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

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':

quoted

quoted

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