[PATCH 03/14] alpha: preserve rolled-back registers for SIGSYS

[Magnus Lindholm]

Record syscall_rollback() in syscall_meta so skipped-syscall handling
preserves r0 and a3. Otherwise seccomp RET_TRAP handlers and kill-action
core dumps lose the original syscall number and fourth argument.

Fixes: 83bd31c2840b ("alpha: enable GENERIC_ENTRY")
Signed-off-by: Magnus Lindholm <linmag7@xxxxxxxxx>
---
 arch/alpha/include/asm/syscall.h     |  4 +++-
 arch/alpha/include/asm/thread_info.h |  1 +
 arch/alpha/kernel/signal.c           | 12 ++++++++++--
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/alpha/include/asm/syscall.h b/arch/alpha/include/asm/syscall.h
index c1394910f584f9ca42367ffc33b1c0ac46cfe112..22315b932f854a71fd3babf6565dda443c6c7a77 100644
--- a/arch/alpha/include/asm/syscall.h
+++ b/arch/alpha/include/asm/syscall.h
@@ -105,14 +105,16 @@ static inline void syscall_set_return_value(struct task_struct *task,
 	}
 }
 
-/* Restore the original syscall nr after seccomp/ptrace modified regs->r1. */
+/* Preserve the original syscall registers for a skipped syscall. */
 
 static inline void syscall_rollback(struct task_struct *task,
 					struct pt_regs *regs)
 {
 	unsigned long nr = task_thread_info(task)->syscall_saved_nr;
 
+	regs->r0 = nr;
 	regs->r1 = nr;
+	task_thread_info(task)->syscall_meta |= ALPHA_SYSCALL_META_ROLLBACK;
 }
 
 static inline bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
diff --git a/arch/alpha/include/asm/thread_info.h b/arch/alpha/include/asm/thread_info.h
index d781ac79106dd1178b9cb0e094333139db531bed..49bc08f3b5a98fbcb1d4ac919001bf70c3db614b 100644
--- a/arch/alpha/include/asm/thread_info.h
+++ b/arch/alpha/include/asm/thread_info.h
@@ -57,6 +57,7 @@ register unsigned long *current_stack_pointer __asm__ ("$30");
 
 #define ALPHA_SYSCALL_META_SKIP			0x2
 #define ALPHA_SYSCALL_META_FORCE_SUCCESS	0x4
+#define ALPHA_SYSCALL_META_ROLLBACK		0x10
 
 /*
  * Thread information flags:
diff --git a/arch/alpha/kernel/signal.c b/arch/alpha/kernel/signal.c
index 441d8dd61055d5ddc321c2221984eab8aca5d206..3fae37b4593f22f07a375ab7bc0b0712d0130a07 100644
--- a/arch/alpha/kernel/signal.c
+++ b/arch/alpha/kernel/signal.c
@@ -587,10 +587,11 @@ alpha_syscall_enter_select(struct pt_regs *regs, long syscall)
 	 * entry work did not already change the return state.
 	 */
 	if (unlikely(!permit || (unsigned long)nr >= NR_syscalls)) {
-		if (regs->r0 == saved_r0 && regs->r19 == saved_r19)
+		if (!(ti->syscall_meta & ALPHA_SYSCALL_META_ROLLBACK) &&
+		    regs->r0 == saved_r0 && regs->r19 == saved_r19)
 			syscall_set_return_value(current, regs, -ENOSYS, 0);
 
-		ti->syscall_meta = ALPHA_SYSCALL_META_SKIP;
+		ti->syscall_meta |= ALPHA_SYSCALL_META_SKIP;
 		return fn;	/* ignored by asm when SKIP is set */
 	}
 
@@ -617,6 +618,13 @@ alpha_finish_syscall_to_user_mode(struct pt_regs *regs, long ret)
 
 	instrumentation_begin();
 
+	if (meta & ALPHA_SYSCALL_META_ROLLBACK) {
+		ti->syscall_saved_nr = 0;
+		instrumentation_end();
+		syscall_exit_to_user_mode(regs);
+		return;
+	}
+
 	if (meta & ALPHA_SYSCALL_META_SKIP) {
 	/*
 	 * Skip-dispatch path: ptrace/seccomp may already have installed
-- 
2.43.0