arm64: ptrace: add NT_ARM_SYSTEM_CALL regset

This regeset is intended to be used to get and set a system call number
while tracing.
There was some discussion about possible approaches to do so:

(1) modify x8 register with ptrace(PTRACE_SETREGSET) indirectly,
    and update regs->syscallno later on in syscall_trace_enter(), or
(2) define a dedicated regset for this purpose as on s390, or
(3) support ptrace(PTRACE_SET_SYSCALL) as on arch/arm

Thinking of the fact that user_pt_regs doesn't expose 'syscallno' to
tracer as well as that secure_computing() expects a changed syscall number,
especially case of -1, to be visible before this function returns in
syscall_trace_enter(), (1) doesn't work well.
We will take (2) since it looks much cleaner.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 8a4ae8e..f576781 100644 (file)
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -551,6 +551,32 @@ static int tls_set(struct task_struct *target, const struct user_regset *regset,
        return ret;
 }
 
+static int system_call_get(struct task_struct *target,
+                          const struct user_regset *regset,
+                          unsigned int pos, unsigned int count,
+                          void *kbuf, void __user *ubuf)
+{
+       int syscallno = task_pt_regs(target)->syscallno;
+
+       return user_regset_copyout(&pos, &count, &kbuf, &ubuf,
+                                  &syscallno, 0, -1);
+}
+
+static int system_call_set(struct task_struct *target,
+                          const struct user_regset *regset,
+                          unsigned int pos, unsigned int count,
+                          const void *kbuf, const void __user *ubuf)
+{
+       int syscallno, ret;
+
+       ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &syscallno, 0, -1);
+       if (ret)
+               return ret;
+
+       task_pt_regs(target)->syscallno = syscallno;
+       return ret;
+}
+
 enum aarch64_regset {
        REGSET_GPR,
        REGSET_FPR,
@@ -559,6 +585,7 @@ enum aarch64_regset {
        REGSET_HW_BREAK,
        REGSET_HW_WATCH,
 #endif
+       REGSET_SYSTEM_CALL,
 };
 
 static const struct user_regset aarch64_regsets[] = {
@@ -608,6 +635,14 @@ static const struct user_regset aarch64_regsets[] = {
                .set = hw_break_set,
        },
 #endif
+       [REGSET_SYSTEM_CALL] = {
+               .core_note_type = NT_ARM_SYSTEM_CALL,
+               .n = 1,
+               .size = sizeof(int),
+               .align = sizeof(int),
+               .get = system_call_get,
+               .set = system_call_set,
+       },
 };
 
 static const struct user_regset_view user_aarch64_view = {

diff --cc include/uapi/linux/elf.h
Simple merge