PTRACE_GET_SYSCALL_INFO is a generic ptrace API that lets ptracer obtain details of the syscall the tracee is blocked in.
There are two reasons for a special syscall-related ptrace request.
Firstly, with the current ptrace API there are cases when ptracer cannot retrieve necessary information about syscalls. Some examples include: * The notorious int-0x80-from-64-bit-task issue. See [1] for details. In short, if a 64-bit task performs a syscall through int 0x80, its tracer has no reliable means to find out that the syscall was, in fact, a compat syscall, and misidentifies it. * Syscall-enter-stop and syscall-exit-stop look the same for the tracer. Common practice is to keep track of the sequence of ptrace-stops in order not to mix the two syscall-stops up. But it is not as simple as it looks; for example, strace had a (just recently fixed) long-standing bug where attaching strace to a tracee that is performing the execve system call led to the tracer identifying the following syscall-exit-stop as syscall-enter-stop, which messed up all the state tracking. * Since the introduction of commit 84d77d3f06e7e8dea057d10e8ec77ad71f721be3 ("ptrace: Don't allow accessing an undumpable mm"), both PTRACE_PEEKDATA and process_vm_readv become unavailable when the process dumpable flag is cleared. On such architectures as ia64 this results in all syscall arguments being unavailable for the tracer.
Secondly, ptracers also have to support a lot of arch-specific code for obtaining information about the tracee. For some architectures, this requires a ptrace(PTRACE_PEEKUSER, ...) invocation for every syscall argument and return value.
PTRACE_GET_SYSCALL_INFO returns the following structure:
struct ptrace_syscall_info { __u8 op; /* PTRACE_SYSCALL_INFO_* */ __u32 arch __attribute__((__aligned__(sizeof(__u32)))); __u64 instruction_pointer; __u64 stack_pointer; union { struct { __u64 nr; __u64 args[6]; } entry; struct { __s64 rval; __u8 is_error; } exit; struct { __u64 nr; __u64 args[6]; __u32 ret_data; } seccomp; }; };
The structure was chosen according to [2], except for the following changes: * seccomp substructure was added as a superset of entry substructure; * the type of nr field was changed from int to __u64 because syscall numbers are, as a practical matter, 64 bits; * stack_pointer field was added along with instruction_pointer field since it is readily available and can save the tracer from extra PTRACE_GETREGS/PTRACE_GETREGSET calls; * arch is always initialized to aid with tracing system calls * such as execve(); * instruction_pointer and stack_pointer are always initialized so they could be easily obtained for non-syscall stops; * a boolean is_error field was added along with rval field, this way the tracer can more reliably distinguish a return value from an error value.
strace has been ported to PTRACE_GET_SYSCALL_INFO. Starting with release 4.26, strace uses PTRACE_GET_SYSCALL_INFO API as the preferred mechanism of obtaining syscall information.
[1] https://lore.kernel.org/lkml/CA+55aFzcSVmdDj9Lh_gdbz1OzHyEm6ZrGPBDAJnywm2LF_... [2] https://lore.kernel.org/lkml/CAObL_7GM0n80N7J_DFw_eQyfLyzq+sf4y2AvsCCV88Tb3A...
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Notes: v8: * Moved syscall_get_arch() specific patches to a separate patchset which is now merged into audit/next tree. * Rebased to linux-next. * Moved ptrace_get_syscall_info code under #ifdef CONFIG_HAVE_ARCH_TRACEHOOK, narrowing down the set of architectures supported by this implementation back to those 19 that enable CONFIG_HAVE_ARCH_TRACEHOOK because I failed to get all syscall_get_*(), instruction_pointer(), and user_stack_pointer() functions implemented on some niche architectures. This leaves the following architectures out: alpha, h8300, m68k, microblaze, and unicore32.
v7: * Rebased to v5.0-rc1. * 5 arch-specific preparatory patches out of 25 have been merged into v5.0-rc1 via arch trees.
v6: * Add syscall_get_arguments and syscall_set_arguments wrappers to asm-generic/syscall.h, requested by Geert. * Change PTRACE_GET_SYSCALL_INFO return code: do not take trailing paddings into account, use the end of the last field of the structure being written. * Change struct ptrace_syscall_info: * remove .frame_pointer field, is is not needed and not portable; * make .arch field explicitly aligned, remove no longer needed padding before .arch field; * remove trailing pads, they are no longer needed.
v5: * Merge separate series and patches into the single series. * Change PTRACE_EVENTMSG_SYSCALL_{ENTRY,EXIT} values as requested by Oleg. * Change struct ptrace_syscall_info: generalize instruction_pointer, stack_pointer, and frame_pointer fields by moving them from ptrace_syscall_info.{entry,seccomp} substructures to ptrace_syscall_info and initializing them for all stops. * Add PTRACE_SYSCALL_INFO_NONE, set it when not in a syscall stop, so e.g. "strace -i" could use PTRACE_SYSCALL_INFO_SECCOMP to obtain instruction_pointer when the tracee is in a signal stop. * Patch all remaining architectures to provide all necessary syscall_get_* functions. * Make available for all architectures: do not conditionalize on CONFIG_HAVE_ARCH_TRACEHOOK since all syscall_get_* functions are implemented on all architectures. * Add a test for PTRACE_GET_SYSCALL_INFO to selftests/ptrace.
v4: * Do not introduce task_struct.ptrace_event, use child->last_siginfo->si_code instead. * Implement PTRACE_SYSCALL_INFO_SECCOMP and ptrace_syscall_info.seccomp support along with PTRACE_SYSCALL_INFO_{ENTRY,EXIT} and ptrace_syscall_info.{entry,exit}.
v3: * Change struct ptrace_syscall_info. * Support PTRACE_EVENT_SECCOMP by adding ptrace_event to task_struct. * Add proper defines for ptrace_syscall_info.op values. * Rename PT_SYSCALL_IS_ENTERING and PT_SYSCALL_IS_EXITING to PTRACE_EVENTMSG_SYSCALL_ENTRY and PTRACE_EVENTMSG_SYSCALL_EXIT * and move them to uapi.
v2: * Do not use task->ptrace. * Replace entry_info.is_compat with entry_info.arch, use syscall_get_arch(). * Use addr argument of sys_ptrace to get expected size of the struct; return full size of the struct.
Dmitry V. Levin (6): nds32: fix asm/syscall.h hexagon: define syscall_get_error() and syscall_get_return_value() mips: define syscall_get_error() parisc: define syscall_get_error() powerpc: define syscall_get_error() selftests/ptrace: add a test case for PTRACE_GET_SYSCALL_INFO
Elvira Khabirova (1): ptrace: add PTRACE_GET_SYSCALL_INFO request
arch/hexagon/include/asm/syscall.h | 14 + arch/mips/include/asm/syscall.h | 6 + arch/nds32/include/asm/syscall.h | 29 +- arch/parisc/include/asm/syscall.h | 7 + arch/powerpc/include/asm/syscall.h | 10 + include/linux/tracehook.h | 9 +- include/uapi/linux/ptrace.h | 35 +++ kernel/ptrace.c | 103 ++++++- tools/testing/selftests/ptrace/.gitignore | 1 + tools/testing/selftests/ptrace/Makefile | 2 +- .../selftests/ptrace/get_syscall_info.c | 271 ++++++++++++++++++ 11 files changed, 471 insertions(+), 16 deletions(-) create mode 100644 tools/testing/selftests/ptrace/get_syscall_info.c