATUS register layout rather than struct pt_regs. The
register set contains r0-r30 followed by pc and unique. Read and update
the syscall number in r1, and set both r0 and the r19/a3 error flag when
synthesizing syscall results.

At syscall entry, r19 still contains the fourth syscall argument, so
callers setting a synthetic return value must also set the syscall
number to -1 to skip execution.

Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
---
Changes in v3:
- Drop the /proc/self/syscall availability check and its commit-message
  paragraph. Alpha selects HAVE_ARCH_TRACEHOOK.
- Document that setting r19/a3 at syscall entry requires callers to also
  set the syscall number to -1.
- Add Matt Turner's Reviewed-by after addressing his review comments.

Changes in v2:
- Use the NT_PRSTATUS layout for PTRACE_GETREGSET/PTRACE_SETREGSET.
- Set the Alpha return value and error flag together.
- Check for /proc/self/syscall at runtime (removed in v3).

Validation on AlphaServer ES40, GCC 15.2.1, running
7.3.0-rc1-es40-tsunami-v5a+:
- Standard v3 and the original baseline binary both report 98 pass,
  1 fail, 12 skip. The common failure is the 30-second timeout in
  user_notification_wait_killable_after_reply.
- A separate diagnostic build extending only that test's timeout to
  180 seconds completes it in 34.938 seconds. The full diagnostic run
  reports 99 pass, 0 fail, 12 skip. No timeout change is in this patch.

 tools/testing/selftests/seccomp/seccomp_bpf.c | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index 0622bc2..79e8eb3 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -136,6 +136,8 @@ struct seccomp_data {
 #  define __NR_seccomp 354
 # elif defined(__x86_64__)
 #  define __NR_seccomp 317
+# elif defined(__alpha__)
+#  define __NR_seccomp 514
 # elif defined(__arm__)
 #  define __NR_seccomp 383
 # elif defined(__aarch64__)
@@ -1754,6 +1756,30 @@ TEST_F(TRACE_poke, getpid_runs_normally)
 # define ARCH_REGS		struct user_regs_struct
 # define SYSCALL_NUM(_regs)	(_regs).orig_eax
 # define SYSCALL_RET(_regs)	(_regs).eax
+#elif defined(__alpha__)
+/* NT_PRSTATUS contains r0-r30, pc and unique, not struct pt_regs. */
+struct alpha_regs {
+	unsigned long regs[33];
+};
+
+# define ARCH_REGS		struct alpha_regs
+/* The kernel keeps the mutable syscall number in r1. */
+# define SYSCALL_NUM(_regs)	((_regs).regs[1])
+# define SYSCALL_RET(_regs)	((_regs).regs[0])
+/*
+ * Alpha returns positive errno in r0 with the r19/a3 error flag set.
+ * At a syscall-entry stop, r19 is still the fourth syscall argument.
+ * Callers must also set the syscall number to -1 to skip the syscall
+ * before replacing r19 with the error flag.
+ */
+# define SYSCALL_RET_SET(_regs, _val)				\
+	do {							\
+		struct alpha_regs *__regs = &(_regs);		\
+		long __ret = (_val);				\
+								\
+		__regs->regs[0] = __ret < 0 ? -__ret : __ret;	\
+		__regs->regs[19] = __ret < 0;			\
+	} while (0)
 #elif defined(__arm__)
 # define ARCH_REGS		struct pt_regs
 # define SYSCALL_NUM(_regs)	(_regs).ARM_r7
-- 
2.43.0