Re: [PATCH v5 24/25] ptrace: add PTRACE_GET_SYSCALL_INFO request

From: Dmitry V. Levin
Date: Mon Dec 10 2018 - 11:09:47 EST


Hi, things are getting too complicated and we need some advice how to deal
with this frame_pointer issue.

On Mon, Dec 10, 2018 at 10:26:50PM +0800, kbuild test robot wrote:
> Hi Elvira,
>
> Thank you for the patch! Yet something to improve:
>
> [auto build test ERROR on linus/master]
> [also build test ERROR on v4.20-rc6]
> [cannot apply to next-20181207]
> [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
>
> url: https://github.com/0day-ci/linux/commits/Dmitry-V-Levin/ptrace-add-PTRACE_GET_SYSCALL_INFO-request/20181210-174745
> config: mips-malta_kvm_defconfig (attached as .config)
> compiler: mipsel-linux-gnu-gcc (Debian 7.2.0-11) 7.2.0
> reproduce:
> wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> chmod +x ~/bin/make.cross
> # save the attached .config to linux build tree
> GCC_VERSION=7.2.0 make.cross ARCH=mips
>
> All errors (new ones prefixed by >>):
>
> kernel/ptrace.c: In function 'ptrace_get_syscall_info':
> >> kernel/ptrace.c:942:20: error: implicit declaration of function 'frame_pointer'; did you mean 'trace_printk'? [-Werror=implicit-function-declaration]
> .frame_pointer = frame_pointer(regs)
> ^~~~~~~~~~~~~
> trace_printk
> cc1: some warnings being treated as errors
>
> vim +942 kernel/ptrace.c
>
> 931
> 932 static int
> 933 ptrace_get_syscall_info(struct task_struct *child, unsigned long user_size,
> 934 void __user *datavp)
> 935 {
> 936 struct pt_regs *regs = task_pt_regs(child);
> 937 struct ptrace_syscall_info info = {
> 938 .op = PTRACE_SYSCALL_INFO_NONE,
> 939 .arch = syscall_get_arch(child),
> 940 .instruction_pointer = instruction_pointer(regs),
> 941 .stack_pointer = user_stack_pointer(regs),
> > 942 .frame_pointer = frame_pointer(regs)
> 943 };
> 944 unsigned long actual_size = offsetof(struct ptrace_syscall_info, entry);
> 945 unsigned long write_size;
> 946
> 947 /*
> 948 * This does not need lock_task_sighand() to access
> 949 * child->last_siginfo because ptrace_freeze_traced()
> 950 * called earlier by ptrace_check_attach() ensures that
> 951 * the tracee cannot go away and clear its last_siginfo.
> 952 */
> 953 switch (child->last_siginfo ? child->last_siginfo->si_code : 0) {
> 954 case SIGTRAP | 0x80:
> 955 switch (child->ptrace_message) {
> 956 case PTRACE_EVENTMSG_SYSCALL_ENTRY:
> 957 actual_size = ptrace_get_syscall_info_entry(child, regs,
> 958 &info);
> 959 break;
> 960 case PTRACE_EVENTMSG_SYSCALL_EXIT:
> 961 actual_size = ptrace_get_syscall_info_exit(child, regs,
> 962 &info);
> 963 break;
> 964 }
> 965 break;
> 966 case SIGTRAP | (PTRACE_EVENT_SECCOMP << 8):
> 967 actual_size = ptrace_get_syscall_info_seccomp(child, regs,
> 968 &info);
> 969 break;
> 970 }
> 971
> 972 write_size = min(actual_size, user_size);
> 973 return copy_to_user(datavp, &info, write_size) ? -EFAULT : actual_size;
> 974 }
> 975

We decided to add .frame_pointer to struct ptrace_syscall_info just for
consistency with .instruction_pointer and .stack_pointer; I must have been
misled by comments in asm-generic/ptrace.h into thinking that
frame_pointer() is universally available across architectures.

Unlike .instruction_pointer and .stack_pointer that are actually needed
in strace, .frame_pointer is not used, so from strace PoV we don't really
need it.

So the question is, does anybody need a
struct ptrace_syscall_info.frame_pointer?

If yes, how can frame_pointer() be defined on MIPS?
Or should we just forget about making sense of frame_pointer() and remove
struct ptrace_syscall_info.frame_pointer from the proposed API?


--
ldv

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