[PATCH 1/2] ptrace: PTRACE_SET_SYSCALL_INFO syscall skipping support

[Renzo Davoli]

This patch extends PTRACE_SET_SYSCALL_INFO with support for skipping a system
call triggered via seccomp.

When the tracer retrieves a ptrace_syscall_info structure with op ==
PTRACE_SYSCALL_INFO_SECCOMP or PTRACE_SYSCALL_INFO_ENTRY, it may choose to skip
the system call by changing op to PTRACE_SYSCALL_INFO_EXIT and populating the
exit union fields (rval and is_error) to define the return value and error
status for the tracee.

Signed-off-by: Renzo Davoli <renzo at cs.unibo.it>
---
 kernel/ptrace.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index d041645d9d17..3877624daf4e 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -1099,7 +1099,7 @@ ptrace_set_syscall_info_seccomp(struct task_struct *child, struct pt_regs *regs,
 
 static int
 ptrace_set_syscall_info_exit(struct task_struct *child, struct pt_regs *regs,
-			     struct ptrace_syscall_info *info)
+			     struct ptrace_syscall_info *info, bool skip_syscall)
 {
 	long rval = info->exit.rval;
 
@@ -1116,6 +1116,9 @@ ptrace_set_syscall_info_exit(struct task_struct *child, struct pt_regs *regs,
 	else
 		syscall_set_return_value(child, regs, 0, rval);
 
+	if (skip_syscall)
+		syscall_set_nr(child, regs, -1);
+
 	return 0;
 }
 
@@ -1125,6 +1128,7 @@ ptrace_set_syscall_info(struct task_struct *child, unsigned long user_size,
 {
 	struct pt_regs *regs = task_pt_regs(child);
 	struct ptrace_syscall_info info;
+	bool skip_syscall = false;
 
 	if (user_size < sizeof(info))
 		return -EINVAL;
@@ -1141,15 +1145,24 @@ ptrace_set_syscall_info(struct task_struct *child, unsigned long user_size,
 	if (info.flags || info.reserved)
 		return -EINVAL;
 
-	/* Changing the type of the system call stop is not supported yet. */
-	if (ptrace_get_syscall_info_op(child) != info.op)
-		return -EINVAL;
+	/*
+	 * Changing the type of the system call stop is not allowed, with the
+	 * following exception:
+	 * PTRACE_SYSCALL_INFO_SECCOMP or PTRACE_SYSCALL_INFO_ENTRY can be changed
+	 * to PTRACE_SYSCALL_INFO_EXIT to skip the system call
+	 */
+
+	if (ptrace_get_syscall_info_op(child) != info.op) {
+		if (info.op != PTRACE_SYSCALL_INFO_EXIT)
+			return -EINVAL;
+		skip_syscall = true;
+	}
 
 	switch (info.op) {
 	case PTRACE_SYSCALL_INFO_ENTRY:
 		return ptrace_set_syscall_info_entry(child, regs, &info);
 	case PTRACE_SYSCALL_INFO_EXIT:
-		return ptrace_set_syscall_info_exit(child, regs, &info);
+		return ptrace_set_syscall_info_exit(child, regs, &info, skip_syscall);
 	case PTRACE_SYSCALL_INFO_SECCOMP:
 		return ptrace_set_syscall_info_seccomp(child, regs, &info);
 	default:
-- 
2.53.0