ptrace: Remove maxargs from task_current_syscall()

task_current_syscall() has a single user that passes in 6 for maxargs, which
is the maximum arguments that can be used to get system calls from
syscall_get_arguments(). Instead of passing in a number of arguments to
grab, just get 6 arguments. The args argument even specifies that it's an
array of 6 items.

This will also allow changing syscall_get_arguments() to not get a variable
number of arguments, but always grab 6.

Linus also suggested not passing in a bunch of arguments to
task_current_syscall() but to instead pass in a pointer to a structure, and
just fill the structure. struct seccomp_data has almost all the parameters
that is needed except for the stack pointer (sp). As seccomp_data is part of
uapi, and I'm afraid to change it, a new structure was created
"syscall_info", which includes seccomp_data and adds the "sp" field.

Link: http://lkml.kernel.org/r/20161107213233.466776454@goodmis.org

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

1 files changed, 25 insertions, 32 deletions

diff --git a/lib/syscall.c b/lib/syscall.c
index 1a7077f20eae..e8467e17b9a2 100644
--- a/lib/syscall.c
+++ b/lib/syscall.c
@@ -5,16 +5,14 @@
 #include <linux/export.h>
 #include <asm/syscall.h>
 
-static int collect_syscall(struct task_struct *target, long *callno,
-                           unsigned long args[6], unsigned int maxargs,
-                           unsigned long *sp, unsigned long *pc)
+static int collect_syscall(struct task_struct *target, struct syscall_info *info)
 {
         struct pt_regs *regs;
 
         if (!try_get_task_stack(target)) {
                 /* Task has no stack, so the task isn't in a syscall. */
-                *sp = *pc = 0;
-                *callno = -1;
+                memset(info, 0, sizeof(*info));
+                info->data.nr = -1;
                 return 0;
         }
 
@@ -24,12 +22,13 @@ static int collect_syscall(struct task_struct *target, long *callno,
                 return -EAGAIN;
         }
 
-        *sp = user_stack_pointer(regs);
-        *pc = instruction_pointer(regs);
+        info->sp = user_stack_pointer(regs);
+        info->data.instruction_pointer = instruction_pointer(regs);
 
-        *callno = syscall_get_nr(target, regs);
-        if (*callno != -1L && maxargs > 0)
-                syscall_get_arguments(target, regs, 0, maxargs, args);
+        info->data.nr = syscall_get_nr(target, regs);
+        if (info->data.nr != -1L)
+                syscall_get_arguments(target, regs, 0, 6,
+                                      (unsigned long *)&info->data.args[0]);
 
         put_task_stack(target);
         return 0;
@@ -38,41 +37,35 @@ static int collect_syscall(struct task_struct *target, long *callno,
 /**
  * task_current_syscall - Discover what a blocked task is doing.
  * @target:             thread to examine
- * @callno:             filled with system call number or -1
- * @args:               filled with @maxargs system call arguments
- * @maxargs:            number of elements in @args to fill
- * @sp:                 filled with user stack pointer
- * @pc:                 filled with user PC
+ * @info:               structure with the following fields:
+ *                       .sp        - filled with user stack pointer
+ *                       .data.nr   - filled with system call number or -1
+ *                       .data.args - filled with @maxargs system call arguments
+ *                       .data.instruction_pointer - filled with user PC
  *
- * If @target is blocked in a system call, returns zero with *@callno
- * set to the the call's number and @args filled in with its arguments.
- * Registers not used for system call arguments may not be available and
- * it is not kosher to use &struct user_regset calls while the system
+ * If @target is blocked in a system call, returns zero with @info.data.nr
+ * set to the the call's number and @info.data.args filled in with its
+ * arguments. Registers not used for system call arguments may not be available
+ * and it is not kosher to use &struct user_regset calls while the system
  * call is still in progress.  Note we may get this result if @target
  * has finished its system call but not yet returned to user mode, such
  * as when it's stopped for signal handling or syscall exit tracing.
  *
  * If @target is blocked in the kernel during a fault or exception,
- * returns zero with *@callno set to -1 and does not fill in @args.
- * If so, it's now safe to examine @target using &struct user_regset
- * get() calls as long as we're sure @target won't return to user mode.
+ * returns zero with *@info.data.nr set to -1 and does not fill in
+ * @info.data.args. If so, it's now safe to examine @target using
+ * &struct user_regset get() calls as long as we're sure @target won't return
+ * to user mode.
  *
  * Returns -%EAGAIN if @target does not remain blocked.
- *
- * Returns -%EINVAL if @maxargs is too large (maximum is six).
  */
-int task_current_syscall(struct task_struct *target, long *callno,
-                         unsigned long args[6], unsigned int maxargs,
-                         unsigned long *sp, unsigned long *pc)
+int task_current_syscall(struct task_struct *target, struct syscall_info *info)
 {
         long state;
         unsigned long ncsw;
 
-        if (unlikely(maxargs > 6))
-                return -EINVAL;
-
         if (target == current)
-                return collect_syscall(target, callno, args, maxargs, sp, pc);
+                return collect_syscall(target, info);
 
         state = target->state;
         if (unlikely(!state))
@@ -80,7 +73,7 @@ int task_current_syscall(struct task_struct *target, long *callno,
 
         ncsw = wait_task_inactive(target, state);
         if (unlikely(!ncsw) ||
-            unlikely(collect_syscall(target, callno, args, maxargs, sp, pc)) ||
+            unlikely(collect_syscall(target, info)) ||
             unlikely(wait_task_inactive(target, state) != ncsw))
                 return -EAGAIN;