Re: [PATCH v9 1/6] arm64: ptrace: add NT_ARM_SYSTEM_CALL regset

From: AKASHI Takahiro
Date: Thu Nov 27 2014 - 00:53:48 EST


On 11/26/2014 09:41 PM, Will Deacon wrote:
Hi Akashi,

On Wed, Nov 26, 2014 at 04:49:46AM +0000, AKASHI Takahiro wrote:
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@xxxxxxxxxx>
---
arch/arm64/kernel/ptrace.c | 35 +++++++++++++++++++++++++++++++++++
include/uapi/linux/elf.h | 1 +
2 files changed, 36 insertions(+)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 8a4ae8e..8b98781 100644
--- 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)
+{
+ struct pt_regs *regs = task_pt_regs(target);
+
+ return user_regset_copyout(&pos, &count, &kbuf, &ubuf,
+ &regs->syscallno, 0, -1);

Does this work for big-endian machines? regs->syscallno is a u64, but the
regset defines it as an int. I think you need to copy to a temporary
register first.

Right. I will fix it.
Do you prefer to use s32, instead of int, like other regsets?


Thanks,
-Takahiro AKASHI

Will

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