PTRACE_SET_SYSCALL_INFO is a generic ptrace API that complements PTRACE_GET_SYSCALL_INFO by letting the ptracer modify details of system calls the tracee is blocked in.

This API allows ptracers to obtain and modify system call details in a straightforward and architecture-agnostic way.

Current implementation supports changing only those bits of system call information that are used by strace, namely, syscall number, syscall arguments, and syscall return value.

Support of changing additional details returned by PTRACE_GET_SYSCALL_INFO, such as instruction pointer and stack pointer, could be added later if needed, by using struct ptrace_syscall_info.flags to specify the additional details that should be set. Currently, "flags" and "reserved" fields of struct ptrace_syscall_info must be initialized with zeroes; "arch", "instruction_pointer", and "stack_pointer" fields are ignored.

PTRACE_SET_SYSCALL_INFO currently supports only PTRACE_SYSCALL_INFO_ENTRY, PTRACE_SYSCALL_INFO_EXIT, and PTRACE_SYSCALL_INFO_SECCOMP operations. Other operations could be added later if needed.

Ideally, PTRACE_SET_SYSCALL_INFO should have been introduced along with PTRACE_GET_SYSCALL_INFO, but it didn't happen. The last straw that convinced me to implement PTRACE_SET_SYSCALL_INFO was apparent failure to provide an API of changing the first system call argument on riscv architecture [1].

ptrace(2) man page:

long ptrace(enum __ptrace_request request, pid_t pid, void *addr, void *data); ... PTRACE_SET_SYSCALL_INFO Modify information about the system call that caused the stop. The "data" argument is a pointer to struct ptrace_syscall_info that specifies the system call information to be set. The "addr" argument should be set to sizeof(struct ptrace_syscall_info)).

[1] https://lore.kernel.org/all/59505464-c84a-403d-972f-d4b2055eeaac@gmail.com/

Notes: v4: * Split out syscall_set_return_value() for hexagon into a separate patch * s390: Change the style of syscall_set_arguments() implementation as requested * Add more Reviewed-by * v3: https://lore.kernel.org/all/20250128091445.GA8257@strace.io/

v3: * powerpc: Submit syscall_set_return_value() fix for "sc" case separately * mips: Do not introduce erroneous argument truncation on mips n32, add a detailed description to the commit message of the mips_get_syscall_arg() change * ptrace: Add explicit padding to the end of struct ptrace_syscall_info, simplify obtaining of user ptrace_syscall_info, do not introduce PTRACE_SYSCALL_INFO_SIZE_VER0 * ptrace: Change the return type of ptrace_set_syscall_info_* functions from "unsigned long" to "int" * ptrace: Add -ERANGE check to ptrace_set_syscall_info_exit(), add comments to -ERANGE checks * ptrace: Update comments about supported syscall stops * selftests: Extend set_syscall_info test, fix for mips n32 * Add Tested-by and Reviewed-by

v2: * Add patch to fix syscall_set_return_value() on powerpc * Add patch to fix mips_get_syscall_arg() on mips * Add syscall_set_return_value() implementation on hexagon * Add syscall_set_return_value() invocation to syscall_set_nr() on arm and arm64. * Fix syscall_set_nr() and mips_set_syscall_arg() on mips * Add a comment to syscall_set_nr() on arc, powerpc, s390, sh, and sparc * Remove redundant ptrace_syscall_info.op assignments in ptrace_get_syscall_info_* * Minor style tweaks in ptrace_get_syscall_info_op() * Remove syscall_set_return_value() invocation from ptrace_set_syscall_info_entry() * Skip syscall_set_arguments() invocation in case of syscall number -1 in ptrace_set_syscall_info_entry() * Split ptrace_syscall_info.reserved into ptrace_syscall_info.reserved and ptrace_syscall_info.flags * Use __kernel_ulong_t instead of unsigned long in set_syscall_info test

v1:

Dmitry V. Levin (7): mips: fix mips_get_syscall_arg() for o32 hexagon: add syscall_set_return_value() syscall.h: add syscall_set_arguments() syscall.h: introduce syscall_set_nr() ptrace_get_syscall_info: factor out ptrace_get_syscall_info_op ptrace: introduce PTRACE_SET_SYSCALL_INFO request selftests/ptrace: add a test case for PTRACE_SET_SYSCALL_INFO

arch/arc/include/asm/syscall.h | 25 + arch/arm/include/asm/syscall.h | 37 ++ arch/arm64/include/asm/syscall.h | 29 + arch/csky/include/asm/syscall.h | 13 + arch/hexagon/include/asm/syscall.h | 21 + arch/loongarch/include/asm/syscall.h | 15 + arch/m68k/include/asm/syscall.h | 7 + arch/microblaze/include/asm/syscall.h | 7 + arch/mips/include/asm/syscall.h | 70 ++- arch/nios2/include/asm/syscall.h | 16 + arch/openrisc/include/asm/syscall.h | 13 + arch/parisc/include/asm/syscall.h | 19 + arch/powerpc/include/asm/syscall.h | 20 + arch/riscv/include/asm/syscall.h | 16 + arch/s390/include/asm/syscall.h | 21 + arch/sh/include/asm/syscall_32.h | 24 + arch/sparc/include/asm/syscall.h | 22 + arch/um/include/asm/syscall-generic.h | 19 + arch/x86/include/asm/syscall.h | 43 ++ arch/xtensa/include/asm/syscall.h | 18 + include/asm-generic/syscall.h | 30 + include/uapi/linux/ptrace.h | 7 +- kernel/ptrace.c | 179 +++++- tools/testing/selftests/ptrace/Makefile | 2 +- .../selftests/ptrace/set_syscall_info.c | 514 ++++++++++++++++++ 25 files changed, 1140 insertions(+), 47 deletions(-) create mode 100644 tools/testing/selftests/ptrace/set_syscall_info.c